Multiple Circuits in Conduit

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  #1  
Old 06-15-07, 01:01 PM
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Question Multiple Circuits in Conduit

I'm planning on running a number of circuits through a conduit... I've been reading this forum and the 2005 NEC.

This is my understanding, corrections very welcome:

Conduit is 1" EMT, about 15-20' long, and wires are 12awg THHN.

#1) According to Table C.1, you can run 26 12awg THHN conductors through the conduit.

#2) Table 310.16 says the allowable ampacity of 12awg THHN in a raceway is 30.

#3) Table 310.15(B)(2)(a) says derate current-carrying (hot) conductors (if more than 3 in a raceway): 4-6: 80%, 7-9: 70%, 10-20: 50%.

#4) 250.122(C) single ground can serve multiple circuits.

Scenario 1:
9 circuits
=> 9 hots, 9 neutrals(*), 1 ground = 19 conductors. (less than 26, OK)
=> 9 hots, so derate: 30 * 0.7 = 21amps
=> So we can run 9 circuits protected by 20amp breakers.

Scenario 2:
10 circuits:
=> 10 hots, 10 neutrals(*), 1 ground = 21 conductors. (less than 26, OK)
=> 10 hots, do derate: 30 * 0.5 = 15amps
=> So, we can run 10 circuits protected by 15 amp breakers.

(*) Some neutrals could potentially be shared, if done right; but if there's a mistake "everything will work great, until the fire..." So there's no reason to go there.

Correct / Close / Dead Wrong ?

Thanks.
 
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  #2  
Old 06-15-07, 01:42 PM
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Wrong. Neutrals are current-carrying conductors too; count them in the de-rating percentage. If you use shared neutrals, the hot-neutral-hot shared group counts as two current-carrying conductors.
 
  #3  
Old 06-15-07, 01:47 PM
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Run three 1/2" EMTs each with three hots, three neutrals and one ground. Each conduit would have room for a fourth 20A circuit as well. This requires no neutral sharing and satisfies derating and conduit fill criteria.
 
  #4  
Old 06-15-07, 01:56 PM
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On the derating, 110-14(c) from 1999 code requires that you use the 310-16 column based on the "weakest link" which may be switches or receptacles or whatever that are rated at 60c. Sorry, I can't cite the later codes. I am sure one of the pros around here will have a better answer for you.

Neutral sharing (multiwire) is not inherently hazardous. In fact when you run into a j-box with many white wires you have to be more careful when splicing so you keep circuits separated properly.

I would recommend using both white and grey for neutrals if you will use anywhere near 1000' of neutral in the whole project, likewise multiple hot colors help a lot, along with those wrap-around wire marker books.

With AFCI and GFCI circuits you cannot share, generally.
 
  #5  
Old 06-15-07, 02:41 PM
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ipbooks, if I'm understanding you:

#5) 310.15(B)(5) A Grounding or bonding conductor shall not be counted when applying provisions of 310.15(B)(2)(a)

Scenario 1 - Corrected:
9 circuits
=> 9 hots, 9 neutrals, 1 ground = 19 conductors. (less than 26, OK)
=> 18 current carrying conductors, so derate: 30 * 0.5 = 15amps
=> So we can run 9 circuits protected by 15amp breakers.

Scenario 2 - Corrected:
10 circuits:
=> 10 hots, 10 neutrals, 1 ground = 21 conductors. (less than 26, OK)
=> 20 current carrying conductors, so derate: 30 * 0.5 = 15amps
=> So, we can run 10 circuits protected by 15 amp breakers.

Scenario 3:
8 circuits
=> 8 hots, 4 neutrals (shared correctly), 1 ground = 13 conductors. (less than 26, OK)
=> 8 current carrying conductors (after "2+1=2" discount), so derate: 30 * 0.5 = 21amps
=> So we can run 8 circuits protected by 20amp breakers.

