Whenever a circuit is extended or rewired, or when any new circuit is
installed, it is critical that the new wiring is made with wire
conductors that are properly sized for the amperage rating of the
circuit. The higher the amperage rating of the circuit, the larger the
wires need to be in order to avoid excess heat that can melt wires and
cause fires. The proper circuit size, as indicated by amperage, is
determined by several factors, including the planned load on the
circuit, the number of outlets or light fixtures, and the length of the
circuit. Once the proper amperage is determined, though, it is critical,
that the wire gauge used in the circuit is appropriate for the amperage
of the circuit breaker.
How Wires Are Sized
If you've shopped for electrical wire, you have likely noticed that there are many types and sizes
of wire to choose from. Different types of wire are intended for
different uses, but with any of these wire types, knowing the right wire
size, or gauge, is key to making the right choice.
Wire is sized by the American Wire Gauge (AWG) system. Wire
gauge refers the physical size of the wire, rated with a numerical
designation that runs opposite to the diameter of the conductors—in
other words, the smaller the wire gauge number, the larger the wire
diameter. Common sizes include 14-, 12-, 10-, 8-, 6-, and 2-gauge wire.
The size of the wire dictates how much current can safely pass through
the wire.
Electrical current is measured in ampacity, and each wire gauge has a maximum safe carrying capacity. For standard NM cable, these amperage capacities are:
These ratings are for standard copper NM sheathed cable, but there
are instances where these amperage ratings vary. For example, there is
aluminum wiring in some homes, and aluminum wires have their own
ampacity-carrying capacity. Aluminum wiring was once widely used, but
because it was found that aluminum had a greater expansion profile under
load, it often loosened wire connections and sometimes caused
electrical fires. That is not to say you are necessarily at risk just
because you have aluminum wiring, because those connections may work
forever if not overloaded. But an evaluation and replacement with copper
wiring may be a good idea.
Stranded vs. Solid Wire
One more thing to keep in mind is to select the style of wire that
best fits your needs. Some wire is stranded, while other wire consists
of a solid copper conductor. In installations using metal conduit, the
solid wire doesn't always pull as easily if the conduit has a large
number of bends. But solid wire is usually easier to secure under screw
terminals, such as those found on standard switches and receptacles. In
standard usage, though, the wire conductors in conduit or NM cable for
household wiring will be 14-, 12- or 10-gauge wire that is a solid
copper conductor.
Why Wire Gauge is Important
While circuit breakers or fuses
offers good protection against overloading wires and overheating them,
they are not absolute protection. Both these devices are designed to
sense current overloads and to trip or "blow"
before the wires can overheat to the danger point. But they are not
foolproof, and it is still important to guard against exceeding the
amperage rating of any given circuit by plugging too many appliances
into them.
There is the potential for danger anytime a device or appliance tries
to draw more power on a circuit than the wire gauge is rated for. For
example, plugging a heater rated for 20 amps into a 15-amp circuit wired
with 14-gauge wire poses a distinct danger. Should the circuit breaker
fail to operate correctly, that heater will draw more current than the
wires can safely handle, and could heat the wires to the point of
melting the insulation around the wires and igniting surrounding
materials.
On the other hand, there is no danger whatsoever by plugging
appliances with mild electrical loads into circuits with heavier gauge
wires and a higher amperage rating. The circuit will draw the power
asked for by whatever is plugged into them and no more. So, for
example, running a laptop computer with a very small amperage demand on a
20-amp circuit wired with 12-gauge wire is perfectly fine.
The potential for danger is most pronounced with the use of light
household extension cords. Many a household fire has occurred when a
light extension cord with 16-gauge wire is used to power a heater or
heating appliance of some sort. Most manufacturers will discourage the
use of any extension cords with portable heaters, but if one must be
used, it has to be a heavy-duty cord with a high amperage rating that
matches the amperage of the appliance and of the circuit it is plugged
into.
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Electrical Wire Size CalculatorThis should be used as a reference only. Please check with your electrical inspector or a licensed electrician for the proper wire size. There are many factors to consider before choosing the proper gauge. |
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