Electrical wiring FAQ (11/94)
Last-modified: Mon Nov 14 01:10:37 EST 1994
Frequently Asked Questions on Electrical Wiring
Copyright 1991, 1992, 1993
Steven Bellovin (email@example.com)
Chris Lewis (firstname.lastname@example.org)
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Subject: Questions answered in this FAQ
What is the NEC? Where can I get a copy?
What is the CEC? Where can I get a copy?
Can I do my own wiring? Extra pointers?
What do I need in the way of tools?
What is UL listing?
What is CSA approval?
What impact does NAFTA have on wiring standards and approvals?
| Are there any cheaper, easier to read books on wiring?
Inspections how and what? Why should I get my wiring inspected?
My house doesn't meet some of these rules and regulations.
A word on voltages: 110/115/117/120/125/220/240
What does an electrical service look like?
What is a circuit?
"grounding" versus "grounded" versus "neutral".
What does a fuse or breaker do? What are the differences?
Breakers? Can't I use fuses?
What size wire should I use?
Where do these numbers come from?
What does "14-2" mean?
What is a "wirenut"/"marrette"/"marr connector". How are they used?
What is a GFI/GFCI?
Where should GFCIs be used?
Where shouldn't I use a GFCI?
What is the difference between a GFCI outlet and a GFCI breaker?
What's the purpose of the ground prong on an outlet, then?
Why is one prong wider than the other? Polarization
How do I convert two prong receptacles to three prong?
Surges, spikes, zaps, grounding and your electronics
Are you sure about GFCIs and ungrounded outlets?
Should the test button work?
What kind of outlets do I need in a kitchen?
Where must outlets and switches be in bathrooms?
General outlet placement rules/line capacities
What is Romex/NM/NMD? What is BX? When should I use each?
Should I use plastic or metal boxes?
Junction box positioning?
Can I install a replacement fixture?
Noisy fluorescent fixtures, what do I do?
| Noisy lights with dimmer switches, what do I do? (NEW)
What does it mean when the lights brighten when a motor starts?
What is 3 phase power? Should I use it? Can I get it in my house?
Is it better to run motors at 110 or 220?
What is this nonsense about 3HP on 110V 15A circuits?
How should I wire my shop?
Doorbell/telephone/cable other service wiring hints
I'm buying a house! What should I do?
What is this weird stuff? Old style wiring
Where do I buy stuff?
Copper wire characteristics table
| Smoke detector guidelines (NEW)
Although we've done a fair bit of wiring, we are not
electricians, and we cannot be responsible for what you do. If
you're at all uncertain about what is correct or safe, *don't
do it*. Contact someone qualified -- a licensed electrician,
or your local electrical inspector. Electricity is no joke;
mistakes can result in shocks, fires, or electrocution.
Furthermore, our discussion is based on the U.S. National
Electrical Code (NEC) and the Canadian Electrical code (CEC).
To the best of our abilities, we have confirmed every detail
with the electrical code, but we don't quote sections
simply to keep this thing readable. If you think we're wrong,
we invite you to correct us, but please - quote references!
The NEC and the CEC do not, in and of themselves, have the
force of law. Many municipalities adopt it en toto. Others,
however, do not. Check your with your local building
department (and <provincial> Hydro Inspection Offices in
Canada) to find out what applies in your area. Also,
your local electrical utility may also have special requirements
for electrical service installation. Bear in mind, too, that
we say here applies primarily to ordinary single-family
residences. Multi-family dwellings, mobile homes, commercial
establishments, etc., are sometimes governed by different
Also note that, contrary to popular belief in the U.S. (and in
some parts of Canada), Canada is not a wholly-owned subsidiary
of the U.S. Consequently, the NEC does not apply in Canada.
Lots of things are the same, including voltages, line
frequencies, and the laws of physics. But there are a number
of crucial differences in the regulations. Where we can, we've
noted them, flagging the relevant passages with ``NEC'' or
Remember that the CEC and NEC are minimal standards. It is often
smart to go beyond their minimal requirements.
Subject: What is the NEC? Where can I get a copy?
The NEC is a model electrical code devised and published by the
National Fire Protection Association, an insurance industry group.
It's revised every three years. The 1993 version has been released.
You can buy a copy at a decent bookstore, or by calling them directly
at 800-344-3555. The code exists in several versions. There's the
full text, which is fairly incomprehensible. There's an abridged
edition, which has only the sections likely to apply to most houses.
And there's the NEC Handbook, which contains the ``authorized
commentary'' on the code, as well as the full text. That's the
recommended version. Unfortunately, there's no handbook for
the abridged edition. And the full handbook is expensive --
US$65 plus shipping and handling.
Subject: What is the CEC? Where can I get a copy?
The Canadian Standards Association is an organization made up
of various government agencies, power utilities, insurance
companies, electrical manufacturers and other organizations.
The CSA publishes CSA Standard C22.1 which is updated every two
or three years. Each province adopts, with some amendments,
this standard and publishes a province-specific code book.
Since each province publishes its own slightly modified
standard, it would be somewhat confusing to obtain the CSA
standard itself. In this FAQ, "CEC" really means the
appropriate provincial standard. In particular, this FAQ is
derived from the Ontario Hydro Electrical Safety Code, 20th
edition (1990). Which is in turn based on CSA C22.1-1990 (16th
edition). While differences exist between the provinces, an
attempt has been made to avoid specific-to-Ontario detail.
The appropriate provincial code can be obtained from electrical
inspection offices of your provincial power authority. In
Ontario, it's Ontario Hydro. The Ontario Hydro book isn't
overly fat. It's about C$25, and includes mailed updates. I
hear that these standards are somewhat easier to read than the
equivalent NEC publications.
Don't bother asking in Quebec - DIY wiring is banned throughout
Subject: Can I do my own wiring? Extra pointers?
In most places, homeowners are allowed to do their own wiring.
In some, they're not. Check with your local electrical
inspector. Most places won't permit you to do wiring on other's
homes for money without a license. Nor are you permitted to do
wiring in "commercial" buildings. Multiple dwellings (eg: duplexes)
are usually considered "semi-commercial" or "commercial". However,
many jurisdictions will permit you to work on semi-commercial
wiring if you're supervised by a licensed electrician - if you can
find one willing to supervise.
If you do your own wiring, an important point:
Do it NEAT and WELL! What you really want to aim for is a better
job than an electrician will do. After all, it's your own home,
and it's you or your family that might get killed if you make
a mistake. An electrician has time pressures, has the skills
and knows the tricks of the trade to do a fast, safe job.
In this FAQ we've consciously given a few recommendations that
are in excess of code, because we feel that it's reasonable,
and will impress the inspector.
The inspector will know that you're an amateur. You have to
earn his trust. The best way of doing this is to spend your
time doing as neat a job as possible. Don't cut corners.
Exceed specifications. Otherwise, the inspector may get extremely
picky and fault you on the slightest transgressions.
Don't try to hide anything from the inspector.
Use the proper tools. Ie: don't use a bread knife to strip
wires, or twist wires with your fingers. The inspector
won't like it, and the results won't be that safe. And it
takes longer. And you're more likely to stick a hunk of
12ga wire through your hand that way.
Don't handle house wire when it's very cold (eg: below -10C
or 16F). Thermoplastic house wire, particularly older types
become very brittle.
Subject: What do I need in the way of tools?
First, there's the obvious -- a hammer, a drill, a few
screwdrivers, both straight and Phillips-head. If you're
lucky enough to live in Canada (or find a source of CSA-approved
devices) you need Robertson ("square recess") screwdrivers
(#1 and #2) instead of phillips.
For drilling a few holes, a 3/4" or 1" spade bit and 1/4" or
3/8" electric drill will do. If you're doing a lot, or
are working with elderly lumber, we recommend a 1/2" drill
(right-angle drills are wonderful. Can be rented) and
3/4" or 1" screw-point auger drill bits. These bits pull
you through, so they're much faster and less fatiguing, even
in 90 year old hardwood timbers.
Screw-driver bits are useful for drills, expecially if you
install your electrical boxes using screws (drywall screws
For stripping wire, use a real wire stripper, not a knife or
ordinary wire cutters. Don't buy the $3 K-mart "combo stripper,
crimper and bottle opener" types. You should expect to pay
$15 to $20 for a good "plier-type" pair. It will have sized
stripping holes, and won't nick or grab the wire - it should
be easy to strip wire with it. One model has a small hole in the
blade for forming exact wire loops for screw terminals. There
are fancier types (autostrip/cut), but they generally aren't
necessary, and pros usually don't use them.
A pair of diagonal side cutter pliers are useful for clipping ends
in constricted places. Don't use these for stripping wire.
You will need linesman pliers for twisting wires for wire nuts.
You should have a pair of needle-nose pliers for fiddling
inside boxes and closing loops, but it's better to form wire
loops with a "loop former hole" on your wire stripper - more
If you're using non-metallic cable, get a cable stripper for
removing the sheath. Or, do what some pros do, they nick the
end of the sheath, grab the ground wire with a pair of pliers,
and simply rip the sheath back using the ground wire as a
"zipper", and cut the sheath off. You shouldn't try to strip
the sheath with a knife point, because it's too easy to
slash the insulation on the conductors. Apparently Stanley
utility knives fitted with linoleum cutters (hooked blades)
can be used to strip sheath, but there is still the possibility
that you'll gouge the conductors.
For any substantial amount of work with armored cable, it's well
worth your while to invest in a rotary cable splitter (~US$ 18).
Hack saws are tricky to use without cutting into the wire
or the insulation.
Three-prong outlet testers are a quick check for properly-wired
outlets. About $6. Multimeters tell you more, but are a lot more
expensive, and probably not worth it for most people. A simple
voltage sensor, which can detect potential through an insulated
wire not supplying any devices, is extremely helpful; they cost
about US$ 10 at Radio Shack.
