Use a chart to determine the correct wire gauge size depending on the amperage a circuit must accommodate. To calculate the correct amperage for a circuit, add the wattage of all of the appliances and fixtures on a particular circuit. Then, divide that number by the system voltage to calculate the necessary amperage for that circuit.
In residential construction, 14-gauge copper wire accommodates 15 amps and a maximum wattage load of 1440 watts at 120 volts, which is enough to support light fixtures. Twelve-gauge copper wire withstands 20 amps, with a maximum wattage load of 1920 watts at 120 volts or 3840 watts at 240 volts. Therefore, a 12-gauge copper wire accommodates lighting fixtures, small appliances and receptacles.
For circuits that power a washing machine or dryer, architects specify 10-gauge copper wire that is safe for a load of 2880 watts at 120 volts or 5760 watts at 240 volts. Very large appliances, such as a central air conditioning system or electric furnace, may require a 6-gauge or 8-gauge copper wire.
Most residential construction uses copper wire, but aluminum wire is also acceptable and was used during the 1950s in response to a copper shortage. However, aluminum and copper wire of the same gauge do not accommodate the same number of amps. For example, a 12-gauge aluminum wire only accommodates 15 volts, rather than the 20 volts a 12-gauge copper wire can handle.