The ability of a wire to transmit an electric current without overheating is a function of its cross-sectional area. Larger wires can transmit larger currents without overheating, whereas smaller wires heat up at lower currents.
What is Amp Load?
Current is measured in amps, abbreviated to the capital letter A. Appliances are rated in watts, abbreviated to a capital W. Watts measure of the load appliances place on the electric supply. If you know the wattage and the voltage (V) you can calculate the current in amps using the formula A = W / V . This shows that when the voltage changes, the load on a wire also changes. In fact, at 110-volts the amp load is double that at 220-volts.
What is Wire Size?
In North America, electrical wire size is measured by its diameter on a scale called the American Wire Gauge, or AWG. It applies only to nonferrous metals such as aluminium, copper and silver. There are 44 standard wire sizes, numbered zero to 40, and three additional extra large sizes numbered 00, 000, and 0000. The numbers get larger as the wires get thinner, so 0000 is the thickest wire, and 44 is the thinnest.
How Are Wire Size and Load Related?
Smaller AWG numbers relate to thicker wires, and the ability to safely carry a current increases with the diameter of a wire. As AWG increases, the physical size of the wire decreases as does its current, or load, carrying capacity.
Wire Size / Load Tables
Tables are available online, and in text books, listing wire sizes and their maximum load ratings. The ratings are for bare wires surrounded by air, unless stated otherwise. Adding insulation or combining wires into a cord reduces their ability to lose heat, and decreases their safe current load. This should be born in mind when selecting a wire size based on its maximum current load.
Example Wire Size To Load Ratings
At 110-volts the load ratings for insulated copper wires in a cord are: 22 gauge wire will cope with 5 amps; 20 gauge wire will cope with 7.5 amps; 18 gauge wire will cope with 10 amps; 16 gauge wire will cope with 13 amps; 14 gauge wire will cope with 17 amps; 12 gauge wire will cope with 23 amps; 10 gauge wire will cope with 33 amps; and 8 gauge wire will cope with 46 amps.