Use a wire gauge amp chart to determine the approximate wire size for an electrical load. There are separate charts for different types of wire. Since the resistance of electricity is dependent on several factors, the chart cannot give the exact size for every application. In the United States, wire is sized using the American wire gauge system.
Wire gauge grows smaller as the diameter of the wire increases, thus a 2 AWG wire carries more electricity safely than a 12-gauge wire. Wire ampacity specifies the amount of electricity a specific gauge wire carries without becoming hot and causing a fire. Electricians often use 14-gauge copper wire, rated at 15 amps for light fixtures and lamps, but they use 12-gauge wire for receptacles, air conditioners, sump pumps and other appliances requiring up to 20 amps. Clothes dryers, water heaters and ovens require 30 amps of electricity, thus the electrician installs 10-gauge wire for these appliances.
The longer a wire the greater the resistance. While the wire gauge amp chart information is adequate for wiring in the typical home, in applications where runs are longer than normal, the electrician must consider the resistance the additional length adds to the circuit to ensure it continues to operate safely without overheating.