Circular mil
A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil (one thousandth of an inch). It corresponds to 5.067×10−4 mm². It is a unit intended for referring to the area of a wire with a circular cross section. As the area in circular mils can be calculated without reference to π, the unit makes conversion between cross section and diameter of a wire considerably easier.
The area in circular mils, A, of a circle with a diameter of d mils, is given by the formula:
In Canada and the United States the National Electrical Code (NEC) uses the circular mil to define wire sizes larger than 0000 AWG. In many NEC publications and uses, large wires may be expressed in thousands of circular mils, which is abbreviated in two different ways: Kcmil or MCM. For example, one common wire size used in the NEC has a cross-section of 250,000 circular mils, written as 250 kcmil or 250 MCM, which is the first size larger than 0000 AWG used within the NEC.
As a unit of area, the circular mil can be converted to other units such as square inches. A 0000 AWG solid wire is defined to have a diameter of exactly 0.46 inch.
Formula 1: Circular Mil
Formula 2: Square Mil
Formula 3: Square Inch
Formula 4: Solving for Circular Mil
Formula 5: Solving for Square Mil
Therefore, the following conversions apply:
1 circular mil is approximately equal to:
1000 circular mils = 1 MCM or 1 kcmil, and is (approximately) equal to:
Therefore, for practical purposes such as wire choice, 2 kcmil ≈ 1 mm² is a reasonable rule of thumb for many applications.
1_circular_mil = π × (1e−3 / 2 × 25.4)2 = 506.7×10−6mm2
1_MCM = 1000 circular_mil = 506.7×10−3mm2
When large diameter wire sizes such as the widely used 250 kcmil and 350 kcmil wires are specified in kcmil, the wire's diameter can be easily determined with the following formula:
Formula 6: diameter
Thus, this wire would have a diameter of a half inch or 12.7 mm.
The formula to calculate the circular mil for any given AWG (American Wire Gauge) size is as follows. An represents the circular mil area for the AWG size n.
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