CM is an abbreviation that can refer to either Communications General Purpose Cable or Circular Mil, depending on the context. The first denotes a cable rating used in low-voltage communication systems, while the second represents a unit of measure for the cross-sectional area of a conductor.
When CM is used to describe Communications General Purpose Cable, it identifies a type of cable that meets specific requirements under the National Electrical Code (NEC) for general-purpose communication applications. CM-rated cables are typically used in commercial or industrial buildings for transmitting data, voice, video, or control signals. While they are suitable for most spaces, CM cables are not designed for use in air-handling or riser environments, where higher fire-resistance ratings such as CMP (plenum) or CMR (riser) are required. The CM rating ensures the cable meets baseline fire safety standards for general-purpose communication wiring.
In contrast, when CM refers to Circular Mil, it denotes a unit of measurement for the cross-sectional area of a wire or conductor. One circular mil equals the area of a circle with a diameter of one mil (one-thousandth of an inch). This measurement is widely used in the electrical and wire manufacturing industries to describe conductor sizes and ensure consistent current-carrying capacity across different wire gauges. As the number of circular mils increases, so does the overall wire size and current capacity.
For Communications General Purpose Cable, regulation falls under the National Electrical Code (NEC) and Underwriters Laboratories (UL), which set standards for cable ratings and fire performance. Circular Mil measurements are standardized through the Institute of Electrical and Electronics Engineers (IEEE) and the National Institute of Standards and Technology (NIST).
The CM cable rating was introduced to simplify classification under the NEC, ensuring that cable manufacturers followed consistent performance and safety requirements. The concept of the circular mil, dating back to the 19th century, was developed as a convenient mathematical unit for defining wire cross-sections without requiring complex formulas for circular geometry.