You should have a voltage detector - to check that the wires are
dead before doing work on them. Neon-bulb version are cheap ($2-3)
and work well. If you get more serious, a "audible alarm" type is
good for tracing circuits without a helper. (Though I've been known
to lock the drill on, and hit breakers until the scream stops ;-)
For running wires through existing walls, you need fish tape.
Often, two tapes are needed, though sometimes, a bent hanger or
a length of thin chain will suffice. Fish tapes can be rented.
Electrical tape. Lots of it ;-) Seriously, a good and competent
wiring job will need very little tape. The tape is useful for
wrapping dicy insulation in repair work. Another use is to wrap
around the body of outlets and switches to cover the termination
screws - I don't do this, but drywall contractors prefer it (to
prevent explosions when the drywall knife collides with a live outlet
that has no cover plate).
Subject: What is UL listing?
The UL stands for "Underwriters Laboratory". It used to be
an Insurance Industry organization, but now it is independent
and non-profit. It tests electrical components and equipment
for potential hazards. When something is UL-listed, that means
that the UL has tested the device, and it meets their requirements
for safety - ie: fire or shock hazard. It doesn't necessarily
mean that the device actually does what it's supposed to, just
that it probably won't kill you.
The UL does not have power of law in the U.S. -- you are
permitted to buy and install non-UL-listed devices. However,
insurance policies sometimes have clauses in them that will
limit their liability in case of a claim made in response to
the failure of a non-UL-listed device. Furthermore, in
many situations the NEC will require that a wiring component
used for a specific purpose is UL-listed for that purpose.
Indirectly, this means that certain parts of your wiring
must be UL-listed before an inspector will approve it and/or
occupancy permits issued.
Subject: What is CSA approval?
Every electrical device or component must be certified by the
Canadian Standards Association (or recognized equivalent) before
it can be sold in Canada. Implicit in this is that all wiring
must be done with CSA-approved materials. They perform testing
similar to the UL (a bit more stringent), except that CSA (or
recognized equivalent) approval is required by law.
Again, like the UL, if a fire was caused by non-CSA-approved
equipment, your insurance company may not have to pay the
Note: strictly speaking, there usually is a legal way around
the lack of a CSA sticker. In some cases (eg: Ontario), a
local hydro inspection prior to purchase, or prior to use, is
acceptable. The hydro inspector will affix a "hydro sticker"
to the unit, which is as good as CSA approval. But it costs
money - last I knew, $75 per unit inspected.
ULC (Underwriters Laboratory of Canada) is an independent
organization that, amongst other things, undertakes the
quarterly inspection of manufacturer's to ensure continued
compliance of UL Listed/Recognized products to Agency reports
and safety standards. This work is done under contract to UL
Inc (Follow-up Services Division). They are not a branch or
subsidiary of UL.
Subject: What impact does NAFTA have on wiring standards and approvals?
The North America Free Trade Agreement came into effect on
January 1st, 1994. NAFTA attempts to bring down trade barriers
between Mexico, Canada and the USA. One of the "barriers" has
been that of approval of material. As of January first, CSA
approval of a device is legally considered equivalent to UL
approval in the USA. Conversely, UL is now accepted as
equivalent to CSA approval in Canada. Theoretically, this
means that devices marked only with UL approval are acceptable
in the CEC, and conversely CSA approval by itself of a device
is accepted by the NEC. This allows much freer trade in
electrical materials between the two countries.
This doesn't affect the electrical codes themselves, so the
differences in practice between the NEC and CEC will remain.
It is also my understanding that bilateral acceptance of
"approval" will only apply when the standards applied are
reasonably the same. As an example, a cable approved by the
NEC for a given purpose may not be acceptable by the CEC for
the same purpose if the standards requirements are different.
Eg: "NMD" ("non-metallic, damp") cable is usually required for
residences in Canada. "NM" cable ("non-metallic, not damp
locations) which is used in the same situations in the US,
would probably not be acceptable in Canada. Also,
municipalities can add additional requirements on top of the
CEC, as they can in the US over the NEC.
Thus, Canadians will probably start seeing UL-only approved
materials in stores, and Americans the same regarding
CSA-only. But some differences will remain. When in doubt on
major items, consult an inspector. At least in Canada, the
fact that the material is available in a store usually means
that it's okay to install.
Subject: Are there any cheaper, easier to read books on wiring?
USA: The following three books were suggested by our readers
by Jeff Markell,
Carlsbad CA for $18.25. ISBN 0-934041-19-9.
Practical Electrical Wiring
Residential, Farm and Industrial, Based on the National
Electrical Code ANSI/NFPA 70
Herbert P. Richter and W. Creighton Schwan
McGraw-Hill Book Co.
H. P. Richter and W. C. Schwan
Park Publishing Co.
| The Electrician's Toolbox Manual
| Rex Miller
| Prentice Hall (ARCO) 1989
| ISBN 0-13-247701-7 $11.00
Try to make sure that the book is based on the latest NEC
revision. Which is currently 1993.
Canada: P.S. Knight authors and publishes a book called
"Electrical Code Simplified". There appears to be a version
published specific to each province, and is very tied into the
appropriate provincial code. It focuses on residential wiring,
and is indispensible for Canadian DIY'ers. It is better to get
this book than the CEC unless you do a lot of wiring (or answer
questions on the net ;-).
It is updated each time the provincial codes are. This book is
available at all DIY and hardware stores for less than C$10.
Subject: Inspections how and what? Why should I get my wiring inspected?
Most jurisdictions require that you obtain a permit and
inspections of any wiring that is done. Amongst other more
mundane bureaucratic reasons (like insurance companies not
liking to have to pay claims), a permit and inspections
provides some assurance that you, your family, your neighbors
or subsequent owners of your home don't get killed or lose
their homes one night due to a sloppy wiring job.
Most jurisdictions have the power to order you to vacate your
home, or order you to tear out any wiring done without a
permit. California, for instance, is particularly nasty about
If fire starts in your home, and un-inspected wiring is at
fault, insurance companies will often refuse to pay the damage
In general, the process goes like this:
- you apply to your local inspections office or building
department for a permit. You should have a sketch or
detailed drawing of what you plan on doing. This is
a good time to ask questions on any things you're not
sure of. If you're doing major work, they may impose
special conditions on you, require loading
calculations and ask other questions. At this point
they will tell you which inspections you will need.
- If you're installing a main panel, you will need to
have the panel and service connections inspected
before your power utility will provide a connection.
This is sometimes done by the local power authority
rather than the usual inspectors.
- After installing the boxes and wiring, but before
the insulation/walls go up, you will need a
- After the walls are up, and the wiring is complete,
you will need a "final inspection".
Subject: My house doesn't meet some of these rules and regulations.
Do I have to upgrade?
In general, there is no requirement to upgrade older dwellings,
though there are some exceptions (ie: smoke detectors in some
cases). However, any new work must be done according to the
latest electrical code. Also, if you do ``major'' work, you
may be required to upgrade certain existing portions or all
of your system. Check with your local electrical inspector.
Subject: A word on voltages: 110/115/117/120/125/220/240
One thing where things might get a bit confusing is the
different numbers people bandy about for the voltage of
a circuit. One person might talk about 110V, another 117V
or another 120V. These are all, in fact, exactly the same
thing... In North America the utility companies are required
to supply a split-phase 240 volt (+-5%) feed to your house.
This works out as two 120V +- 5% legs. Additionally, since there
are resistive voltage drops in the house wiring, it's not
unreasonable to find 120V has dropped to 110V or 240V has dropped
to 220V by the time the power reaches a wall outlet. Especially
at the end of an extension cord or long circuit run. For a number
of reasons, some historical, some simple personal orneryness,
different people choose call them by slightly different numbers.
This FAQ has chosen to be consistent with calling them "110V" and
"220V", except when actually saying what the measured voltage will
be. Confusing? A bit. Just ignore it.
One thing that might make this a little more understandable
is that the nameplates on equipment ofen show the lower (ie: 110V
instead of 120V) value. What this implies is that the device
is designed to operate properly when the voltage drops that
208V is *not* the same as 240V. 208V is the voltage between
phases of a 3-phase "Y" circuit that is 120V from neutral to any
hot. 480V is the voltage between phases of a 3-phase "Y"
circuit that's 277V from hot to neutral.
In keeping with 110V versus 120V strangeness, motors intended
to run on 480V three phase are often labelled as 440V...
Subject: What does an electrical service look like?
There are logically four wires involved with supplying the
main panel with power. Three of them will come from the utility
pole, and a fourth (bare) wire comes from elsewhere.
The bare wire is connected to one or more long metal bars pounded
into the ground, or to a wire buried in the foundation, or sometimes
to the water supply pipe (has to be metal, continuous to where
the main water pipe entering the house. Watch out for galvanic
action conductivity "breaks" (often between copper and iron pipe).
This is the "grounding conductor". It is there to make sure that
the third prong on your outlets is connected to ground. This wire
normally carries no current.
One of the other wires will be white (or black with white or
yellow stripes, or sometimes simply black). It is the neutral wire.
It is connected to the "centre tap" (CEC; "center tap" in the
NEC ;-) of the distribution transformer supplying the power. It
is connected to the grounding conductor in only one place (often
inside the panel). The neutral and ground should not be connected
anywhere else. Otherwise, weird and/or dangerous things may happen.
Furthermore, there should only be one grounding system in
a home. Some codes require more than one grounding electrode.
These will be connected together, or connected to the neutral
at a common point - still one grounding system. Adding additional
grounding electrodes connected to other portions of the house
wiring is unsafe and contrary to code.
If you add a subpanel, the ground and neutral are usually
brought as separate conductors from the main panel, and are
not connected together in the subpanel (ie: still only one
neutral-ground connection). However, in some situations
(certain categories of separate buildings) you actually do
have to provide a second grounding electrode - consult your
The other two wires will usually be black, and are the "hot"
wires. They are attached to the distribution transformer as
The two black wires are 180 degrees out of phase with each
other. This means if you connect something to both hot wires,
the voltage will be 220 volts. If you connect something to the
white and either of the two blacks you will get 110V.
