Posted-By: auto-faq 3.3.1 (Perl 5.006)
This FAQ has not been updated or posted in a number of years. It
is intended that it will be updated to bring it more into compliance
with current code. Comments welcome at wirefaq(@)ferret.ocunix.on.ca
Frequently Asked Questions on Electrical Wiring
Steven Bellovin (smb(at)research.att.com)
Chris Lewis (clewis(at)ferret.ocunix.on.ca)
Redistribution for profit, or in altered content/format
prohibited without permission of the authors.
Redistribution via printed book or CDROM expressly
prohibited without consent of the author. Any other
redistribution must include this copyright notice and
Comments to: email@example.com
The latest FAQ can always be obtained from:
This FAQ is formatted as a digest. Most news readers can skip from one
question to the next by pressing ^G.
Answers to many other topics related to houses can be obtained from the
misc.consumers.house archive; send an empty piece of mail to
firstname.lastname@example.org for information.
Changes to previous issue marked with "|" in left column. Watch
particularly for "NEW" in the Questions list for new or substantively
changed answers. "g^|" will get you to the changed sections quickly in
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?
Other Resources 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 "wire-nut"/"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?
Grounding electrode system (NEW)
Bonding requirements (NEW)
Testing grounding conductors and grounding electrodes. (NEW)
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?
How to wire 3-way and 4-way switches
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
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 $20 for a good "plier-type" pair. It will have sized
stripping holes, and won't nick or grab the wire - it should
be easy to strip wire with it. One model has a small hole in the
blade for forming exact wire loops for screw terminals. There
are fancier types (autostrip/cut), but they generally aren't
necessary, and pros usually don't use them.
A pair of diagonal side cutter pliers are useful for clipping ends
in constricted places. Don't use these for stripping wire.
You will need linesman pliers for twisting wires for wire connectors.
You should have a pair of needle-nose pliers for fiddling
inside boxes and closing loops, but it's better to form wire
loops with a "loop former hole" on your wire stripper - more
If you're using non-metallic cable, get a cable stripper for
removing the sheath. Or, do what some pros do, they nick the
end of the sheath, grab the ground wire with a pair of pliers,
and simply rip the sheath back using the ground wire as a
"zipper", and cut the sheath off. You shouldn't try to strip
the sheath with a knife point, because it's too easy to
slash the insulation on the conductors. Apparently Stanley
utility knives fitted with linoleum cutters (hooked blades)
can be used to strip sheath, but there is still the possibility
that you'll gouge the conductors.
For any substantial amount of work with armored cable, it's well
worth your while to invest in a rotary cable splitter (~US$ 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
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: Other Resources on Wiring
http://homewiring.tripod.com is a truly excellent site.
It contains a fairly wide range of very detailed information.
If you need pictures on how to do common things (like 3 or 4
way switches, ceiling fan installation etc), this is a great
place to go. It doesn't cover as broad variety of things
as this FAQ, but it's much more detailed, and more up to date
in some areas.
It's particularly good for figuring out the wiring of complicated
Note that this site is 1999 NEC specific. Which means that
if you're not in the USA, you will have to be very careful about
taking the rules as gospel. For example, the section on kitchens
is entirely wrong for Canada.
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 to 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
[Important Note: In the US, the NEC used to permit a circuit
similar to (2) be used for stoves and dryers - namely, three
conductor wiring, with a ground wire doing dual duty as a neutral.
As of the 1996 revision to the NEC, this is NO LONGER PERMITTED.]
(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 "wire-nut"/"marrette"/"marr connector"? How are they
A twist-on wire connector is a cone shaped threaded plastic
thingummy that's used to connect wires together. "Marrette",
"Marr connector", "IDEAL Wire-nut(R)" 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 connector on.
While some connectors advertise that you don't need to twist the
wire, do it anyways - it's more mechanically and electrically
secure. Unless the instructions specifically state otherwise...
There are many different sizes of wire connector. You should check
that the connector 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 connector properly.
After twisting the connector on, give each wire a tug, and
make sure that nothing is loose.
Some people wrap the "open" end of the connector 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 connector - 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, kitchen counters 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: Grounding electrode system
Note that full coverage of how to install a grounding electrode
system is well beyond the scope of this FAQ. The comments made
here are primarily so that the reader understands what it is
for, and some of its characteristics.
The grounding electrode system is a method by which the neutral
and grounding conductors are connected to the common "earth"
reference. The connection from the electrical system to the
grounding system is made in only one place to avoid ground
The grounding electrode system is _not_ intended to carry much
current. Ground faults (Ie: hot to grounded case short) are
conducted down the ground wire to where it is interconnected
with the neutral and hopefully the breaker/fuse trips. The
grounding electrode does not participate in such a situation.
