ohm circular mil per foot
Electrical resistivityexact by definition
The ohm circular mil per foot is the North American wire table's resistivity unit, about 1.6624e-9 ohm metres. It is built so that resistance comes out directly from a wire's dimensions in trade units: resistance equals this resistivity times length in feet divided by area in circular mils. Annealed copper is about 10.4 Ω·cmil/ft at 20 degrees Celsius and 12.9 at the 75 degree conductor temperature the NEC uses. The value is exact by definition, since the inch and the foot are exact and the circular mil is exactly π/4 square mils, though the π makes the decimal non-terminating.
Watch out: A circular mil is not a square mil. One circular mil is π/4, about 0.7854, square mils, so a conductor's area in circular mils is always the larger number. Mixing the two undersizes a conductor by 27 percent.
| 1 Ω·cmil/ft | 1.6624261e-09 Ω·m |
The circular mil exists to delete a factor of π from wire arithmetic. It is defined as the area of a circle one mil in diameter, so the area of any round wire in circular mils is simply its diameter in mils squared. A 0.1 inch wire is 100 mils across and 10000 circular mils in area, with no multiplication by π anywhere, which is why AWG tables come out in whole numbers a field electrician can use.