CerroWire.com, AllenElectric.com, Gore.com, LugsDirect.com and Powerstream.com have ampacity charts that show the correct wire gauge to use for the intended amperage draw. Calculate the amperage draw by adding the wattage of each electrical device attached to the circuit, and then divide the sum by the voltage used.
In typical residential applications, the divisor is either 110 or 220 volts. Wire gauge requirements depend upon the type of conductor being used, with copper needing less thickness than aluminum to transfer the same amperage. Most American ampacity charts show only the American Wire Gauge sizes; however, PowerStream.com also lists metric wire sizes.