Which unit is used to measure the cross-sectional area of a conductor?

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

Answer: (A)

Circular mils are the unit used to express the cross-sectional area of a circular conductor in electrical practice. A circular mil is the area of a circle with a diameter of 1 mil (one-thousandth of an inch). This means the cross-sectional area in circular mils equals the diameter in mils squared, which makes it convenient because many electrical properties, like resistance and current-carrying capability, scale with the conductor’s diameter. While square millimeters or square inches are used in metric or general area contexts, circular mils are the standard for wire sizing in North America. For example, a wire with a diameter of 80 mils has an area of 80 × 80 = 6,400 circular mils.