Multiple circuits through a single conduit pipe

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Old 04-14-13, 09:22 PM
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Multiple circuits through a single conduit pipe

Hey all,

Dad and I put in a 3/4" EMT conduit from his subpanel to his new compressor, a 240V 15A unit, pulling a pair of 12AWG hots, a 12AWG ground, and a 12AWG neutral (that's currently unused, pulled for future expansion).

I realized after we were done that we want to serve that wall with a 120V 20A circuit and that it would be trivially easy to use the unused neutral and pull another 12AWG for another hot, and to share ground with the existing compressor wiring. I'd then splice to some romex and come out of the metal box, distributing to the other boxes on the wall...

Any issues with this?
 
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Old 04-14-13, 09:27 PM
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I don't see any issue with that since both circuits originate from the same panel. I would identify the wire you intend to use for the 120V circuit at both ends with either a different color wire or tape.

Make the compressor red and black. Make the receptacle blue.
 
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Old 04-14-13, 09:29 PM
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Gotchya. Right now the wires are taped red, black, and green, with the neutral white-unlabelled.
 
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Old 04-14-13, 10:23 PM
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Two issues. One is that when you add that third hot wire you have installed more than three current-carrying conductors in the receway (conduit). The ampacity of the conductors must then be rated at 80%. The #12 AWG copper will need to be protected at 15 amps. If you want to have 20 amp protection you will need to pull #10 AWG.

Right now the wires are taped red, black, and green, with the neutral white-unlabelled.
Wire smaller than #4 cannot be re-designated. You will need two black wires and one red wire, or one black wire and two reds, plus a white and a green.
 
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Old 04-14-13, 10:25 PM
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*nod* good to know. So, the derating of the #12 for the compressor is currently fine, because the compressor is being serviced by a 15A breaker, right?
 
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Old 04-14-13, 10:30 PM
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So, the derating of the #12 for the compressor is currently fine, because the compressor is being serviced by a 15A breaker, right?
Yep. Right as rain and legal as sunshine. The only issue with that circuit is wire color.

Having said that, I may be over-interpreting the code. An ungrounded conductor that is still an ungrounded conductor may be fine with any funny-colored tape on it.
 
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Old 04-14-13, 10:34 PM
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*nod* Last time we get advice at the home improvement store.
 
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Old 04-14-13, 10:42 PM
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Last time we get advice at the home improvement store.
Ya think? .
 
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Old 04-15-13, 10:59 AM
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One is that when you add that third hot wire you have installed more than three current-carrying conductors in the receway (conduit). The ampacity of the conductors must then be rated at 80%. The #12 AWG copper will need to be protected at 15 amps.
Nash, are you sure about that? I thought that since 12ga wire on a 20A breaker is already 'derated' by the NEC... 12ga THHN is rated at 30A (but limited to 20A in residences). So derating 80% still keeps you in the 20A range.

I was under the impression that you could have up to 9 current-carrying 12ga wires on 20A circuits before they needed to be upsized to 10ga (or breaker downsized).

I'm still learning how to read the language in the NEC, so it's quite possible I've misinterpreted...
 
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Old 04-15-13, 12:26 PM
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Z.... I'm pretty sure it is 9 before derating.
 
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Old 04-15-13, 01:56 PM
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I'm pretty sure it is 9 before derating.
I remember it that way too. Maybe it got changed in one of the recent cycles? Anyway, the relevant section now says
Table 310.15(B)(3)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable
It is four or more. But, that said, I may have been incorrect in stating that adding the third hot wire kicked this assembly over the threshold. I hadn't read this section in awhile:
310.15(B)(5) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a).
Something got to nagging at me about the neutrals so I went back and checked. OTOH, I might argue that since the neutral in this application is only being used with one ungrounded conductor it should be counted.

Guess what. This one is up to the AHJ, as always.
 
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Old 04-16-13, 01:46 AM
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The "nine conductor" phrase is not correct. It just happens to work out that way for #12 THHN conductors down-rating from the 90 degree C. column. #12 THHN at 90 degrees C. is rated for 30 amperes, derating to 70% for seven to nine conductors gives you nine current-carrying conductors at 21 amperes. Protect at 20 amperes and you are good.

Not counting the neutral only works for three-wire 240/120 single phase circuits and for single-phase loads on four-wire three-phase circuits. Running both a 240 volt load (the compressor) and a separate 120 volt load ALL conductors are current-carrying and must be considered.
 
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Old 04-16-13, 07:00 PM
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Furd is correct. Derating does not start to matter until you get over 9 wires in a pipe for smaller conductors. Same may be true for larger ones, but I have never needed to calculate it out.
 
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