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:
- 14-gauge wire: 15 amps
- 12-gauge wire: 20 amps
- 10-gauge wire: 30 amps
- 8-gauge wire: 40 amps
- 6-gauge wire: 55 amps
- 4-gauge wire: 70 amps
- 3-gauge wire: 85 amps
- 2-gauge wire: 95 amps
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.
|Wire Use||Rated Ampacity||Wire Gauge|
|Low-voltage lighting and lamp cords||10 amps||18-gauge|
|Extension cords||13 amps||16-gauge|
|Light fixtures, lamps, lighting runs||15 amps||14-gauge|
|Receptacles, 120-volt air conditioners, built-in ovens, electric water heaters||20 amps||12-gauge|
|Electric clothes dryers, 220-volt window air conditioners, built-in ovens, electric water heaters||30 amps||10-gauge|
|Electric furnaces, large electric heaters||60 amps||6-gauge|
|Electric furnaces, large electric water heaters, sub panels||80 amps||4-gauge|
|Service panels, sub panels||100 amps||2-gauge|
|Service entrance||150 amps||1/0-gauge|
|Service entrance||200 amps||2/0-gauge|