# Dumb QotD: MCM=kcmil ?



## derekleffew

When discussing wire sizing, "kcmil" means the same thing as "MCM," correct?


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## STEVETERRY

derekleffew said:


> When discussing wire sizing, "kcmil" means the same thing as "MCM," correct?



That is correct.

ST


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## jwl868

A pseudo-metrification, I suspect. MCM – Thousand Circular Mils. “M” for “thousand”. (Ah, the good old days, not “M” for “mega”, and when “MM” was “million”.) C for, well, Circular. And M for mil, or 1/1,000 inch. And it’s essentially an area concept, but with a unit of length. Now that’s an Imperial unit.

kcmil – “k” for “kilo” (1,000), “c” is still circular, and “mil” is still “mil” (1/1,000 inch). Metric prefix attached to an Imperial unit. Elegant.

Joe


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## derekleffew

Thanks, jwl868. I had it until you interjected. 1000 C 1/1000. One-thousand one-thousandths. Wouldn't that be ONE? But 500 MCM is not 500 square inches; I don't think it's one-half square inches either, though that's closer.


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## philhaney

From this web site:

_MCM is thousand circular mils. Circular mils is a measurement of area: 
1 CM = .000001 square inch. 
1 MCM = .001 square inch._

So 500 MCM wire has an area of approx. 0.5 or 1/2 square inch, but according to the chart, it has a nominal diameter of 0.813 inches.

To answer your question, kcmil does equal MCM (one kilo of circular mills does equal 1,000 circular mills).


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## jwl868

derekleffew said:


> Thanks, jwl868. I had it until you interjected. 1000 C 1/1000. One-thousand one-thousandths. Wouldn't that be ONE? But 500 MCM is not 500 square inches; I don't think it's one-half square inches either, though that's closer.



Like I said, it's a peculiarly defined unit: The mil is length, but MCM or kcmil is area. So you can't mutiply the M (1,000) by the 1/1,000 to get 1. Worse, I have two conversions for this, the one in the link that philhaney provided, but another in an engineering handbook. A circular mil is either 0.000001 square inch per the philhaney link, or 7.854x10^-7 (or 0.0000007854) square inch per Perry's Handbook. The latter rounds to the former, though. 

From "engineeringtoolbox" website:
A circular mil is the equivalent area of a circle whose diameter is 0.001 (10-3) inch, or approximately 0.7854 millionths of a square inch. 


Joe


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