Some panels seem to only have three wires coming into them.
This is either because the neutral and ground are connected
together at a different point (eg: the meter or pole) and one
wire is doing dual-duty as both neutral and ground, or in some
rare occasions, the service has only one hot wire (110V only
Subject: What is a circuit?
Inside the panel, connections are made to the incoming wires.
These connections are then used to supply power to selected
portions of the home. There are three different combinations:
1) one hot, one neutral, and ground: 110V circuit.
2) two hots, no neutral, and ground: 220V circuit.
3) two hots, neutral, and ground: 220V circuit + neutral,
and/or two 110V circuits with a common neutral.
(1) is used for most circuits supplying receptacles and
lighting within your house. (3) is usually used for supplying
power to major appliances such as stoves, and dryers - they
often have need for both 220V and 110V, or for bringing several
circuits from the panel box to a distribution point. (2) is
usually for special 220V motor circuits, electric heaters, or
[Note: In the US, the NEC frequently permits a circuit similar
to (2) be used for stoves and dryers - namely, that there
are two hot wires, and a wire that does dual duty as neutral
and ground, and is connected to the frame as well as providing
the neutral for 110V purposes - three prong plugs instead
of four (*only* for stoves/dryers connected to the main panel.
When connected to most sub-panels, 4 prong plugs and receptacles
are required). In our not-so-humble opinion this is crazy, but
the NFPA claims that this practice was re-evaluated for the 1992 NEC,
and found to be safe. Check your local codes, or inquire as to
local practice -- there are restrictions on when this is
(1) is usually wired with three conductor wire: black for hot,
white for neutral, and bare for grounding.
(2) and (3) have one hot wire coloured red, the other black, a
bare wire for grounding, and in (3) a white wire for neutral.
You will sometimes see (2) wired with just a black, white and ground
wire. Since the white is "hot" in this case, both the NEC and CEC
requires that the white wire be "permanently marked" at the ends
to indicate that it is a live wire. Usually done with paint, nail
polish or sometimes electrical tape.
Each circuit is attached to the main wires coming into the
panel through a circuit breaker or fuse.
There are, in a few locales, circuits that look like (1), (2)
or (3) except that they have two bare ground wires. Some places
require this for hot tubs and the like (one ground is "frame ground",
the other attaches to the motor). This may or may not be an
alternative to GFCI protection.
Subject: "grounding" versus "grounded" versus "neutral".
According to the terminology in the CEC and NEC, the
"grounding" conductor is for the safety ground, i.e., the green
or bare or green with a yellow stripe wire. The word "neutral"
is reserved for the white when you have a circuit with more than
one "hot" wire. Since the white wire is connected to neutral and
the grounding conductor inside the panel, the proper term is
"grounded conductor". However, the potential confusion between
"grounded conductor" and "grounding conductor" can lead to
potentially lethal mistakes - you should never use the bare wire
as a "grounded conductor" or white wire as the "grounding conductor",
even though they are connected together in the panel.
[But not in subpanels - subpanels are fed neutral and ground
separately from the main panel. Usually.]
Note: do not tape, colour or substitute other colour wires for the
safety grounding conductor.
In the trade, and in common usage, the word "neutral" is used
for "grounded conductor". This FAQ uses "neutral" simply to
avoid potential confusion. We recommend that you use "neutral"
too. Thus the white wire is always (except in some light
switch applications) neutral. Not ground.
Subject: What does a fuse or breaker do? What are the differences?
Fuses and circuit breakers are designed to interrupt the power
to a circuit when the current flow exceeds safe levels. For
example, if your toaster shorts out, a fuse or breaker should
"trip", protecting the wiring in the walls from melting. As
such, fuses and breakers are primarily intended to protect the
wiring -- UL or CSA approval supposedly indicates that the
equipment itself won't cause a fire.
Fuses contain a narrow strip of metal which is designed to melt
(safely) when the current exceeds the rated value, thereby
interrupting the power to the circuit. Fuses trip relatively
fast. Which can sometimes be a problem with motors which have
large startup current surges. For motor circuits, you can use
a "time-delay" fuse (one brand is "fusetron") which will avoid
tripping on momentary overloads. A fusetron looks like a
spring-loaded fuse. A fuse can only trip once, then it must be
Breakers are fairly complicated mechanical devices. They
usually consist of one spring loaded contact which is latched
into position against another contact. When the current flow
through the device exceeds the rated value, a bimetallic strip
heats up and bends. By bending it "trips" the latch, and the
spring pulls the contacts apart. Circuit breakers behave
similarly to fusetrons - that is, they tend to take longer to
trip at moderate overloads than ordinary fuses. With high
overloads, they trip quickly. Breakers can be reset a finite
number of times - each time they trip, or are thrown
when the circuit is in use, some arcing takes place, which
damages the contacts. Thus, breakers should not be used in
place of switches unless they are specially listed for the
Neither fuses nor breakers "limit" the current per se. A dead
short on a circuit can cause hundreds or sometimes even
thousands of amperes to flow for a short period of time, which
can often cause severe damage.
Subject: Breakers? Can't I use fuses?
Statistics show that fuse panels have a significantly higher
risk of causing a fire than breaker panels. This is usually
due to the fuse being loosely screwed in, or the contacts
corroding and heating up over time, or the wrong size fuse
being installed, or the proverbial "replace the fuse with a
Since breakers are more permanently installed, and have better
connection mechanisms, the risk of fire is considerably less.
Fuses are prone to explode under extremely high overload. When
a fuse explodes, the metallic vapor cloud becomes a conducting
path. Result? From complete meltdown of the electrical panel,
melted service wiring, through fires in the electrical
distribution transformer and having your house burn down.
[This author has seen it happen.] Breakers won't do this.
Many jurisdictions, particularly in Canada, no longer permit
fuse panels in new installations. The NEC does permit new
fuse panels in some rare circumstances (requiring the special
inserts to "key" the fuseholder to specific size fuses)
Some devices, notably certain large air conditioners, require fuse
protection in addition to the breaker at the panel. The fuse
is there to protect the motor windings from overload. Check the
labeling on the unit. This is usually only on large permanently
installed motors. The installation instructions will tell you
if you need one.
Subject: What size wire should I use?
For a 20 amp circuit, use 12 gauge wire. For a 15 amp circuit,
you can use 14 gauge wire (in most locales). For a long run,
though, you should use the next larger size wire, to avoid
voltage drops. 12 gauge is only slightly more expensive than
14 gauge, though it's stiffer and harder to work with.
Here's a quick table for normal situations. Go up a size for
more than 100 foot runs, when the cable is in conduit, or
ganged with other wires in a place where they can't dissipate
We don't list bigger sizes because it starts getting very dependent
on the application and precise wire type.
Subject: Where do these numbers come from?
There are two considerations, voltage drop and heat buildup.
The smaller the wire is, the higher the resistance is. When
the resistance is higher, the wire heats up more, and there is
more voltage drop in the wiring. The former is why you need
higher-temperature insulation and/or bigger wires for use in
conduit; the latter is why you should use larger wire for long
Neither effect is very significant over very short distances.
There are some very specific exceptions, where use of smaller
wire is allowed. The obvious one is the line cord on most
lamps. Don't try this unless you're certain that your use fits
one of those exceptions; you can never go wrong by using larger
Subject: What does "14-2" mean?
This is used to describe the size and quantity of conductors
in a cable. The first number specifies the gauge. The second
the number of current carrying conductors in the wire - but
remember there's usually an extra ground wire. "14-2" means
14 gauge, two insulated current carrying wires, plus bare ground.
-2 wire usually has a black, white and bare ground wire. Sometimes
the white is red instead for 220V circuits without neutral. In
the latter case, the sheath is usually red too.
-3 wire usually has a black, red, white and bare ground wire.
Usually carrying 220V with neutral.
Subject: What is a "wirenut"/"marrette"/"marr connector"? How are they
A wire nut is a cone shaped threaded plastic thingummy that's used
to connect wires together. "Marrette" or "Marr connector"
are trade names. You'll usually use a lot of them in DIY wiring.
In essence, you strip the end of the wires about an inch, twist them
together, then twist the wirenut on.
Though some wirenuts advertise that you don't need to twist the
wire, do it anyways - it's more mechanically and electrically
There are many different sizes of wire nut. You should check
that the wire nut you're using is the correct size for the
quantity and sizes of wire you're connecting together.
Don't just gimble the wires together with a pair of pliers or
your fingers. Use a pair of blunt nose ("linesman") pliers,
and carefully twist the wires tightly and neatly. Sometimes
it's a good idea to trim the resulting end to make sure it
goes in the wirenut properly.
Some people wrap the "open" end of the wirenut with electrical
tape. This is probably not a good idea - the inspector may
tear it off during an inspection. It's usually done because
a bit of bare wire is exposed outside the wire nut - instead
of taping it, the connection should be redone.
Subject: What is a GFI/GFCI?
A GFCI is a ``ground-fault circuit interrupter''. It measures
the current current flowing through the hot wire and the
neutral wire. If they differ by more than a few milliamps, the
presumption is that current is leaking to ground via some other
path. This may be because of a short circuit to the chassis of
an appliance, or to the ground lead, or through a person. Any
of these situations is hazardous, so the GFCI trips, breaking
GFCIs do not protect against all kinds of electric shocks. If,
for example, you simultaneously touched the hot and neutral
leads of a circuit, and no part of you was grounded, a GFCI
wouldn't help. All of the current that passed from the hot
lead into you would return via the neutral lead, keeping the
The two pairs of connections on a GFCI outlet are not symmetric.
