Wire gauge units – definition of a circular mil In electrical engineering practice, a “circular mil” is defined as the area of a circular wire having which of the following geometric properties?

Introduction:
The circular mil (cmil) is a traditional North American unit for wire cross-sectional area used in ampacity and resistance tables. Although SI units use mm^2, the circular mil remains common in legacy specifications and cable catalogs.

Given Data / Assumptions:

• 1 mil = 0.001 inch.
• Area refers to a circle's cross-section of a round wire.
• Definition should use diameter, not radius or circumference.

Concept / Approach:

By definition, 1 circular mil is the area of a circle having a diameter of exactly 1 mil. Hence, the area in circular mils is simply the diameter in mils squared (A_cmil = d_mils^2). This simplifies mental arithmetic for wire sizes without using π explicitly, since A = (π/4) d^2 in inch-based units corresponds to a constant when converted from circular mils.

Step-by-Step Solution:

Verification / Alternative check:

Wire tables list areas directly in kcmil (MCM), consistent with the diameter-squared rule, confirming the use of diameter in the definition.

Why Other Options Are Wrong:

Radius- or circumference-based definitions are nonstandard; 1 millimetre is not 1 mil and would be a vastly larger diameter.

Common Pitfalls:

Confusing mil with millimetre; forgetting that “circular mil” is based on diameter, not radius.

Final Answer:

having a diameter equal to 1 mil