The United States uses AWG for indicating the sizes of cables. AWG is the acronym for American Wire Gauge. AWG sizes increase as the actual size of the wire decreases. For very large wires, the gauge system is not used and instead, the cross sectional area is used. Electricians, as well as electrical engineers, will need to know the size of the wires so that the conduit that carries those wires from place to place can be sized correctly. Most often, a table can be used to look up this information, but when that isn't available, then the size can be calculated using the AWG.
Obtain the wire that you will be using. Look to see what the AWG is for that wire. This is usually found on the wire itself, printed sideways along the insulation, or on the packaging for the wire.
Calculate the diameter of your wire using the following formula: D = [92^((36-AWG)/39)].005, where D is the diameter and AWG is the gauge of the wire. The result will be in inches. For example, using a 10-gauge wire, the formula would be as follows: D = [92^((36-10)/39)].005 = 0.101897 inches. For the very large wires that have gauges of 0, 00, and 000, use -1, -2, and -3 for the AWG variable in the equation.
Convert the answer from Step 2 to mils by multiplying the number of inches by 1000. Most wires are too small for inches to be used. In the example: 0.101897in * 1000 = 101.897 mils. 1 mil = 1/1000 inch.
Calculate the cross-sectional area of your wire or cable by squaring the answer from Step 3. In the example: 101.897 mils ^ 2 = 10383.0221 circular mils. 1 circular mil = the area of a circle with a diameter of 1 mil. This number is often expressed in kcmils, which is obtained by dividing circular mils by 1000. In the example, 10383.0221/1000 = 10.4 kcmils. 1 kcmil = 1/1000 circular mil.
Always refer to local codes and the National Electric Code (NEC) before starting any project.