Circular mil

Unit of area

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 or ). It corresponds to approximately 5.067. 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 , it is easy to calculate area values in circular mils when the diameter in mils is known. 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 Canadian Electrical Code (CEC) and the National Electrical Code (NEC), respectively, use 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 conductor diameter of 0.5 inches, or 500 mils, and thus a cross section of 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 approximately 0.5067, so for many purposes, a ratio of 2 MCM ≈ 1 mm2 can be used with negligible (1.3%) error.

Likewise, since 1 inch is defined as exactly 25.4mm, 1mil is equal to exactly 0.0254mm, so a similar conversion is possible from circular mils to square millimetres:

A 0000 AWG solid wire is defined to have a diameter of exactly The cross sectional area of this wire is:

Some tables give conversions to circular millimetres (cmm). 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.

Standard sizes are from 250 to 400 in increments of 50kcmil, 400 to 1000 in increments of 100kcmil, and from 1000 to 2000 in increments of 250kcmil. The diameter in the table below is that of a solid rod with the given conductor area in circular mils. Stranded wire is larger in diameter to allow for gaps between the strands, depending on the number and size of strands. + Standard kcmil wire sizes& solid copper equivalents Area Diameter NEC copper wireampacity with60/75/90 °Cinsulation (A) (kcmil, MCM) (mm2) (in) (mm) 250 126.7 0.500 12.70 215 255 290 300 152.0 0.548 13.91 240 285 320 350 177.3 0.592 15.03 260 310 350 400 202.7 0.632 16.06 280 335 380 500 253.4 0.707 17.96 320 380 430 600 304.0 0.775 19.67 355 420 475 700 354.7 0.837 21.25 385 460 520 750 380.0 0.866 22.00 400 475 535 800 405.4 0.894 22.72 410 490 555 900 456.0 0.949 24.10 435 520 585 1000 506.7 1.000 25.40 455 545 615 1250 633.4 1.118 28.40 495 590 665 1500 760.1 1.225 31.11 520 625 705 1750 886.7 1.323 33.60 545 650 735 2000 1013.4 1.414 35.92 560 665 750
Note: For smaller wires, consult .