Conduit fill Question

[Zeno]

I've got 1" EMT attached to the panel with appx 20 or so THHN 12 guage wires coming in. Inspector said that was "too many" and that I have to derate or remove some of the wires from the conduit.

Annex C of 2008 NEC shows I can have up to 26 12 AWG THHN conductors in a 1" EMT....BUT...table 310.15(B)(2)(a) states that I'd have to derate the conductors to 50% (this is assuming I actually have 20 conductors, I didn't count them yet.)

So how do I reconcile these two tables? And does this mean that if I were to have a 3" EMT with 4-6 conductors that I would STILL have to derate them to 80%??? :blink:

Can I show the inspector Annex C and say I'm ok? What if he shows ME table 310.15(B)(2)(a)?

Thanx for your help.

Zeno

 

[Dennis Alwon]

You can fill the conduit with conductors to as many as is allow in Annex C however you would still have to derate. Say you need a 15 amp circuit and you have to derate to 50%. You can run #12 wire at 90C, which is good for 30 amps and derate 50%. This will leave you with a conductor rated 15 amps.

 

[George Stolz]

So how do I reconcile these two tables?

By realizing that one is concerned with you damaging the wires by packing too much physically into a conduit, and the other is concerned with the heat from several conductors damaging each other thermally, after they've been safely installed in the pipe.

And does this mean that if I were to have a 3" EMT with 4-6 conductors that I would STILL have to derate them to 80%??? :blink:

Yes, and if you were to put 10 #12's in a 4" EMT you would also have to derate those to 15A as well.

You could put 443 #12's into a 4" EMT if you wanted to, but they would only be able to carry 10.8A.