What are Circular Mils?
There are many different wire gauge systems used to specify electrical wire size. In North America the two most common systems are the SAE (Society of Automotive Engineers) and AWG (American Wire Gauge). A 14 SAE wire is smaller than a 14 AWG wire and therefore, for a given insulation temperature rating, capable of carrying less electrical current.
In many ABYC tables and wire suppliers catalogs the cryptic term "Circular Mils" will be shown as part of the wire specification. Circular Mils is the term created for the measurement system devised to measure wire conductor (without the insulation) sizes in absolute terms. By specifying how many Circular Mils a 14 SAE wire has compared to the Circular Mils in a 14 AWG wire it is possible to determine the relative amounts of copper in each wire.
Unfortunately the term Circular Mils does little to clarify the situation for the average person because it is a unit of measure not used commonly and it doesn't necessarily have anything to do with circles!
One Circular Mil is a unit of area equal to that of a circle .001" in diameter (.0005" radius). The actual area of a Circular Mil is:
A =Pi x r²
A = 3.14159 x (.0005)² inches
A = .0000007857 square inches
When wires get very large their sizes are sometimes shown as "MCM", such as 250MCM. MCM means Thousand (M) Circular (C) Mils (M). A 250MCM wire is wire with an area equal to 250 x 1000 Circular Mils
The illustrations show that a circle can be 1 Circular Mil, but a square can be one too!
A = .0000007857 square inches / A = .0000007857 square inches
These are both 1 Circular Mil!
