Conductor sizes are expressed in American Wire Gauge (AWG) and which other unit?

• A. Circular mils
• B. Square inches
• C. Square millimeters
• D. Ohms per kilometer

Answer: (A)

When you look at conductor sizes in AWG, what’s being described is the cross-sectional area of the wire. The unit commonly used for that area is circular mils. A circular mil represents the area of a circle whose diameter is one mil (one-thousandth of an inch). This unit is preferred because the cross-sectional area—and thus the current-carrying capability and resistance—scales with the square of the diameter, so expressing size in circular mils makes comparisons straightforward and calculations easier.

Other units like square inches or square millimeters exist for area, but they aren’t the standard way AWG-sized conductors are described. Ohms per kilometer is a measure of resistance, not size. So circular mils is the unit that accompanies AWG in expressing conductor size. Quick rule: circular mils roughly equal the square of the diameter in mils.