Ambient Temperature

[George Stolz]

Would ambient temperature on an outdoor installation be best found by taking into account the hottest 3 months of the year's high temperatures, since derating would need to take place for 86 degrees F and above?

Just curious.

 

[jtester]

Re: Ambient Temperature

In New Mexico, the State has directed us to derate wires inside a conduit on a roof to the hottest temperature on the hottest day, about 111 degrees. I personally don't think that is enough, but that's the State's intepretation of ambient.
Jim T

 

[tom baker]

Re: Ambient Temperature

There is a new FPN in the 2005 NEC about derating for conduits installed above a roof, and it requires a 30 Deg F derating. I don't have the electronic version of the 05 working yet, so I can't get you the section.

 

[bob]

Re: Ambient Temperature

requires a 30 Deg F derating

Tom
Did you mean a 130 degree F derating?

 

[charlie]

Re: Ambient Temperature

From the 2005 NEC.

310.10 Temperature Limitation of Conductors.

FPN No. 2: Conductors installed in conduit exposed to direct sunlight in close proximity to rooftops have been shown, under certain conditions, to experience a temperature rise of 17?C (30?F) above ambient temperature on which the ampacity is based.

 

[charlie b]

Re: Ambient Temperature

This is one of my favorite topics of discussion, mostly because I don?t know the answer , and because the NEC itself does not give the answer.

For my part, I am a fan of the ?year-round average temperature? basis for derating. I have said so on this Forum before, so I?ll not give my reasons again. It should be easy enough to find. But I have not yet had the chance to present my argument to an Inspector who has a different point of view.