228 Electrical Power Systems Technology
Area (in square mils)
Area (in emils) = ——————————
0.7854
This formula allows us to convert the dimensions of a conductor to square
mils, and then to an equivalent value in cmils. For example, if a conductor
is 1/2 inch × 3/4% inch (12.7 mm × 19.05 mm), the cmil area may be found
using the following method. Again, convert the fractional inches into deci-
mal values, and then into equivalent mil values. Thus, 1/2 inch equals 0.5
inch, which equals 500 mils, and 3/4 inch is 0.75 inch, which equals 750
mils. Using the formula for square mils and substituting, you have:
Area (in square mils)
Area = —————————
0.7854
500 mils × 750 mils
= —————————
0.7854
375,000 mils^2
= ——————
0.7854
= 477,463.7 cmils
We can also use the cmil area of a conductor, which may be found in
any standard conductor table, to find the diameter of a conductor. If a ta-
ble shows that the cmil area of a conductor is equal to 16,510 cmils (a No.
8 AWG conductor), its diameter is found by the following method. Since
D^2 = cmil
then:
D = cmil
= 16,510 cmils
= 128.5 mils
= 0.1285 inch