Conduit fill and derating ampacity?

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  #1  
Old 11-28-04, 04:51 PM
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Location: Tujunga, CA, USA
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Conduit fill and derating ampacity?

Background:
My service panel is surface mounted on the exterior of the house. My entire house is currently served by only 2 circuits (100amp panel w/10 spaces for breakers - 20 circuits if filled with twins). I am considering breaking the circuits up but I need to get additional wires into the attic. The panel sits on stucco and on the inside of the house is tub enclosure. So, I thought that the easiest way to get more wires into the attic would be to run conduit about 5ft up the side of the house and go directly into the attic. The house wiring is a combination of flex and old 2 wire romex (which should be replaced).

Question:
I'd like to know how many current carrying conductors can be put in the conduit without losing too much ampacity. All of the circuits will be 15 or 20 amp and I'd like to use 14 & 12 awg. Besides conduit fill, I probably need to derate for ambient temp (So. CA - outside might peak a little above 105F) Because of aesthetics, I'd like to keep the number of pipes running up the side of the house to a minimum. So I am looking for advice on derating and any other recommendations??

I still might consider breaking some stucco to fish cables in the wall up to the attic. It is a hip roof, so there isn't room to drill the top plate from inside the attic. The access in the attic right above the panel is real tight.

Thanks
Phil
 
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  #2  
Old 11-28-04, 05:19 PM
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Location: Oregon
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Table 310.15(B)(2)(a)
1-3 conductors: 100%
4-6 conductors: 80%
7-9 conductors:70%
10-20 conductors: 50%
21-30 conductors: 45%
31-40 conductors: 40%

Table 310.16
running at 104F: 91%
Ampacity 14ga THHN: 25
12ga THHN: 30
10ga THHN: 40

So with 14ga wire, you are still good with a 15A breaker with up to 9 current carrying conductors in a raceway.

With 12ga wire, you are still good with a 20A breaker with up to 6 conductors. Check with your AHJ about the thermal derating that they suggest; most will say that for pipes on an outside wall you don't have to derate for temperature, in which case 12ga would be good on a 20A breaker with up to 9 current carrying conductors in the raceway.

For normal 15 and 20A circuits, you don't have much problem with derating because other code rules require that the conductors be oversized. 12ga THHN is rated for 30A, but you are only allowed to protect it with a 20A breaker. If because of derating a particular piece of 12ga is only good for 21A, then you still get to protect it with a 20A breaker.

Derating becomes a serious problem when you have larger circuits (say a 40A range circuit) in the same pipe.

-Jon
 
  #3  
Old 11-28-04, 10:50 PM
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Thanks Jon - That is exactly the information I was looking for.
Phil
 
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