What is the standard unit for measuring the cross-sectional area of a wire?

• A. Circular mil
• B. Square millimeter
• C. Circular inch
• D. Square inch

Answer: (A)

Wires are sized by cross-sectional area expressed in circular mils. A circular mil is the area of a circle with a diameter of 1 mil (one-thousandth of an inch). This unit is convenient because the area of a circular wire relates directly to its diameter: if you know the wire diameter in mils, the cross-sectional area in circular mils is simply the diameter squared. That simple relationship keeps numbers easy to work with and avoids extra factors that would come from using other area units.

In other words, a wire 10 mils in diameter has an area of 100 circular mils. Using square mils or square millimeters would complicate things with conversions and extra constants, and square inches or circular inches are impractical for the small sizes typical of wiring. Circular mils are the standard for quick, practical sizing in electrical practice.