The cross-sectional area of round wire is measured in circular mils.

• A. True
• B. False
• C. Square mils
• D. Square inches

Answer: (A)

For round conductors, cross-sectional area is expressed in circular mils. A circular mil is a unit defined so that the area of a circle with a diameter measured in mils equals the diameter squared, giving a direct link between diameter and area without extra factors. Since a mil is one thousandth of an inch, using circular mils lets you read the area from the diameter in mils as simply d^2. This convention is standard in electrical practice because it makes resistance, current capability, and related calculations straightforward. For example, a wire with a 20 mil diameter has an area of 400 circular mils. Using square mils would require multiplying by pi/4 to convert, and square inches are far too large for typical wire sizes. So the statement is true.