T115   10 0209 

Reference Sheet JB09-39 Circular Mils
OEET115 

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1 mil = 0.001 inch = 1 thousandth of an inch.
"cmil" is the abbreviation for circular mil.
"kcmil" is the abbreviation for one thousand circular mils.

1 circular mil = the area of a circle 1 mil in diameter

The area in circular mils is given by this formula: A = d2, where d is the diameter of the wire in mils.

If you know the area, and want to find the diameter, take the square root of both sides of the A = d2 equation. This gives you  (square root of A) = d, or
d = (square root of A), where A is in circular mils, and d is in mils.

Examples
  • A wire 2 mils in diameter has an area of 22 = 4 cmil.
  • 0000 AWG (usually written as 4/0 AWG) wire has a diameter of 0.46 inches, or 460 mils. Let's calculate the area of the wire's cross section:
    • A = d2
    • A = (460 mil)2  = 211,600 cmil = 211.6 kcmil., which is aproximately 212 kcmil. 
    • Thus, you could refer to the wire as 212 kcmil wire instead of 4/0 AWG wire.
    • However, the National Electrical Code uses AWG numbers for wires sizes up to 4/0 AWG, and kcmils for wires with diameters larger than 4/0 AWG.
  • After 4/0 AWG, the next larger size wire listed in the NEC is 250 kcmil wire. Let's calculate it's diameter:
d = (square root of A)
d = (square root of 250 kcmil)
d = (square root of 250,000 mil)
d = 500 mil = 0.5 inches
0.5 inches is just a little larger than 0.46 inches (which is the diameter of 4/0 AWG wire).
 
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