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 is a convenient unit for referring to the area of a wire with a circular cross section, because the area in circular mils can be calculated without reference to pi ($\pi$). The area in circular mils, A, of a circle with a diameter of d mils, is given by the formula: Electricians in Canada and the United States are familiar with the circular mil because 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: 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, which is the first size larger than 0000 AWG used within the NEC.
Circular Mil