Pt 3/4: Electrical Wiring FAQ [Part 1/2]
Chris Lewis
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
replaced.
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
purpose.
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.
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
penny" trick.
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.
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
heat easily:
Gauge Amps
14 15
12 20
10 30
8 40
6 65
We don't list bigger sizes because it starts getting very dependent
on the application and precise wire type.
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
runs.
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
wire.
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.
used?
A wire nut is a cone shaped threaded plastic thingummy that's used
to connect wires together. "Marrette" or "Marr connector"
are trade names. You'll usually use a lot of them in DIY wiring.
In essence, you strip the end of the wires about an inch, twist them
together, then twist the wirenut on.
Though some wirenuts advertise that you don't need to twist the
wire, do it anyways - it's more mechanically and electrically
secure.
There are many different sizes of wire nut. You should check
that the wire nut you're using is the correct size for the
quantity and sizes of wire you're connecting together.
Don't just gimble the wires together with a pair of pliers or
your fingers. Use a pair of blunt nose ("linesman") pliers,
and carefully twist the wires tightly and neatly. Sometimes
it's a good idea to trim the resulting end to make sure it
goes in the wirenut properly.
Some people wrap the "open" end of the wirenut with electrical
tape. This is probably not a good idea - the inspector may
tear it off during an inspection. It's usually done because
a bit of bare wire is exposed outside the wire nut - instead
of taping it, the connection should be redone.
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
the circuit.
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
GFCI happy.
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.
The NEC mandates GFCIs for 110V, 15A or 20A single phase
outlets, in bathrooms, kitchens within 6' of the sink, wet-bar
sinks, roof outlets, garages, unfinished basements or crawl spaces,
outdoors, near a pool, or just about anywhere else where you're likely
to encounter water or dampness. There are exceptions for inaccessible
outlets, those dedicated to appliances ``occupying fixed space'',
typically refrigerators and freezers, and for sump pumps and
laundry appliances.
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?
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.
For most situations, you can use either a GFCI outlet as the
f