Ambient Temperature Derating Follow-Up

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Old 10-29-04, 01:51 PM
Henry V
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Ambient Temperature Derating Follow-Up

In an earlier post, I raised the issue of how to apply the ambient temperature derating table in the Code for a circuit that runs through an attic. I directed the question to my local inspection office, and the head inspector advised me that the Code provision is advisory on this point and not mandatory. Since this is not an enforcement issue, he did not suggest how to apply the table in the event I want to follow the Code's advice. He did, however, suggest ways to limit the effect of the heat in the event I am significantly concerned, such as increasing the wire size, using a smaller breaker, running wires on top of the insulation, keeping wires apart, monitoring the attic temperature on hot days, etc.

Overall, this did not seem to be a big concern, although I am of course interested in safety above and beyond the code. Therefore, I will follow the recommendations as much as possible and try to limit derating within the house due to bundling.

John and Hex, thanks again for your input. This may be boring stuff for you, but I find it rather interesting.
 
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Old 10-29-04, 02:49 PM
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Originally Posted by Henry V
I directed the question to my local inspection office, and the head inspector advised me that the Code provision is advisory on this point and not mandatory. Since this is not an enforcement issue, he did not suggest how to apply the table in the event I want to follow the Code's advice.


That's an interesting opinion.

As far a I know, the correction factors tables are part of the Allowable Ampacity Tables.

Does your state, city, etc., have its own electrical code?
 
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Old 10-29-04, 02:59 PM
Henry V
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I believe that we adopted the Code verbatim, with no changes. I've searched pretty far and wide on the internet, including various other discussion forums, and nowhere have I found any indication that derating is applied to attic runs. The closest I found was a discussion on an electrical contractor board where the participants seemed to all acknowledge that they did not derate based on attic heat without agreeing as to the justification.
 
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Old 10-29-04, 03:27 PM
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Henry, why is this an issue for you? Is your attic any hotter than anyone else's?
 
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Old 10-29-04, 09:07 PM
Henry V
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Speedy Petey

Speedy, I am just trying to understand the code. If you read my earlier post, you will see the issue I thought I might be facing. That is, if you bundle wires on a circuit for 24 inches or more and derate by .80 as a result, and then run part of the same circuit through an attic, then ambient temperature derating could present some serious issues. The question is what derating factor you apply. If you look to the hottest reasonably expected termperature, then you could easily be derating by another 50% (roughly). Looking to average annual temperatures would not seem appropriate, since averages mean little if you face a lengthy heat wave and your wires overheat and burn down your house. That's an exaggeration, hopefully, but I don't want to take anything for granted.
 
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