conduit and fill calcs

Old 04-08-03, 08:48 PM
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conduit and fill calcs

Same barn project, different questions. I have also been reading a lot of old posts today about running wire in conduit and need to make sure I understand what I read.

Are the ampacity de-rating calculations completely separate from the fill calculations? I understand from previous posts that when you add the 10th #12 current carrying conductor in a conduit you have to reduce from 20 to 15 amp breakers, but does the size of the conduit play any factor in this? I ask this because it seems to me those same 10 conductors would be much more capable of dissipating heat in a 6" conduit than in a 1" conduit.

Another question on wire . . . Is the cross-sectional area for #12 THW, #12 THWN, and #12 THHN the same? Somebody have these numbers handy they could post for me?

Thanks for all the help,
Old 04-08-03, 09:02 PM
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Yes, ampacity derating considerations are a completely different topic from conduit fill considerations. Let's not discuss them at the same time or we'll get hopelessly confused as to what we are talking about.

The size of the conduit plays no role in ampacity derating. Wires tend to all stack in the bottom of the conduit anyway. The only way to avoid ampacity derating is to have some way to enforce separation of the wire, and conduit has no such properties.

Most people don't bother computing fill from cross-sectional area, since the NEC has tables that makes looking thing up easier. But since you asked: #12 THWN and THHN are both 0.0133 square inches, and #12 THW is 0.0181 square inches.
Old 04-09-03, 11:18 AM
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I have felt overstuffed 1/2 and 3/4 inch EMT conduits that were very hot to the touch. Derating the ciruits probably wouldn't have "cured" the heat problem, but not exceeding allowable number of conductors in the conduits probably would have.

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