Box/conduit fill Cubic inches and Square inches

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brother

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I'm a liittle rusty on some of my math. I know the NEC already gives cubic inches for most wire TABLE 314.16(B) ie.. #12 2.25 cube inches.
#12 THHN 0.0133 square inches TABLE 5 Chapter 9. I'm just curious if you had to convert to cubic inches from square inches, how would you do that??

I know measures volume height, length, width(cubic inches), and the other measures area length and width (square inches).

I was looking at that way I found the 'cable' fill in this post.
http://forums.mikeholt.com/showthread.php?t=126646

brother said:
I want to make sure Im doing this right in my calculation, there are 2 cables , each cable has 19 pairs. The o.d. on one cable is .603 inches. this is NOT square inches so I had to convert. I came up with using 1 1/2" emt conduit NEC 2008 TABLE 4 chapter nine.

Area= 3.14 * R(squared)
R= D/2

D= .603

R= .603/2=0.3015

Area= 3.14 X .3015 X.3015=.285

2X .285=.570 1 1/2 EMT conduit size needed. (0.631 sqr inches for emt)

since there are 2 cables and that means I have 2 conductors and have to use the 2 wires 31% fill.

Did I do this right?? I dont deal with multi cables much, just regular wire!
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So if I had to size a box for this cable, how would one do it since boxes are in cubic inches. 2008 NEC 314.28 deals with #4 and larger for the 'pulls' that lets you know what size box to have by going off the largest size conduit and multiplying by 6 or 8. So what about cable that is not that big. ;)
 
What kind of box? If it's one of the types listed in table 314.16(A) you would use the box volume in that table. If the box has the volume marked in it, then you can used the marked volume.
 
brother said:
Ok, so how do they come up with 2.25 cubic inches for #12. ?? Just curious as to where they get this info, since i could use it for multiconductor cable when all I have is area(square inches)
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I think they pulled it out of the air.

2.25 ci is not the actual volume of the conductor (unless it is long) it is just the amount of space someone decided is needed in a box for that AWG.
 
I don't think they necessarily pulled the number out of the air. They determined an average wire length, then measured the volume of that wire.

The square inch measurement you find in the code book is for determining conduit fill because the volume is not definite. That's because the code doesn't know how long your conduit run is going to be. So you measure conduit fill by taking a cross section of the conduit which gives you two dimensions.
 
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