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
Chris Lewis and Steven Bellovin
Redistribution for profit, or in altered content/format
prohibited without permission of the authors.
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The latest FAQ can always be obtained from:
Subject: How to wire 3-way and 4-way switches
The above is a truly excellent site with pictures and very
detailed code analysis. Warning, it's NEC-only-specific.
So, if you're outside of the USA, you will need to be careful.
For example, the rules for kitchens are considerably different
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,
New rule (1996 NEC): all counter outlets must be GFCI protected.
(Old NEC rule for historical purposes) Non-dedicated outlets within
6' of a sink *must* be protected by a GFCI.
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
. . . .
. . . .
. . . .
. . . .
... ... ~ -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
... ... ~ +160V
| . | .
| . | .
| . | .
| . | .
... ... ~ -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
connectors 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.
[Note that regulations are changing rapidly in this area.
Suggest that you discuss any work with an inspector if you're
going to do more than one or two connections.]
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.
There's a useful article at http://inspect-ny.com/aluminum.htm
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 - if the wire is visible, check for "UF"
or "NMW" markings.
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
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"
In Canada, the day before the standard/daylight time change
(a Saturday) now seems to be officially called "smoke detector
battery day" ;-)
We don't recommend waiting for the detector to tell you that the
battery is dead, unless you manually test the detector monthly.
Subject: Other links