For house wiring, the cross sectional area is specified by the American Wire Gauge (AWG). The gauge numbers run from "0" for the largest to 20 for the smallest. For AWG numbers 0 - 6, the wires are required to be stranded, several smaller wires twisted together. AWG numbers 8 through 20 may be single wires. The lower gauges will have lower resistance and hence can carry greater amounts of current. They are used for main feeder lines and typically can carry hundreds of amperes of current. The higher gauges should supply current only to a few small appliances. AWG gauges 16, 18 and 20 are used only for very low current service such as for thermostats or extension cords.

Roughly speaking, you need a 12 gauge wire to carry 20 A of current for a wire length of 100 ft or less. Click here for an AWG Table of AWG numbers with corresponding diameters and cross sections. For more detailed information about how to determine the gauge based upon current requirement and wire length, check out the Epanorama site (cache).

Standard house wiring (also called by the brand name Romex) always comes with three or more wires encased in an insulator such as neoprene. (Flexible metal armor is also available.) For the three-wire cable, two of the wires will be insulated. They are called the "hot" wire (usually black or red) and the "return" wire (usually white). The third wire is typically a bare copper wire. The 120 VAC potential will found on the hot wire, while the return wire should be close to zero potential. Current will flow from the hot wire to the device and return along the return wire. No current flows without a return path. There should always be the same current flowing in the return as there is in the hot wire. You can think of the power company as if it were a giant battery. The hot and return wires are connected to the two terminals of the battery.

But the return is not always at zero potential relative to your "local ground". For safety, there should always be a local ground. This is the purpose of the bare copper wire. It should be connected at one end to a conductor that is buried into the ground. All metal electrical casings and electrical outlets should be connected to this wire.

Often you will also find four-wire house wiring that contains both a red and black wire. Both are "hot" but they are not the same. Both are 120 VAC from the return wire, but they are 240 VAC relative one another. More on this in the Magnetism and Generators/Motors section!

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