One is labeled LOAD; the other, LINE. The incoming power feed
*must* be connected to the LINE side, or the outlet will not be
protected. The LOAD side can be used to protect all devices
downstream from it. Thus, a whole string of outlets can be
covered by a single GFCI outlet.
Subject: Where should GFCIs be used?
The NEC mandates GFCIs for 110V, 15A or 20A single phase
outlets, in bathrooms, kitchens within 6' of the sink, wet-bar
sinks, roof outlets, garages, unfinished basements or crawl spaces,
outdoors, near a pool, or just about anywhere else where you're likely
to encounter water or dampness. There are exceptions for inaccessible
outlets, those dedicated to appliances ``occupying fixed space'',
typically refrigerators and freezers, and for sump pumps and
The NEC now requires that if your replace an outlet in a
location now requiring GFCI, you must install GFCI protection.
Note in particular - kitchen and bathroom outlets.
When using the "fixed appliance" rule for avoiding GFCI outlets,
single outlet receptacles must be used for single appliances,
duplex receptacles may be used for two appliances.
The CEC does not mandate as many GFCIs. In particular, there
is no requirement to protect kitchen outlets, or most garage or
basement outlets. Basement outlets must be protected if you
have a dirt floor, garage outlets if they're near the door to
outside. Bathrooms and most exterior outlets must have GFCIs,
as do pools systems and jacuzzi or whirlpool pumps.
There are many rules about GFCIs with pools and so on. This
is outside of our expertise, so we're not covering it in
detail. See your inspector.
When replacing an outlet, it must now be GFCI-protected if
such would now be required for a new installation. That is,
a kitchen outlet installed per the 1984 code need not have
been protected, but if that outlet is ever replaced, GFCI
protection must now be added (under NEC). This is explicit
in the 1993 NEC, and inspector-imposed in Canada.
Even if you are not required to have GFCI protection, you may
want to consider installing it anyway. Unless you need a GFCI
breaker (see below), the cost is low. In the U.S., GFCI
outlets can cost as little as US$8. (Costs are a bit higher in
Canada: C$12.) Evaluate your own risk factors. Does your
finished basement ever get wet? Do you have small children?
Do you use your garage outlets to power outdoor tools? Does
water or melted snow ever puddle inside your garage?
Subject: Where shouldn't I use a GFCI?
GFCIs are generally not used on circuits that (a) don't pose a
safety risk, and (b) are used to power equipment that must run
unattended for long periods of time. Refrigerators, freezers,
and sump pumps are good examples. The rationale is that GFCIs
are sometimes prone to nuisance trips. Some people claim that
the inductive delay in motor windings can cause a momentary
current imbalance, tripping the GFCI. Note, though, that most
GFCI trips are real; if you're getting a lot of trips for no
apparent reason, you'd be well-advised to check your wiring
before deciding that the GFCI is broken or useless.
Subject: What is the difference between a GFCI outlet and a GFCI breaker?
For most situations, you can use either a GFCI outlet as the
first device on the circuit, or you can install a breaker with
a built-in GFCI. The former is generally preferred, since GFCI
breakers are quite expensive. For example, an ordinary GE
breaker costs ~US$5; the GFCI model costs ~US$35. There is one
major exception: if you need to protect a ``multi-wire branch
circuit'' (two or more circuits sharing a common neutral wire),
such as a Canadian-style kitchen circuit, you'll need a
multi-pole GFCI breaker. Unfortunately, these are expensive;
the cost can range into the hundreds of dollars, depending on
what brand of panel box you have. But if you must protect such
a circuit (say, for a pool heater), you have no choice.
One more caveat -- GFCI outlets are bulky. You may want to use
an oversize box when installing them. On second thought, use
large (actually deep) boxes everywhere. You'll thank yourself
Incidentally, if you're installing a GFCI to ensure that one
specific outlet is protected (such as a bathroom), you don't
really have to go to all of the trouble to find the first
outlet in the circuit, you could simply find the first outlet
in the bathroom, and not GFCI anything upstream of it. But
protecting the whole circuit is preferred.
When you install a GFCI, it's a good idea to use the little
"ground fault protected" stickers that come with it and mark
the outlets downstream of the GFCI. You can figure out which
outlets are "downstream", simply by tripping the GFCI with the
test button and see which outlets are dead.
Note that the labels are mandatory for GFCI-protected-but-ungrounded
three prong outlets according to the NEC.
Subject: What's the purpose of the ground prong on an outlet, then?
Apart from their use in electronics, which we won't comment on,
and for certain fluorescent lights (they won't turn on without
a good ground connection), they're intended to guard against
insulation failures within the device. Generally, the case of
the appliance is connected to the ground lead. If there's an
insulation failure that shorts the hot lead to the case, the
ground lead conducts the electricity away safely (and possibly
trips the circuit breaker in the process). If the case is not
grounded and such a short occurs, the case is live -- and if
you touch it while you're grounded, you'll get zapped. Of
course, if the circuit is GFCI-protected, it will be a very
tiny zap -- which is why you can use GFCIs to replace
ungrounded outlets (both NEC and CEC).
There are some appliances that should *never* be grounded. In
particular, that applies to toasters and anything else with
exposed conductors. Consider: if you touch the heating
electrode in a toaster, and you're not grounded, nothing will
happen. If you're slightly grounded, you'll get a small shock;
the resistance will be too high. But if the case were
grounded, and you were holding it, you'd be the perfect path to
Subject: Why is one prong wider than the other? Polarization
Nowadays, many two-prong devices have one prong wider than the
other. This is so that the device could rely (not guaranteed!)
on one specific wire being neutral, and the other hot.
This is particularly advantageous in light fixtures, where the
the shell should neutral (safety), or other devices which want to
have an approximate ground reference (ie: some radios).
Most 2-prong extension cords have wide prongs too.
This requires that you wire your outlets and plugs the right
way around. You want the wide prong to be neutral, and the
narrow one hot. Most outlets have a darker metal for the
hot screw, and lighter coloured screw for the neutral.
If not, you can usually figure out which is which by which
prong the terminating screw connects to.
Subject: How do I convert two prong receptacles to three prong?
Older homes frequently have two-prong receptacles instead
of the more modern three. These receptacles have no safety
ground, and the cabling usually has no ground wire. Neither
the NEC or CEC permits installing new 2 prong receptacles anymore.
There are several different approaches to solving this:
1) If the wiring is done through conduit or BX, and the
conduit is continuous back to the panel, you can connect
the third prong of a new receptacle to the receptacle
box. NEC mainly - CEC frowns on this practice.
2) If there is a copper cold water pipe going nearby, and
it's continuous to the main house ground point, you can
run a conductor to it from the third prong.
NEC: this can only be done if the point of attachment
is within 5 feet of where the pipe enters the ground.
3) Run a ground conductor back to the main panel.
4) Easiest: install a GFCI receptacle. The ground lug
should not be connected to anything, but the GFCI
protection itself will serve instead. The GFCI
will also protect downstream (possibly also two prong
outlets). If you do this to protect downstream outlets,
the grounds must not be connected together. Since it
wouldn't be connected to a real ground, a wiring fault
could energize the cases of 3 prong devices connected
to other outlets. Be sure, though, that there aren't
indirect ground plug connections, such as via the sheath
on BX cable.
The CEC permits you to replace a two prong receptacle with a three
prong if you fill the U ground with a non-conducting goop.
Like caulking compound. This is not permitted in the NEC.
The NEC requires that three prong receptacles without ground
that are protected by GFCI must be labelled as such.
See the next section about computers on GFCI-protected groundless
Subject: Surges, spikes, zaps, grounding and your electronics
Theoretically, the power coming into your house is a perfect AC
sine wave. It is usually quite close. But occasionally, it
won't be. Lightning strikes and other events will affect the
power. These usually fall into two general categories: very
high voltage spikes (often into 1000s of volts, but usually
only a few microseconds in length) or surges (longer duration,
but usually much lower voltage).
Most of your electrical equipment, motors, transformer-operated
electronics, lights, etc., won't even notice these one-shot
events. However, certain types of solid-state electronics,
particularly computers with switching power supplies and MOS
semiconductors, can be damaged by these occurances. For
example, a spike can "punch a hole" through an insulating layer
in a MOS device (such as that several hundred dollar 386 CPU),
thereby destroying it.
The traditional approach to protecting your electronics is to
use "surge suppressors" or "line filters". These are usually
devices that you plug in between the outlet and your
Roughly speaking, surge suppressors work by detecting
overvoltages, and shorting them out. Think of them as voltage
limiters. Line filters usually use frequency-dependent
circuits (inductors, capacitors etc.) to "tune out" undesirable
spikes - preventing them from reaching your electronics.
So, you should consider using suppressors or filters on your
These devices come in a very wide price range. From a couple
of dollars to several hundred. We believe that you can protect
your equipment from the vast majority of power problems by
selecting devices in the $20-50 range.
A word about grounding: most suppressors and EFI filters
require real grounds. Any that don't are next to useless.
For example, most surge suppressors use MOVs (metal oxide
varistors) to "clamp" overvoltages. Yes, you can have a
suppressor that only has a MOV between neutral and hot to
combat differential-mode voltage excursions, but that isn't
enough. You need common-mode protection too. Good suppressors
should have 3 MOVs, one between each pair of wires. Which
means you should have a good solid ground. Eg: a solidly
connected 14ga wire back to the panel. Not rusty BX armour or
galvanized pipe with condensation turning the copper connection
Without a ground, a surge or spike is free to "lift" your
entire electronics system well away from ground. Which is
ideal for blowing out interface electronics for printer ports
Secondly, static electricity is one of the major enemies of
electronics. Having good frame grounds is one way of
protecting against static zaps.
If you're in the situation of wanting to install computer
equipment on two wire groundless circuits take note:
Adding a GFCI outlet to the circuit makes the circuit safe for
you. But it doesn't make it safe for your equipment - you need
a ground to make surge suppressors or line filters effective.
