What Is, and Why Is There, a Circular Mil (CM)?

Nigel Calder | Professional BoatBuilder Magazine


As you’re wiring or rewiring a boat, you’ll need to include wire diameters in the calculations necessary to determine compliance with ABYC and ISO standards. The AWG conductor-sizing system assigns a diameter to any given gauge size. The ancient Greeks figured out that if you want to calculate a circular area (for example, the cross-sectional area of a conductor), you divide the diameter in half, square it, and multiply by pi (π). This gives us the πr² formula we all learned in school. For some reason lost in the fog of history, the U.S. decided to ditch this formula when calculating conductor cross-sectional areas (CSAs) and instead substituted D² (diameter squared). Typically, conductor diameters are a fraction of an inch, the squaring of which results in numbers that are difficult to work with. For example, the diameter of a 16 AWG conductor is 0.0508. If we square this, we get 0.0025806. In the CM system, we multiply the diameter by 1,000 to create a whole number (50.8) which, when squared, gives us another whole number (2,580), the CMs for this conductor size. If we convert the CM to square inches or square millimeters, we find this methodology always yields a calculated CSA that is higher than the actual CSA. ​

The ISO applies the πr² formula and gives us the actual CSA in mm².