What unit is used to express the cross-sectional area of a round wire?

• A. Circular mils
• B. Square mils
• C. Square inches
• D. Square centimeters

Answer: (A)

Cross-sectional area for a round wire grows with the square of its diameter, so using a unit that makes that relationship direct is helpful. Circular mils do exactly that: one circular mil is the area of a circle with a diameter of 1 mil. With a wire whose diameter is D mils, the area in circular mils is simply D^2. This makes sizing and electrical calculations straightforward, since no pi/4 factors appear in the conversion. For example, a 2-mil diameter wire has 4 circular mils of cross-sectional area. Other units would either introduce extra factors or be impractically large for small wires, so circular mils is the standard choice.