What Is a Circular Mil?

A circular mil (cmil) is a measure of cross-sectional area of a cable. One circular mil is a very small area. If a circle has a diameter that is exactly a thousand times smaller than 1 inch (0.03 m), the area of that circle is 1 circular mil.

Different countries may tend to choose a specific unit for the cross-sectional area of cables. When international projects are implemented, there are agreements on which standards will be enforced. The principles involved in the various ways of specifying cable size should be understood when choosing a standard. It is a good idea for people to know how to convert wire sizes in different units. There are conversion tables readily available to convert circular mils to American wire gauge (AWG) to square millimeter and other measurement units.

A circular mil is often the choice unit for specifying sizes of wires and cables. There are English system and metric system references to cross-sectional areas of wires and cables. The circular mil references an area of a circle with a diameter of 0.001 inch (0.0254 mm), while the metric system reference is the square millimeter. A million circular mils is equivalent to about 0.7854 square inches (1.99 square cm).

The circular mil applies to the size of the cable, or the cross-sectional area of the cable. For electrical cables, there could be a single circular cross-section or several smaller cables to form a bigger cable, which is referred to as a stranded cable. A 1,000 cmil solid cable and a 100 cmil x 10 stranded cable may have the same electrical ampere capacity but different mechanical specifications. Insulation is another matter for electrical cables. Whether the cable is to meet insulation at 300 volts alternating current (VAC), for instance, or at 1,000 volts direct current (VDC) has to be thoroughly studied.

Electrical charges on an energized cable travel at right angles to the cross-sectional area mentioned, which is very similar to vehicles traveling in a direction at right angles to the width of the freeway. The wider the freeway, the more vehicles may travel per unit of time. On the electrical cable, the wider or bigger the cross-sectional area, the more electrical charge may travel per unit time. In other words, the larger the cross-section the greater the ampere rating.