Do you know where "If you use shared neutrals, the hot-neutral-hot shared group counts as two current-carrying conductors" is in the NEC? I have not memorized the whole thing yet. :-}

The 1" EMT conduit is already installed, so I'm "somewhat" attached to running my circuits through it...

Thanks
 
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Old 06-15-07, 03:31 PM
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"Do you know where "If you use shared neutrals, the hot-neutral-hot shared group counts as two current-carrying conductors" is in the NEC?"

310.15(B)(4) 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)(2)(a).




"I have not memorized the whole thing yet. :-}"

Good luck with that!
 
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Old 06-15-07, 03:34 PM
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Arg, for the purposes of derating we use the actual ampacity of the conductor. For #12 THHN this is 30 amps.

For figuring CIRCUIT ampacity we use the "weakest link" theory.
 
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Old 06-15-07, 03:36 PM
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=> 8 current carrying conductors (after "2+1=2" discount), so derate: 30 * 0.7 = 21amps

I think that's what you meant. Correct.

Yes, your scenarios are correct now; but do you need 15A circuits or 20A circuits? Also, any circuits to bedrooms cannot utilize shared neutrals as AFCI breakers are incompatible with a shared neutral. If any of these circuits are feeding a kitchen, bathroom or laundry the circuit size must be 20A. Why so many circuits in this conduit? Perhaps a subpanel at the terminal end may be more appropriate?

> Do you know where...is in the NEC?

Sorry I don't have an article, but the concept is that the neutral carries the difference in current between the hots. If you work the math, it's only possible for there to be 2 wires' worth of current between the three wires. Both hots at 20A, 20-20= 0A in neutral. One hot at 20A, one at 0A: 20-0 = 20A in neutral. One hot at 15A, one at 3A: 15 - 3 = 12A in neutral. No combination of the hots and neutral current exceeds 40A.

> The 1" EMT conduit is already installed, so I'm "somewhat" attached to
> running my circuits through it...

Another option is to upsize all wiring to #10. You can pull 15 conductors for a total of 7 non-shared neutral 20A circuits. What size j-box does this conduit terminate at? The box must be quite large to accommodate all of the proposed circuits.
 
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Old 06-15-07, 03:38 PM
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Oh, and Short. IMO scenario 3 is your best bet.
 
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Old 06-15-07, 03:41 PM
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ib, Cutler Hammer BR series and others ARE available in AFCI two pole breakers for use with multi-wire circuits.
 
  #11  
Old 06-15-07, 04:21 PM
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Thanks for the education! This forum is great!

*) Do I need 15A or 20A circuits? -- 20A since some will go to kitchen/bathroom.

*) Why so many circuits in this conduit? -- It's an older house in need of upgrades; I had an electrician upgrade the service and panel, and he recommended installing a HUGE j-box under the house (crawlspace) and running the conduit to it to facilitate "easy" rewiring with that j-box as a "hub" of sorts. I went with that recommendation.

I'll have to run some more conduit in order to completely rewire the house, but this setup (I'll either use scenario 3, or ibpooks' upsized 7-circuit solution) will cover a big chunk of the job.

I have some more thinking and planning to do before I actually start pulling the wires...

Again, thank you! (I'll be back with more questions, I'm sure.)
 
  #12  
Old 06-15-07, 04:34 PM
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just remember that you need to use 12 wire for these 15 amp circuits.
 
  #13  
Old 06-15-07, 09:49 PM
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> 2-pole AFCI

I didn't realize there were any actually available. I'll have to keep that in mind. Thanks.
 
  #14  
Old 06-16-07, 06:05 AM
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Originally Posted by Speedy Petey View Post
Arg, for the purposes of derating we use the actual ampacity of the conductor. For #12 THHN this is 30 amps.

For figuring CIRCUIT ampacity we use the "weakest link" theory.
Thanks for the correction. I spent hours trying to figure this out years ago and then realized that my state code waived the derating requirement, so I never did come to terms with the NEC on the issue.
 
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