How to measure octagonal j-boxes

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  #1  
Old 12-29-07, 04:10 PM
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How to measure octagonal j-boxes

I have an "octagonal" j-box... at least I think it is... it is basically square w/ rounded corners... anyway, assuming this is the definition of octagonal... the NEC lists standard box sizes and mine here is 3.5" across side to side x 1.5" deep. But it is not stamped w/ volume and it is UL listed, so I am wondering if by 4 x 1.5" in NEC do they mean measuring from octagonal corner to octagonal corner? That is 4" on my box.

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Old 12-29-07, 10:46 PM
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from table 314.16(A) - for 4" x 1.5" octagonal box is 15.5 cu. inches
 
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Old 12-30-07, 11:14 AM
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Originally Posted by bob100 View Post
from table 314.16(A) - for 4" x 1.5" octagonal box is 15.5 cu. inches
Granted, but is the box I am looking at a 4" x 1.5" box? That's what I don't know! If I were stating the size of the octagonal box I'd say it was 3.5 x 3.5 x 1.5--but that would make it a nonstandard size box and that would imply that it should have the volume stamped on it; which it does not. So... are the box sizes stated in the NEC and given as (for example) 4 x 1.5 meaning 4" diagonal (for an octagonal)?
 
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Old 12-30-07, 11:21 AM
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Box Size

The "diagonal" measurement of 4 in. is correct.
 
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Old 12-30-07, 11:38 AM
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It sounds like you have a 3 1/2" round box to me. octagon boxes are measured and calculated like round boxes are.

Check 3 1/2 x 1 1/2 octagon box in this T&B steel city cat for volumes

http://www.tnb.com/contractor/docs/sc_indoorboxes.pdf
 
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