Conduit fill and derating questions

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Old 03-22-16, 05:17 PM
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Conduit fill and derating questions

OK I am adding an attic junction box and 1 new circuit which involves messing around with two 1/2" EMT conduits.

I have pulled out the existing #12 solid conductors and plan on using all new stranded conductors which are easier to pull.

Between the two 1/2" EMT conduits I need to feed five (6) circuits - all for standard receptacles and lighting purposes. I will be using tandem breakers and sharing the neutral conductors between circuits on opposite legs.

So circuits 1, 2, 3, 4, 5, 6 that would be six hot conductors. circuits 1, 2 will have a common neutral, 3 and 4 one neutral, and 5 and 6 one neutral. No need for ground conductors for EMT. Altogether I will have six hot and three neutral conductors between two 1/2" EMT conduits going to the same attic junction box. I think the only way I can do this is to put 3 conductors in one (circuits 1 & 2) and 6 conductors in the other (circuits 3, 4, 5, 6) right?

Which lead to the question, is six #12 stranded THHN conductors too much for a 1/2" EMT? The box fill chart says I can put in 9 #12s but that's before derating kicks in. The way I am reading it, if I put four circuits (six conductors) in one 1/2" EMT, and the code says that 4 to 6 current carrying conductors would need to derate by 20%, which means in my case my circuits 3, 4, 5, 6 canNOT be 20A but 15A breakers correct?

IF the above is correct, then would it be better for me to divide up the six circuits EQUALLY between the two 1/2" conduits? In other words, put circuits 1, 2, 3 in one conduit (circuit 1 has it's own neutral, circuits 2,3 MWBC), and circuits 4, 5, 6 in the other conduit (circuit 4 has it's own neutral, circuits 5, 6 MWBC). The total number of conductors between the two conduits is actually more (10 total instead of 9) but in each conduit will be 3 hot and 2 neutral. If I do it this way, do I still need to worry abiut derating? Do the neutral conductors count as "current carry conductors"?
 
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Old 03-22-16, 06:07 PM
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is six #12 stranded THHN conductors too much for a 1/2" EMT?
No. You can put up to 9 #12 in a 1/2" pipe, Derating does not become an issue until you go over 9 wires because you are using THHN wire which is rated at 30 amps. Up to 9 wires you derate 70% which gives you 21 amps. That is still higher then allowed by 240.4(D) so you can still use a 20 amp breaker. If you want you can still pull the circuits as you discribe in different pipes for ease of pulling that is fine too.

Be sure to use two pole breakers for your multi-wire circuits. MWBC circuits need to have the hots on opposite legs.
 
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Old 03-22-16, 06:21 PM
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Conduit fill and conductor derating are two separate subjects.

A half-inch EMT conduit may contain a maximum of nine #12 THHN conductors so that shoots down your idea of having twelve conductors.

Conductors that carry ONLY the unbalanced current of a multi-wire branch circuit are not counted when calculating the number of current-carrying conductors.

Equipment grounding conductors are not counted when calculating the number of current-carrying conductors but ARE counted for conduit fill purposes.

When derating conductors for more than three current-carrying conductors in a single conduit the maximum temperature rating current is used. For type THHN that means derating from the 90[SUP]o[/SUP] C. column. Example: the 90o C. column rates type THHN as being able to carry 30 amperes. Having six #12 type THHN current-carrying conductors would require derating to 80% of maximum or 80% of 30 amperes which would equal 24 amperes. Having nine such conductors would require derating to 70% of maximum or 21 amperes. Since #12 type THHN is specifically limited to 20 amperes that means that you may have up to nine #12 type THHN conductors in the same 1/2 inch EMT conduit and still protect each circuit with a 20 ampere circuit breaker or fuse.

I suggest that you purchase a copy of UGLY'S Electrical Reference as it has all this information.
 
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Old 03-22-16, 06:38 PM
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I will add only a small point to the detailed explanations above: if you split up the runs, the hot and neutral for a given circuit are supposed to be in the same conduit, so you can't put the hot for circuit 1 in conduit 1 and the neutral for circuit 1 in conduit 2.
 
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Old 03-22-16, 06:53 PM
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Got it, thanks Tolyn, Furd and Carbide Tipped for the explanations. Obviously I was trying to read the code, but sometimes reading out of context I came away with a misunderstanding.

You can put up to 9 #12 in a 1/2" pipe
I don't think I will. I never had too much luck with pulling conductors. Last time I tried pulling 4 solid #12 conductors through a 1/2" and I had a hard time even with only two 90 degree bends. Although it was used not new...I needed to add one more conductors and I had to pull out existing three add that new one and pull back all four.

so that shoots down your idea of having twelve conductors.
I don't plan on using 12 conductors. It was either 9 or 10 between the two EMTs.

if you split up the runs, the hot and neutral for a given circuit are supposed to be in the same conduit, so you can't put the hot for circuit 1 in conduit 1 and the neutral for circuit 1 in conduit 2.
Understood. I will either do 3 hots in each conduit, or 2 in one and 4 in the other.
 
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