Cerro Wire, a manufacturer of copper electrical building wire, provides a wire size and amp ratings chart on its website. On the chart, Cerro identifies the maximum amp load for both copper and aluminum wire of varying wire gauge sizes.
An ampere, or amp, is a measure of electrical charge passing through a circuit. The correct wire size is necessary to prevent overheating of the electrical wire. For example, standard residential electrical outlets support 20 amps of current, and this setup requires 12-gauge NM-B copper wire. Most lighting circuits only require 15 amps of current and call for 14-gauge NM-B wire.
Adding together the wattage of all electrical devices on an individual circuit and then dividing by the circuit's voltage amount – either 110 or 120 volts – calculates total amperage.