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#202022 - 07/10/11 10:36 PM Ampacity adjustment  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Does 310.15(A)(2) allow me to ignore 310.15(B)(2)(c) adjustment of ampacity for heat in rooftop conduits if the run on the roof is less than 10% of the total run?


George Little

2017 / 2014 NEC & Related Books and Study Guides

#202030 - 07/11/11 02:27 PM Re: Ampacity adjustment [Re: George Little]  
HotLine1  Offline


Member
Joined: Apr 2002
Posts: 6,885
Brick, NJ USA
George:

Does this commentary from the 2008 Handbook resolve your inquiry:

"Three 500-kcmil THW conductors in a rigid conduit are run from a motor control center for 12 ft past a heat-treating furnace to a pump motor located 150 ft from the motor control center. Where run in a 78F to 86F ambient temperature, the conductors have an ampacity of 380 amperes, per Table 310.16. The ambient temperature near the furnace, where the conduit is run, is found to be 113F, and the length of this particular part of the run is greater than 10 ft and more than 10 percent of the total length of the run at the 78F to 86F ambient. Determine the ampacity of total run in accordance with 310.15(A)(2).
Solution
In accordance with the correction factors for temperature at the bottom of Table 310.16, the ampacity is 0.82 380 amperes, or 311.6 amperes. This, therefore, is the ampacity of the total run, in accordance with 310.15(A)(2).
Had the run near the furnace at the 113F ambient been 10 ft or less in length, the ampacity of the entire run would have been 380 amperes, in accordance with the exception to 310.15(A)(2). The heat-sinking effect of the run at the lower ambient temperature would have been sufficient to reduce the temperature of the conductor near the furnace."



John

#202031 - 07/11/11 03:59 PM Re: Ampacity adjustment [Re: George Little]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,142
Estero,Fl,usa
To answer the question, I would say no, you need to derate. The roof top is specifically set aside as needing special attention.

I am still not sure I actually understand this "heat sinking" thing anyway. They say I can run 10 feet by a hot furnace without a problem but heat sinking does not happen in Romex running an inch and a half through insulation.


Greg Fretwell

#202032 - 07/11/11 04:37 PM Re: Ampacity adjustment [Re: George Little]  
HotLine1  Offline


Member
Joined: Apr 2002
Posts: 6,885
Brick, NJ USA
Greg:
I agree with you 100% on you have to derate.

I also have a tough time trying to get a handle on heat sinking...but...I'm not the PE.

Remember...this is a George question!


John

#202033 - 07/11/11 05:36 PM Re: Ampacity adjustment [Re: George Little]  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Greg & John, I agree with both of you because that's my point. I am not PE either and each time I bring up this issue, I get kinda a vague response. I think that Greg is eluding to the code in 334.80 about NM cable and I agree with him. John, the handbook has an explanation of how to handle the situation when wiring passes through a high temperature area and I can understand that also. For my example, it's likely that the wiring could be in the high temperature area and terminate in the high temperature area. I think the code panel has some explaining to do. I submitted this same scenario to Charlie Trout's Code Question of the Day and I haven't gotten a response as of yet. And yes, John it's a "George question".


George Little

#202036 - 07/11/11 07:55 PM Re: Ampacity adjustment [Re: George Little]  
HotLine1  Offline


Member
Joined: Apr 2002
Posts: 6,885
Brick, NJ USA
George:

I would like to read Charlie's response. I personally really love your questions!!

I spoke with a PE this afternoon that is re-submitting plans with additional derating re: rooftop raceway/conductors at 28' +/-. No argument, debate, or discussion required.


John


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