Subject: Are you sure about GFCIs and ungrounded outlets?
Should the test button work?
The NEC, section 210-7(d), and CEC, section 26-700(9), are quite
explicit that GFCIs are a legal substitute for a grounded outlet
in an existing installation where there is no ground available in
the outlet box.
But your local codes may vary. As for the TEST button -- there's
a resistor connecting the LOAD side of the hot wire to the LINE
side of the neutral wire when you press the TEST button. Current
through this resistor shows up as an imbalance, and trips the GFCI.
This is a simple, passive, and reliable test, and doesn't require
a real ground to work. If your GFCI does not trip when you press
the TEST button, it is very probably defective or miswired. Again:
if the test button doesn't work, something's broken, and potentially
dangerous. The problem should be corrected immediately.
The instructions that come with some GFCIs specify that the ground
wire must be connected. We do not know why they say this. The
causes may be as mundane as an old instruction sheet, or with the
formalities of UL or CSA listing -- perhaps the device was never
tested without the ground wire being connected. On the other hand,
UL or CSA approval should only have been granted if the device
behaves properly in *all* listed applications, including ungrounded
outlet replacement. (One of us called Leviton; their GFCIs are
labeled for installation on grounded circuits only. The technician
was surprised to see that; he agreed that the NEC does not require
it, and promised to investigate.)
Chris Lewis: _Una confibula non sat est_
Phone: Canada 613 832-0541
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Last-modified: Mon Nov 14 01:03:29 EST 1994
Copyright 1991, 1992, 1993
Chris Lewis and Steven Bellovin
Redistribution for profit, or in altered content/format
prohibited without permission of the authors. Other
redistribution must contain this copyright notice,
The latest FAQ can always be obtained from:
Subject: What kind of outlets do I need in a kitchen?
The NEC requires at least two 20 amp ``small appliance
circuits'' for kitchen counters. The CEC requires split-duplex
receptacles. Outlets must be installed such that no point is more
than 24" (NEC) (900 mm CEC) from an outlet. Every counter wider
than 12" (NEC) or 300 mm (CEC) must have at least one outlet.
The circuit these outlets are on may not feed any outlets except
in the kitchen, pantry, or dining room. Furthermore, these circuits
are in addition to any required for refrigerators, stoves, microwaves,
lighting, etc. Non-dedicated outlets within 6' of a sink *must* be
protected by a GFCI (NEC only).
Split duplex receptacles are fed with a 220V circuit. The tab
is broken on the hot side of the outlet, and one hot goes to
the upper outlet, and the other hot goes to the lower outlet.
The neutral connects to both outlets through one screw. When
"carrying through" to another outlet, the neutral must be
pigtailed, such that removing the outlet, or having the neutral
connection fall off or burn out doesn't cause the neutral to
disconnect from downstream outlets ("loose neutral" problems -
see "What does it mean when the lights brighten...").
Subject: Where must outlets and switches be in bathrooms?
There must be at least one outlet in each bathroom, adjacent to
the sink, in addition to any outlet that may be incorporated in
the light fixture. All such outlets *must* be GFCI-protected.
The NEC says that switches may not be installed inside bathtubs
or showers. The CEC says that switches may not be installed
"within reach" of bathtubs or showers (consult an inspector
if you can't make it at least four feet).
Subject: General outlet placement rules/line capacities
We paraphrase CEC 26-702 (NEC: 210-52 through 210-63)
Note: In laying out receptacle outlets, consideration shall be
given to the placement of electrical baseboards, hot air
registers, hot water or steam registers, with a view of
eliminating cords having to pass over hot or conductive
surfaces wherever possible.
NEC: You're not allowed to put outlets over electric
baseboards. That, coupled with the spacing requirements, more
or less mandates the use of baseboards with integral outlets.
Note that such outlets are fed by a different branch circuit
than the heating elements.
2. Except as otherwise required, receptacles shall be installed
in the finished walls of every room or area, other than
kitchens, bathrooms, hallways, laundry rooms, utility rooms or
closets, so that no point along the floor line of any usable
wall space is more than 1.8m (6') horizontally from a
receptacle in that or an adjoining space, such distance being
measured along the floor line of the wall spaces involved.
Fixed dividers, counters, etc., are considered wall space.
Floor outlets do not satisfy the requirement unless they are
``near'' the wall. Insofar as practical, outlets should be
3. At least one duplex receptacle shall be provided in each
enclosed area such as a balcony or porch that is not classified
as a finished room or area.
[NEC doesn't seem to have this rule.]
4. The receptacles referred to in (2) and (3) shall be duplex
receptacles or equivalent number of single receptacles.
5. "Usable wall space" is defined as any wall space 900mm (3',
NEC 2') or more in width, not to include doorways, areas
occupied by a door when fully opened, windows which extend to
the floor, fireplaces or other permanent installations that
would limit the use of the wall space.
6. See kitchen counter requirements. At least one duplex
receptacle in eat-in dining area.
[We don't think the latter part is in the NEC. Also, the NEC
says that the two 20-amp small appliance circuits can't go
outside of the kitchen, dining room, pantry, etc., nor can they
be used for anything else, except for things like clock
outlets, stove accessory outlets, etc.]
7. Receptacles shall not be mounted facing up in the work
surfaces or counters of the kitchen or dining area.
8. No point in a hallway within a dwelling unit shall be more
than 4.5m (15', NEC 10') from a duplex receptacle as measured
by the shortest path which the supply cord of an appliance
connected to the receptacle would follow without passing
through an openning fitted with a door. (vacuum-cleaner
9. At least one duplex receptacle shall be provided: in laundry
room, utility room and any unfinshed basement area
[NEC: see GFCI requirements. There must be a dedicated 20 amp
laundry receptacle, with no other outlets, plus an additional
unfinished basement receptacle. Any attic or crawl space with
heating or air conditioning equipment must have a receptacle.
(this is probably in the CEC too.)]
10, 11, 12, 13: See bathroom requirements, GFCI, washing
machine outlet placement.
14, 15. Outlets shall not be placed in ironing cabinets,
cupboards, wall cabinets, nor in similar enclosures except
where they're for specific non-heating appliances (including
microwave) in the enclosure.
[NEC: No such requirement. Are you sure Steven?]
16, 17. For each single-family dwelling, at least one duplex
receptacle shall be installed outdoors to be readily available
from ground level (see GFCI requirements). Appendix B
(additional notes) suggests front and back outlets to be
controlled by an interior switch.
[NEC: One in front, one in back. No discussion of them being
18. At least one duplex receptacle shall be provided for each
car space in a garage or carport.
[NEC: For an attached garage, or detached garage with electric
service -- but there is no requirement that detached garages
have power. This remark is probably relevant to CEC as well.]
19. For the purposes of this rule, all receptacles shall be of
the grounding type, configuration 5-15R (standard 110V/15A 3
20. Any receptacle that is part of a lighting fixture or
appliance that is > 1.7m (5 feet) above the floor, or in
cabinets or cupboards, is not counted in the above rules.
21. Where a switched duplex outlet is used in lieu of a light
outlet and fixture, the receptacle shall be considered one of
the wall mounted receptacles required here.
22. At least one duplex receptacle shall be provided for a
central vacuum system if the ducting is installed.
[NEC: couldn't find an equivalent rule.]
Capacities: Knight recommends no more than 10 outlets per
circuit. Some US references talk about a limit of 12. There
appears to be a wattage/area/outlet count calculation somewhere
in the NEC. 20A circuits may have different rules.
It is open to considerable debate whether you should mix
general lighting and outlets on individual circuits. Knight
recommends it. Some netters don't. I tend towards the former
for load balancing reasons.
NEC: There's a new rule on outdoor outlets. If exposed to the
weather, and if used for unattended equipment (pool filters,
outdoor lighting, etc.), the outlet must still be weatherproof
even when the device is plugged in.
Subject: What is Romex/NM/NMD? What is BX? When should I use each?
Romex is a brand name for a type of plastic insulated wire.
Sometimes called non-metallic sheath. The formal name is NM.
This is suitable for use in dry, protected areas (ie: inside
stud walls, on the sides of joists etc.), that are not subject
to mechanical damage or excessive heat. Most newer homes are
wired almost exclusively with NM wire. There are several
different categories of NM cable.
BX cable -- technically known as armored cable or "AC" has a
flexible aluminum or steel sheath over the conductors and is
fairly resistant to damage.
TECK cable is AC with an additional external thermoplastic
Protection for cable in concealed locations: where NM or AC cable
is run through studs, joists or similar wooden members, the outer
surface of the cable must be kept at least 32mm/1.25" (CEC & NEC)
from the edges of the wooden members, or the cable should be protected
from mechanical injury. This latter protection can take the form of
metal plates (such as spare outlet box ends) or conduit.
[Note: inspector-permitted practice in Canada suggests that armored
cable, or flexible conduit can be used as the mechanical protection,
but this is technically illegal.]
Additional protection recommendations: [These are rules in the
Canadian codes. The 1993 NEC has many changes that bring
it close to these rules. These are reasonable answers to the
vague "exposed to mechanical damage" in both the NEC and CEC.]
- NM cable should be protected against mechanical damage
where it passes through floors or on the surface of walls
in exposed locations under 5 feet from the floor.
Ie: use AC instead, flexible conduit, wooden guards etc.
- Where cable is suspended, as in, connections to furnaces
or water heaters, the wire should be protected. Canadian
practice is usually to install a junction or outlet
box on the wall, and use a short length of AC cable
or NM cable in flexible conduit to "jump" to the appliance.
Stapling NM to a piece of lumber is also sometimes used.
- Where NM cable is run in close proximity to heating
ducts or pipe, heat transfer should be minimized by
means of a 25mm/1" air space, or suitable insulation
material (a wad of fiberglass).
