Circular mils and Table 8

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Bob Anchorite

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On table 8, page 70-625 of the 2002 NEC there is a listing of circular mils. I was taught that to find circular mils one takes the diameter in inches and divide that by .001, and then square the result to get circlular mils.

In table 8 it shows that #12 awg has a diameter in inches of .081

.081/.001 = 81. If you square that then you get 6561 cir. mils. However the number of cir. mils that Table 8 gives for #12 awg is 6530. Now either my calcs are wrong, an humbly maybe I am way off... but shouldn't the codes assigned diameters map out to match their cir. mils???
 
Bob Anchorite said:
On table 8, page 70-625 of the 2002 NEC there is a listing of circular mils. I was taught that to find circular mils one takes the diameter in inches and divide that by .001, and then square the result to get circlular mils.

In table 8 it shows that #12 awg has a diameter in inches of .081

.081/.001 = 81. If you square that then you get 6561 cir. mils. However the number of cir. mils that Table 8 gives for #12 awg is 6530. Now either my calcs are wrong, an humbly maybe I am way off... but shouldn't the codes assigned diameters map out to match their cir. mils???
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Bob,

The #12AWG diameter of 0.081" has been rounded. The nominal diameter is 0.08081"
 
Source of cir mils measurement

Source of cir mils measurement

To Smart

Thank you. Can you tell me what source or book is giving you the non-rounded numbers? When I take a micrometer and measure the diameter of three different brands of #12 solid awg I get anywhere from .07905 to .07850, but nothing at .80.

Bob
 
Bob,

My source is the American Electricians' Handbook, Thirteenth Edition, Division 2: Properties and Splicing of Conductors, Section 89: Solid Bare Copper Conductors, a table furnished to the authors/compilers by Anaconda Wire and Cable Co.

Another table in the book (2-85) which references AWG diameters is a comparison of wire and sheet metal gage diameters lists #12AWG at 0.0810", but has a footnote stating the AWG sizes have been rounded off to about the usual limits of commercial accuracy.
 
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