A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil. A mil is one thousandth of an inch. A circular mil is a unit for referring to the area of the cross section of a wire or cable with a circular cross section, as the area in circular mils can be calculated without reference to pi.
The area in circular mils, A, of a circle with a diameter of d mils, is given by:
- A = d2
Electricians in the USA are familiar with the circular mil because the National Electrical Code uses the circular mil to define wire sizes larger than 4/0 AWG. In many NEC publications and uses, large wires may be expressed in thousands of circular mils, which is abbreviated in two different ways: MCM or kcmil. 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.
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Equivalence to other units of area
1 circular mil (1 cmil) is equal to square mils ÷ π × 4, or square inches ÷ π × 4,000,000. Therefore, the following formulas apply:
- circular mil = square mils ÷ π × 4
- circular mil = square inches ÷ π × 4,000,000
1 circular mil is approximately equal to:
- 0.7854 square mils (1 square mil is about 1.273 circular mils)
- 0.7854 millionths of a square inch (1 square inch is about 1.273 million circular mils)
- 5.067×10-10 square metres
- 506.7 μm²
1000 circular mils = 1 MCM or 1 kcmil, and is (approximately) equal to:
- 0.5067 mm², so 2 kcmil ≈ 1 mm² (a 1.3% error)
Therefore, for practical purposes such as wire choice, 2 kcmil = 1 mm² is a reasonable rule of thumb for many applications.
AWG circular mil formula
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.
- For example, a number 12 gauge wire would use n = 12; and the calculated result would be 6529.946789 circular mils
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 n = −(m−1) = 1−m in the above formula.
- For example, the number 0000 gauge or 4/0 gauge, would use n = −3; and the calculated result would be 211,600 circular mils
See also
References
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