Box Fill - NEC Cubic Inches vs Real World Inches

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Conduit Phil

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Why does a 1-1/4" 4" square box have a cubic inch capacity of 18? 1.25*4*4=20. Where did the extra 2 cubic inches go? A 2-1/8" box is 30.3 vs 34, etc...

Also, why doesn't a raised ground screw bump take up any room in a box fill calculation?
 
My guess is that the numbers listed in Article 314 have more to do with the averages of the same style boxes made by various manufacturers. Some boxes have rounded corners while other have completely square corners. The rounded ones should have a slightly smaller volume. Unless the manufacturer stamps the cu in amount in the box you get to use the number listed in table 314.16(A).
 
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