While the conductors involved in this are relatively large, they're
sized for lightning strikes and other extremely short duration
events. The grounding electrode system is specifically _not_
expected to have enough conductivity to trip a 15A breaker.
The grounding electrode often has a moderately high
resistance. For example, according to the NEC, an acceptable
ground electrode system may have 25 ohms of resistance - only
5A at 120V, not enough to trip a 15A breaker.
A grounding electrode system usually consists of a primary
grounding electrode, plus possibly a secondary electrode. A
primary electrode can be (if in direct contact with the earth):
10' of ground rod. 10' of well casing or metallic water pipe
(must be connected within 5' of pipe entrance to house). 20'
of copper wire buried in the bottom of the footings. A
secondary electrode will be required if the primary is a water
pipe or (NEC) if the primary electrode is >25 ohms to the
Subject: Bonding requirements
All "metallic systems" in a home that are capable of being
energized are required to be bonded to the grounding system.
This is usually taken to mean: metallic water supply, metallic
drain-waste-vent pipe, metal ducting, gas lines, and sometimes
metallic structural elements (eg: metal framing systems).
The rationale for this is simple: if somehow a hot conductor contacts
a water pipe, say, you don't want every plumbing fixture in your
home to become live. The bonding attempts to ensure that you have
a low resistance path to the ground system at the panel, and thence
to the neutral - ensuring that this ground fault is stopped by
a breaker or fuse tripping. Remember that this is independent of
the grounding electrode system's conductivity.
Normally the bonding of most of these systems are done by the
equipment involved. Furnace ducting is grounded by the furnace
connection. Gas line grounding is done by the gas man ;-)
So we'll mainly talk about water line grounding here.
The NEC appears to insist that each electrically isolated section
of metallic water pipe must be jumpered together. Take particular
note that you are required to provide a jumper wire that bypasses the
main water meter (especially if you're using the water supply line
as a grounding electrode), and a jumper between hot and cold if the
water heater is an electrical insulator. The CEC, for example,
also requires that the frame of your clothes washer is bonded to the
cold water supply pipe.
Exact details of how this bonding should be done is beyond the scope of
this FAQ. It tends to be a 6ga wire running from the grounding terminal
of the panel to a convenient copper pipe. If the water supply is used
as a grounding electrode, the rules become stricter (5' rule applies
in NEC etc.)
Subject: Testing grounding conductors and grounding electrodes.
Testing grounds is a tricky and somewhat dangerous process.
Testing for continuity is not enough. Nor is simple resistance
testing. We will outline some possible approaches, but if
you're the slightest bit uncomfortable, don't even think of
trying these procedures.
For a ground conductor to be good, the resistance must be
"low". It must also be robust enough to withstand an overload
long enough to allow the fuse or breaker to trip. The
electrical code states, as a general principle, that the
resistance of the grounding conductor be such that 4-5 times
the current of the breaker rating will flow. For example, if
your breaker is 15A, the grounding conductor's resistance
should be low enough to permit 60-75A to flow - around 2 ohms
maximum at 120V. For comparative purposes, 1000' of 14ga wire
is 2.5 ohms.
The difficulty in older homes is that the grounding conductor's
condition may be that even though the resistance is < 2 ohms, a
ground connection may blow out before the fuse/breaker goes,
leaving the case of the appliance that just shorted out live.
Therefore, you have to measure both the resistance and it's
ability to stand up to load.
One simple way to perform a "real" test is dead short the hot
to ground and see if the fuse or breaker trips. This is,
unfortunately, _extremely_ dangerous. The fuse might explode.
The breaker may malfunction. You may get sprayed with molten
copper. You may start a fire. You may get electrocuted or
blinded. So don't even think of trying this.
One moderately safe approach is to connect a 100W lightbulb
between hot and the ground you wish to test. The lamp should
light fully. If you have a voltmeter, test the voltage between
the ground and the neutral. You should see less than 2 volts.
If the voltage is much higher, or the lamp dims, disconnect it
quickly - the ground may be overheating somewhere. The ground
should be checked for poor connections.
Testing a grounding electrode is a somewhat different matter.
The codes aim for a dirt-to-electrode resistance of 25 ohms or
better. One moderately safe way is:
- turn off the main panel
- turn off all of the breakers
- disconnect the grounding electrode from the rest of
the system. (often just a bolt in the panel)
- connect a 5A fuse between the output of one 15A breaker
and the grounding electrode. (use a 5A automotive fuse
in a pigtail holder)
- turn on the main breaker and the single breaker connected
to the 5A fuse.
- if the 5A fuse blows, your ground is good.
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 metallic cold water pipe going nearby, and
it's electrically continuous to the main house ground
point, you can run a conductor to it from the third
prong. You MUST NOT assume that the pipe is continuous,
unless you can visually check the entire length and/or
test it. Testing grounds is tricky - see "Testing
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.)