- NM cable shall be supported within 300mm/1' of every box
or fitting, and at intervals of no more than 1.5m/5'.
Holes in joists or studs are considered "supports".
Some slack in the cable should be provided adjacent to
each box. [while fishing cable is technically in violation,
it is permitted where "proper" support is impractical]
- 2 conductor NM cable should never be stapled on edge.
[Knight also insists on only one cable per staple, referring
to the "workmanship" clause, but this seems more honoured
in the breach...]
- cable should never be buried in plaster, cement or
similar finish, except were required by code [Ie: cable
burial with shallow bedrock.].
- cable should be protected where it runs behind baseboards.
- Cable may not be run on the upper edge of ceiling joists
or the lower edges of rafters where the headroom is more
than 1m (39").
Whenever BX cable is terminated at a box with a clamp, small
plastic bushings must be inserted in the end of the cable to
prevent the clamps forcing the sharp ends of the armor through
Whenever BX cable is buried in thermal insulation, 90C
wire should be selected, but derated in current carrying
capacity to 60C.
BX is sometimes a good idea in a work shop unless covered by
solid wall coverings.
In places where damage is more likely (like on the back wall of
a garage ;-), you may be required to use conduit, a
UL- (or CSA-) approved metal pipe. You use various types of
fittings to join the pipe or provide entrance/exit for the
Service entrances frequently use a plastic conduit.
In damp places (eg: buried wiring to outdoor lighting) you will
need special wire (eg: CEC NMW90, NEC UF). NMW90 looks like
very heavy-duty NMD90. You will usually need short lengths of
conduit where the wire enters/exits the ground. [See underground
Thermoplastic sheath wire (such as NM, NMW etc.) should not be
exposed to direct sunlight unless explicitly approved for that
Many electrical codes do not permit the routing of wire through
furnace ducts, including cold air return plenums constructed
by metal sheeting enclosing joist spaces. The reason for this
is that if there's a fire, the ducting will spread toxic gasses
from burning insulation very rapidly through the building.
Teflon insulated wire is permitted in plenums in many areas.
Canada appears to use similar wire designations to the US,
except that Canadian wire designations usually include the
temperature rating in Celsius. Eg: "AC90" versus "AC".
In the US, NM-B is 90 degrees celcius.
NOTE: local codes vary. This is one of the items that changes
most often. Eg: Chicago codes require conduit *everywhere*.
There are very different requirements for mobile homes.
Check your local codes, *especially* if you're doing anything
that's the slightest out of the ordinary.
Wire selection table (incomplete - the real tables are enormous,
uncommon wire types or applications omitted)
Condition Type CEC NEC
Exposed/Concealed dry plastic NMD90 NM
armor AC90 AC
Exposed/Concealed damp plastic NMD90 NMC
Exposed/Concealed wet plastic NMWU90
Exposed to weather plastic NMWU
Direct earth burial/ plastic NMWU* UF
Service entrance RWU
[* NMWU not for service entrance]
Subject: Should I use plastic or metal boxes?
The NEC permits use of plastic boxes with non-metallic cable
only. The reasoning is simple -- with armored cable, the box
itself provides ground conductor continuity. U.S. plastic
boxes don't use metal cable clamps.
The CEC is slightly different. The CEC never permits cable
armor as a grounding conductor. However, you must still
provide ground continuity for metallic sheath. The CEC also
requires grounding of any metal cable clamps on plastic boxes.
The advantage of plastic boxes is comparatively minor even for
non-metallic sheathed cable -- you can avoid making one ground
connection and they sometimes cost a little less. On the other
hand, plastic boxes are more vulnerable to impacts. For
exposed or shop wiring, metal boxes are probably better.
Metal receptacle covers must be grounded, even on plastic
boxes. This may be achieved by use of a switch with ground
Subject: Junction box positioning?
A junction box is a box used only for connecting wires together.
Junction boxes must be located in such a way that they're accessible
later. Ie: not buried under plaster. Excessive use of junction
boxes is often a sign of sloppy installation, and inspectors may
Subject: Can I install a replacement light fixture?
In general, one can replace fixtures freely, subject to a few
caveats. First, of course, one should check the amperage
rating of the circuit. If your heart is set on installing half
a dozen 500 watt floodlights, you may need to run a new wire
back to the panel box. But there are some more subtle
constraints as well. For example, older house wiring doesn't
have high-temperature insulation. The excess heat generated by
a ceiling-mounted lamp can and will cause the insulation to
deteriorate and crack, with obvious bad results. Some newer
fixtures are specifically marked for high temperature wire
only. (You may find, in fact, that your ceiling wiring already
has this problem, in which case replacing any devices is a real
Other concerns include providing a suitable ground for some
fluorescent fixtures, and making sure that the ceiling box and
its mounting are strong enough to support the weight of a heavy
chandelier or ceiling fan. You may need to install a new box
specifically listed for this purpose. A 2x4 across the ceiling
joists makes a good support. Metal brackets are also available
that can be fished into ceilings thru the junction box hole and
mounted between the joists.
There are special rules for recessed light fixtures such as
"pot" lamps or heat lamps. When these are installed in
insulated ceilings, they can present a very substantial fire
hazard. The CEC provides for the installation of pot lamps in
insulated ceilings, provided that the fixture is boxed in a
"coffin" (usually 8'x16"x12" - made by making a pair of joists
12" high, and covering with plywood) that doesn't have any
insulation. (Yes, that's 8 *feet* long)
NEC rules are somewhat less stringent. They require at least
3" clearance between the fixture and any sort of thermal
insulation. The rules also say that one should not obstruct
free air movement, which means that a CEC-style ``coffin''
might be worthwhile. Presumably, that's up to the local
inspector. [The CEC doesn't actually mandate the coffin
per-se, this seems to be an inspector requirement to make
absolutely certain that the fixture can't get accidentally
buried in insulation. Ie: if you have insulation blown in
There are now fixtures that contain integral thermal cutouts
and fairly large cases that can be buried directly in
insulation. They are usually limited to 75 watt bulbs, and are
unfortunately, somewhat more expensive than the older types.
Before you use them, you should ensure that they have explicit
UL or CSA approval for such uses. Follow the installation
instructions carefully; the prescribed location for the sensor
There does not yet appear to be a heat lamp fixture that is
approved for use in insulation. The "coffin" appears the only
Subject: Noisy fluorescent fixtures, what do I do?
Many fluorescent fixtures tend to buzz, objectionably so when used in
residential (rather than warehouse or industrial) situations. This
tends to be the result of magnetic/physical resonances at the
(low) frequencies that standard fixture ballasts operate. You
can eliminate this problem by switching to electronic ballasts,
which operate at a higher (inaudible) frequency. Unfortunately,
these are quite expensive.
Subject: Noisy lights with dimmer switches, what do I do?
| Often, after installing a dimmer switch, or replacing bulbs controlled
| by a dimmer, you'll start hearing objectionable buzzing or humming
| from the bulb. Sometimes it even interferes with televisions or radios.
| A little theory first. The voltage on the wiring in your house looks
| like this - a sine wave (forgive the lousy ASCII graphics ;-):
| ... ... ~ +160V
| . . . .
| . . . .
| ------------------------------------ 0V
| . . . .
| . . . .
| ... ... ~ -160V
| Most dimmers work by having a solid-state switch called a triac
| in series with the light bulb. Whenever the voltage passes through
| zero (it does this 120 times per second), the triac turns itself off.
| The control circuitry in the dimmer provides an adjustable delay
| before the triac turns back on. So, the resulting wave form looks
| like this:
| ... ... ~ +160V
| | . | .
| | . | .
| ------------------------------------ 0V
| | . | .
| | . | .
| ... ... ~ -160V
| As you can see, by varying the turn-on point, the amount of
| power getting to the bulb is adjustable, and hence the light
| output can be controlled. Voila, a dimmer!
| This is where it gets interesting. Note the sharp corners.
| According to the Nyquist theorem, those corners effectively
| consist of 60Hz plus varying amounts of other frequencies that
| are multiples of 60Hz. In some cases up to 1Mhz and more. The
| wiring in your house acts as an antenna and essentially
| broadcasts it into the air. Hence TVs and radios can be
| effected. This is called EMI (Electromagnetic Interference).
| As far as the bulbs are concerned, a bulb consists of a series
| of supports and, essentially, fine coils of wire. When you
| run current through a coil, it becomes a magnet right? If
| there's any other metal nearby, it'll move. Just like a
| solenoid. Further, when the amount of current flow abruptly
| changes the magnetism change can be much stronger than it is on
| a simple sine wave. Hence, the filaments of the bulb will tend
| to vibrate more with a dimmer chopping up the wave form, and
| when the filaments vibrate against their support posts, you
| will get a buzz.
| Worse, some dimmers only do half-wave switching, such that the
| one half of the chopped wave form will be absent. Which means
| that the current flow during the present half will have to be
| much stronger to produce the same amount of light - more EMI
| and more tendency to buzz.
| Solving buzzing problems: If you have buzzing, it's always
| worth trying to replace the bulb with a different brand. Some
| cheap bulb brands have inadequate filament support, and simply
| changing to a different brand may help. Try "rough service" or
| "farm service" bulbs. They're usually much stronger and better
| Chance are, however, that switching bulbs won't make that much
| of a difference. Perhaps the buzzing will go away at some
| dimmer settings, but not at all.
| Buzzing bulbs are usually a sign of a "cheap" dimmer. Dimmers
| are supposed to have filters in them. The filter's job is to
| "round off" the sharp corners in the chopped waveform, thereby
| reducing EMI, and the abrupt current jumps that can cause
| buzzing. In cheap dimmers, they've economized on the
| manufacturing costs by cost-reducing the filtering, making it
| less effective. Perhaps the dimmer will be okay at some
| settings, but not others. Or be very picky about what bulbs to
| It is our belief that most buzzing problems can be traced down
| to cheap (<$15 dimmers), and most effectively solved by going
| to mid-range ($25-$35) dimmers from respected companies, such
| as Leviton. One of the authors of this FAQ, after learning
| this lesson, will still use $.89 outlets, but insists on better
| dimmers. By all means, try a different bulb first. You may
| get lucky. If not, it's time to swap dimmers.
| If you have EMI problems, it's almost certain to be a cheap
Subject: What does it mean when the lights brighten when a motor starts?
