A circular mil (CM) is a unit of measurement used to define the cross-sectional area of a round electrical conductor. It represents the area of a circle with a diameter of one-thousandth of an inch (0.001 in).
The circular mil is a fundamental unit in the electrical and wire manufacturing industries, providing a standardized way to compare and calculate conductor sizes based on cross-sectional area rather than diameter. This measurement simplifies the process of determining current-carrying capacity, voltage drop, and electrical resistance for round conductors. One circular mil equals approximately 7.854 × 10⁻⁷ square inches, and the total cross-sectional area of a wire increases as the number of circular mils increases.
Using circular mils allows engineers and manufacturers to evaluate and specify conductors for industrial and commercial applications with precision. Because resistance and ampacity are directly related to a wire’s cross-sectional area, the circular mil measurement provides a consistent means of predicting performance across different materials and conductor types. For example, doubling the number of circular mils roughly halves the resistance, assuming identical conductor materials.
In the American Wire Gauge (AWG) system, wire sizes are derived from circular mil calculations. Each successive AWG size represents a decrease in circular mil area, which corresponds to a reduction in the conductor’s ability to carry current. For larger conductors, sizes are often specified directly in circular mils, such as 250,000 CM (250 kcmil) or 500,000 CM (500 kcmil), rather than by AWG number.
The use of circular mils is recognized and standardized by organizations such as the National Electrical Code (NEC), IEEE (Institute of Electrical and Electronics Engineers), and UL (Underwriters Laboratories). These entities define guidelines for conductor sizing, ampacity, and safe electrical system design.
The circular mil was first introduced in the 19th century to simplify wire gauge measurement for round conductors. Prior to this standardization, measurements were inconsistent, with various regional methods for describing conductor sizes. The adoption of the circular mil provided a unified and mathematically consistent standard for the growing electrical industry, particularly in the United States.