A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil. It corresponds to approximately. It is a unit intended for referring to the area of a wire with a circular cross section. As the definition of the unit contains pi|, it is easy to calculate area values in circular mils knowing the diameter in mils. 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 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. 1000 circular mil equals, so for most purposes, a ratio of 2 MCM ≈ 1 mm2 can be used with negligible error.
1000 circular mils = 1 MCM or 1 kcmil, and is equal to:
0.5067 mm, so 2 kcmil ≈ 1 mm
Therefore, for practical purposes such as wire choice, 2 kcmil ≈ 1 mm is a reasonable rule of thumb for many applications. 1 cmil = π × = 5.067 × 10−4 mm 1 kcmil = 1 MCM = 1000 cmil = 506.7 × 10−3 mm 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.
Metric equivalent
Some tables give conversions to circular millimetres. The area in cmm is defined as the square of the wire diameter in mm. However, this unit is rarely used in practice. One of the few examples is in a patent for a bariatric weight loss device.
AWG circular mil formula
The formula to calculate the circular mil for any given AWG size is as follows. An represents the circular mil area for the AWG size n. For example, a number 12 gauge wire would use n = 12: Sizes with multiple zeros are successively larger than the number 0 gauge size and can be denoted using "number of zeros/0"; for example 4/0 for the number 0000 gauge. For an m/0 AWG wire size, use For example, the number 0000 gauge or 4/0 gauge, would use n = −3; and the calculated result would be 211,600 circular mils.
Standard sizes
Standard sizes are from 250 to 400 in increments of 50 kcmil, 400 to 1000 in increments of 100 kcmil, and from 1000 to 2000 in increments of 250 kcmil. The diameter in the table below is that of a solid rod with the given conductor area in circular mils. Stranded wire is around 5% larger in diameter to allow for gaps between the strands, depending on the number and size of strands. Note: For smaller wires, consult.