This usually means that the neutral wire in the panel is
loose. Depending on the load balance, one hot wire may end up
being more than 110V, and the other less than 110V, with
respect to ground. This is a very hazardous situation - it can
destroy your electronic equipment, possibly start fires, and in
some situations electrocute you (ie: some US jurisdictions
require the stove frame connected to neutral).
If this happens, contact your electrical authority immediately
and have them come and check out the problem. If you say "loose
neutral", they will come.
Note: a brief (< 1 second) brightening is sometimes normal with
lighting and motors on the same 220V with neutral circuit. A
loose main panel neutral will usually show increased brightness
far longer than one second. In case of doubt, get help.
Subject: What is 3 phase power? Should I use it? Can I get it in my house?
Three phase power has three "hot" wires, 120 degrees out of
phase with each other. These are usually used for large motors
because it is more "efficient", provides a bit more starting torque,
and because the motors are simpler and hence cheaper.
You're most likely to encounter a 3 phase circuit that shows
110 volts between any hot and ground, and 208 volts between
any two hots. The latter shows the difference between a normal
220V/110V common neutral circuit, which is 240 volts between the
two hots. There are 3 phase circuits with different voltages.
Bringing in a 3 phase feed to your house is usually
ridiculously expensive, or impossible. If the equipment you
want to run has a standard motor mount, it is *MUCH* cheaper to
buy a new 110V or 220V motor for it. In some cases it is
possible to run 3 phase equipment on ordinary power if you have
a "capacitor start" unit, or use a larger motor as a
(auto-)generator. These are tricky, but are a good solution if
the motor is non-standard size, or too expensive or too big to
replace. The Taunton Press book ``The Small Shop'' has an
article on how to do this if you must.
Note that you lose any possible electrical efficiency by using
such a converter. The laws of thermodynamics guarantee that.
Subject: Is it better to run motors at 110 or 220?
Theoretically, it doesn't make any difference. However, there
is a difference is the amount of power lost in the supply
wiring. All things being equal, a 110V motor will lose 4 times
more power in the house wiring than a 220V motor. This also
means that the startup surge loss will be less, and the motor
will get to speed quicker with 220V. And in some circumstances,
the smaller power loss will lead to longer motor life.
This is usually irrelevant unless the supply wires are more
than 50 feet long.
Subject: What is this nonsense about 3HP on 110V 15A circuits?
It is a universal physical law that 1 HP is equal to 746
watts. Given heating loss, power factor and other inefficiencies,
it is usually best to consider 1 HP is going to need 1000-1200
watts. A 110V 15A circuit can only deliver 1850 watts to a motor,
so it cannot possibly be more than approximately 2 HP. Given rational
efficiency factors, 1.5HP is more like it.
Some equipment manufacturers (Sears in particular, most router
manufacturers in general ;-) advertise a HP rating that is far
in excess of what is possible. They are giving you a "stall
horsepower" or similar. That means the power is measured when
the motor is just about to stop turning because of the load.
What they don't mention is that if you kept it in that
condition for more than a few seconds your motor will melt - the
motor is drawing far more current than its continuous rating.
When comparing motors, compare the continuous horsepower. This
should be on the motor nameplate. If you can't find that figure,
check the amperage rating, which is always present.
Subject: How should I wire my shop?
As with any other kind of wiring, you need enough power for all
devices that will be on simultaneously. The code specifies
that you should stay under 80% of the nominal capacity of the
circuit. For typical home shop use, this means one circuit for
the major power tools, and possibly one for a dust collector or
shop vac. Use at least 12 gauge wire -- many power tools have
big motors, with a big start-up surge. If you can, use 20 amp
breakers (NEC), though CEC requires standard 20A receptacles
which means you'd have to "replug" all your equipment. Lights
should either be on a circuit of their own -- and not shared
with circuits in the rest of the house -- or be on at least two
separate circuits. The idea is that you want to avoid a
situation where a blade is still spinning at several thousand
RPM, while you're groping in the dark for the OFF switch.
Do install lots of outlets. It's easier to install them in the
beginning, when you don't have to cut into an existing cable.
It's useful if at least two circuits are accessible at each
point, so you can run a shop vac or a compressor at the same
time as the tool you really want. But use metal boxes and
plates, and maybe even metal-sheathed cable; you may have
objects flying around at high speeds if something goes a bit
Note that some jurisdictions have a "no horizontal wiring"
rule in workshops or other unfinished areas that are used
for working. What this means is that all wiring must be
run along structural members. Ie: stapled to studs.
Other possible shop circuits include heater circuits, 220V
circuits for some large tools, and air compressor circuits.
Don't overload circuits, and don't use extension cords if you
can help it, unless they're rated for high currents. (A coiled
extension cord is not as safe as a straight length of wire of
the same gauge. Also, the insulation won't withstand as much
heat, and heat dissipation is the critical issue.)
If your shop is located at some remove from your main panel,
you should probably install a subpanel, and derive your shop
wiring from it. If you have young children, you may want to
equip this panel with a cut-off switch, and possibly a lock.
If you want to install individual switches to ``safe''
particular circuits, make sure you get ones rated high enough.
For example, ordinary light switches are not safely able to
handle the start-up surge generated by a table saw. Buy
``horsepower-rated'' switches instead.
Finally, note that most home shops are in garages or unfinished
basements; hence the NEC requirements for GFCIs apply. And
even if you ``know'' that you'd never use one of your shop
outlets to run a lawn mower, the next owner of your house might
have a different idea.
Note: Fine Woodworking magazine often carries articles on shop
wiring. April 1992 is one place to start.
Subject: Doorbell/telephone/cable other service wiring hints.
Auxiliary services, such as cable, telephone, doorbell, furnace
control circuits etc. are generally considered to be "class 2"
wiring by both the CEC and NEC.
What this generally means is:
1) class 2 and house power should not share conduit or
2) class 2 and house power should be 12" apart in walls
except where necessary.
3) cross-over should be at 90 degrees.
While the above may not be strictly necessary to the code, it
is advantageous anyways - paralleling house power beside telephone
lines tends to induce hum into the telephone. Or could interfere
with fancier furnace control systems.
With telephone wiring, twisted pair can alleviate these problems,
and there are new cable types that combine multiple services into
one sheath. Consult your inspector if you really want to violate
the above recommendations.
Subject: Underground Wiring
You will need to prepare a trench to specifications, use
special wire, protect the wire with conduit or special plastic
tubing and possibly lumber (don't use creosoted lumber, it rots
thermoplastic insulation and acts as a catalyst in the corrosion
of lead). The transition from in-house to underground wire is
generally via conduit. All outdoor boxes must be specifically
listed for the purpose, and contain the appropriate gaskets,
fittings, etc. If the location of the box is subject to immersion
in water, a more serious style of water-proof box is needed. And
of course, don't forget the GFCIs.
The required depths and other details vary from jurisdiction to
jurisdiction, so we suggest you consult your inspector about
your specific situation.
A hint: buy a roll of bright yellow tape that says "buried power
line" and bury it a few inches above where the wire has been placed.
Subject: Aluminum wiring
During the 1970's, aluminum (instead of copper) wiring became
quite popular and was extensively used. Since that time,
aluminum wiring has been implicated in a number of house fires,
and most jurisdictions no longer permit it in new installations.
We recommend, even if you're allowed to, that do not use it for new
But don't panic if your house has aluminum wiring. Aluminum
wiring, when properly installed, can be just as safe as copper.
Aluminum wiring is, however, very unforgiving of improper
installation. We will cover a bit of the theory behind potential
problems, and what you can do to make your wiring safe.
The main problem with aluminum wiring is a phenomenon known as
"cold creep". When aluminum wiring warms up, it expands. When
it cools down, it contracts. Unlike copper, when aluminum goes
through a number of warm/cool cycles it loses a bit of tightness each
time. To make the problem worse, aluminum oxidises, or corrodes
when in contact with certain types of metal, so the resistance
of the connection goes up. Which causes it to heat up and corrode/
oxidize still more. Eventually the wire may start getting very hot,
melt the insulation or fixture it's attached to, and possibly even
cause a fire.
Since people usually encounter aluminum wiring when they move
into a house built during the 70's, we will cover basic points
of safe aluminum wiring. We suggest that, if you're
considering purchasing a home with aluminum wiring, or have
discovered it later, that you hire a licensed electrician or
inspector to check over the wiring for the following things:
1) Fixtures (eg: outlets and switches) directly attached to
aluminum wiring should be rated for it. The device will
be stamped with "Al/Cu" or "CO/ALR". The latter supersedes
the former, but both are safe. These fixtures are somewhat
more expensive than the ordinary ones.
2) Wires should be properly connected (at least 3/4 way around
the screw in a clockwise direction). Connections should be
tight. While repeated tightening of the screws can make the
problem worse, during the inspection it would pay off to snug
up each connection.
Note that aluminum wiring is still often used for the
main service entrance cable. It should be inspected.
3) "push-in" terminals are an extreme hazard with aluminum wire.
Any connections using push-in terminals should be redone with
the proper screw connections immediately.
4) There should be no signs of overheating: darkened connections,
melted insulation, or "baked" fixtures. Any such damage should
5) Connections between aluminum and copper wire need to be
handled specially. Current Canadian codes require that the
wire nut used must be specially marked for connecting
aluminum to copper. The NEC requires that the wire be
connected together using special crimp devices, with an
anti-oxidant grease. The tools and materials for the latter
are quite expensive - not practical to do it yourself unless
you can rent the tool.
6) Any non-rated receptacle can be connected to aluminum wiring
by means of a short copper "pigtail". See (5) above.
7) Shows reasonable workmanship: neat wiring, properly stripped
(not nicked) wire etc.
If, when considering purchasing a home, an inspection of the wiring
shows no problems or only one or two, we believe that you can consider
the wiring safe. If there are signs of problems in many places,
we suggest you look elsewhere. If the wrong receptacles are used,
you can replace them with the proper type, or use pigtails - having
this professionally done can range from $3 to $10 per receptacle/
switch. You can do this yourself too.
Subject: I'm buying a house! What should I do?
Congratulations. But... It's generally a good idea to hire
an inspector to look through the house for hidden gotchas.
Not just for wiring, but plumbing and structural as well. If an
inspection of the wiring shows no problems or only one or two minor
ones, we believe that you can consider the wiring safe (after any
minor problems are fixed). If there are signs of problems in many
places, we suggest you look elsewhere.
Here's some hints on what to look for:
Obvious non-code wiring can include:
- Zip cord wiring, either concealed or nailed to walls
- Hot wiring on the identified (neutral) conductor without
- Ungrounded grounding outlets (except when downstream of
- Splices hanging in mid-air (other than proper knob-and-tube)
- Switched neutrals
- Unsecured Romex swinging about like grapevines
Certain wiring practices that are actually to code (or were at one
time) sometimes reveal DIY wiring that may have hidden violations:
- Switches that seem to control nothing (abandoned, perhaps
not properly terminated wiring)
- A wall switch that controls things that you think it
shouldn't, for instance mysteriously removing power
from lights or outlets in other rooms.
- Switches and outlets in bizarre locations
- Great numbers of junction boxes without outlets or lamps
- Junction boxes with great numbers of wires going into them
- Wiring that passes through a closet instead of a wall or
- Backwrapped grounding wires (ground wire wrapped around
the incoming cable insulation outside the box).
- A breaker or fuse for outside wiring that is near the bottom
of the breaker panel or in an add-on fusebox. The outdoor
wiring may have been homeowner-installed after the house was
built, and was not buried deep enough or was done with the
wrong kind of wire.
Subject: What is this weird stuff? Old style wiring
In the years since Edison "invented" electricity, several different
wiring "styles" have come and gone. When you buy an older home you
may encounter some of this stuff. This section describes the old
methods, and some of their idiosyncrasies.
The oldest wiring system you're likely to encounter is called
"knob and tube" (K&T). It is made up of individual conductors with
a cloth insulation. The wires are run along side structural
members (eg: joists or studs) using ceramic stand-offs (knobs).
Wire is run through structural members using ceramic tubes. Connections
were made by twisting the wire together, soldering, and wrapping
with tape. Since the hot and neutral were run separately,
the wiring tends to be rather confusing. A neutral often runs
down the centre of each room, with "taps" off to each fixture.
The hot wire tended to run from one fixture to the next. In some
cases K&T isn't colour-coded, so the neutral is often the same
colour as the hot wires.
You'll see K&T in homes built as late as the 40's.
Comments on K&T:
- the people installing K&T were pretty paranoid about
electricity, so the workmanship tends to be pretty good.
- The wire, insulation and insulators tend to stand up
very well. Most K&T I've seen, for example, is in
quite good condition.
- No grounding. Grounding is usually difficult to install.
- boxes are small. Receptacle replacement (particularly with
GFCI) can be difficult. No bushing on boxes either,
so wiring changes need special attention to box entry.
- Sometimes the neutral isn't balanced very well between
separately hot circuits, so it is sometimes possible to
overload the neutral without exceeding the fusing on
- In DC days it was common to fuse both sides, and no
harm was done. In fact, it was probably a Good Thing.
The practise apparently carried over to K&T where
you may find fused neutrals. This is a very bad
- Building code does not usually permit insulation in
walls or ceilings that contains K&T. Some jurisdictions
| will allow it under some circumstances (eg: engineer's
- Connection to existing K&T from new circuits can be
tricky. Consult your inspector.
- Modern wiring practice requires considerably more
outlets to be installed than K&T systems did.
Since K&T tends to be in pretty decent condition it generally
isn't necessary to replace it simply because it's K&T. What
you should watch out for is renovations that have interfered
with it and be cautious about circuit loading. In many cases
it's perfectly reasonable to leave existing K&T alone, and add
new fixtures on new circuits using modern techniques.
After K&T, they invented multi-conductor cable. The first type
you will see is roughly a cloth and varnish insulation. It
looks much like the romex cable of the last decade or two.
This stuff was used in the 40's and 50's. Again, no grounding
conductor. It was installed much like modern wiring. Its
major drawback is that this type of insulation embrittles.
We've seen whole systems where the insulation would fracture
and fall off at a touch. BX cable of the same vintage has
similar problems. It is possible for the hot conductor to
short out to the cable jacket. Since the jacket is rusted, it
no longer presents a low resistance return path for the current
flow, but rather more acts like a resistance heater. In
extreme cases the cable jacket will become red hot without
blowing the fuse or circuit breaker. The best thing to do with
old style BX is to replace it with modern cable whenever it's
encountered and there's any hint of the sheath rusting.
This stuff is very fragile, and becomes rather hazardous if the
wires become bare. This wiring should be left untouched as
much as possible - whenever an opportunity arises, replace it.
A simple receptacle or switch replacement can turn into a
several hour long frustrating fight with electrical tape or
After this wiring technique, the more modern romex was
invented. It's almost a asphalt impregnated cloth. Often a
bit sticky. This stuff stands up reasonably well and doesn't
present a hazard and is reasonably easy to work with. It does
not need to be replaced - it should be considered as safe as
the "modern" stuff - thermoplastic insulation wire. Just don't
abuse it too much.
Subject: Where do I buy stuff?
Try to find a proper electrical supply outlet near you. Their
prices will often be considerably better than chain hardware stores or
DIY centres, have better quality materials, have wider variety
including the "odd" stuff, and have people behind the counter that
know what you're talking about. Cultivate friendly knowledgeable
sales people. They'll give you much valuable information.
Subject: Copper wire characteristics table
These are taken from the Amateur Radio Relay Handbook, 1985.
AWG dia circ open cable ft/lb ohms/
mils mils air A Amp bare 1000'
10 101.9 10380 55 33 31.82 1.018
12 80.8 6530 41 23 50.59 1.619
14 64.1 4107 32 17 80.44 2.575
We don't show specs for 8ga or larger because they're
Mils are .001". "open air A" is a continuous rating for
a single conductor with insulation in open air. "cable amp"
is for in multiple conductor cables. Disregard the amperage
ratings for household use.
To calculate voltage drop, plug in the values:
V = DIR/1000'
Where I is the amperage, R is from the ohms/1000' column
above, and D is the total distance the current travels (don't
forget to add the length of the neutral and hot together - ie:
usually double cable length). Design rules in the CEC call
for a maximum voltage drop of 6% (7V on 120V circuit)
Subject: Smoke detector guidelines
| Many (most?) building codes now require the installation of
| smoke detectors in homes. In fact, this has been made
| retroactive in many municipalities.
| There are many different types of smoke detectors. Ionization,
| photo-cell, battery-powered, AC-powered etc. The only thing
| we're concerned with here, is AC versus battery powered, other
| than to comment that most building codes are based around
| ionization detectors, photocell units being usually for
| somewhat more specialized purposes. All things being equal, in
| a residential setting with the "ordinary fire", an ionization
| detector will detect smoke before a photo-cell will - indeed,
| in some fires, the smoke is almost invisible, and less likely
| to trip a photo-cell.
| There is another type of fire detectors - "heat detectors".
| These work usually by a small piece of special metal melting at
| 110F or so. These are much better at avoiding false trips.
| But they usually take much longer to trip than a smoke detector, and
| should usually only be considered for triggering sprinkler
| devices (where the consequences of a false trip are quite
| severe). Heat detectors should not be used as primary fire
| Most building codes that mandate detectors mandated AC-powered
| ones for new construction. This is because the statistics show
| that, in houses equipped with smoke detectors, a lot more
| people were getting killed in houses with battery-only
| detectors that had dead batteries than were getting killed in
| houses where the breakers tripped and killed an AC-only
| detector. It's also worth noting that some battery detectors
| are quite sensitive about battery condition. Some even refuse
| to work if the battery is zinc-carbon (standard cheap battery)
| instead of alkaline (more expensive).
| Our building code discourages the installation of smoke
| detectors on circuits used for other purposes. This means that
| only a main-panel breaker trip can kill the detectors. A
| main-panel trip is unlikely even in a fire started by an
| electrical fault until well after the fire has really engulfed the
| These codes also usually require that the AC detectors be
| interconnected so that if one triggers, they all sound the
| alarm. This is usually done by an additional wire between the
| The above suggests that the best way of doing things is to have
| one circuit dedicated for smoke detectors, and you run 14-3
| between each of the detectors - the red wire being the "gang
| trip" control.
| If you're still concerned about losing power and thereby losing
| your detectors, we suggest either the use of detectors that run
| off AC power with battery backup, OR, adding battery detectors
| into a system that's already adequately covered with AC detectors.
| Battery-only detectors should only be considered a stopgap
| measure in putting detectors into a house that doesn't have any
| detectors at all, or adding redundancy into a system that already
| has AC detectors.
| We also suggest that, if you have battery detectors, you make
| changing the battery a yearly (or semi-yearly) scheduled event.
| Some people change the batteries on their birthdays. Others
| change the batteries during a "daylight/standard time change"
| maintenance pass.
| We don't recommend waiting for the detector to tell you that the
| battery is dead, unless you manually test the detector monthly.
Chris Lewis: _Una confibula non sat est_
Phone: Canada 613 832-0541
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