iwire said:
Where in the NEC tells us we can use the average temperature? :?:
It doesn't. Since you brought it up, I looked closer:
310.10, FPN No.1, (1) Ambient temperature ? ambient temperature may vary along the conductor length as well as from time to time.
Unfortunately, it doesn't provide much assistance in decision making. :?
IMO we have to use the maximum expected temperature.
If so, we're still okay. A 14-2 NM cable in a 140?F attic has an ampacity of 17.25 amps. The same cable in a 158? attic has an ampacity of 14.5 amps. Provided the load does not exceed 14.5 amps for less than three hours, or 11.6 amps for over three hours, then we're still okay.
When derating conductors for fill do you use the highest ampacity a conductor may carry or do you use the average ampacity the conductor may carry?
Neither. The expected load does not factor into derating, until you have finished the derating process. Once we're done derating, then we can turn our attention back to the load, and overcurrent protection.
To me it is the same issue we are dealing with heat damaging the insulation.
But the damage isn't a foregone conclusion. I think what's messing with us is that we're trying to determine mathematically
if a conductor is going to be damaged under certain conditions. If we assume that the damage is guaranteed, then we would be inclined to follow the "max temperature" approach. If we're not sure that it's guaranteed, then we turn
our attention to what "the conductor can withstand over a prolonged time period without serious degradation." If we're looking to the big picture, we'd need to look at the environment over a prolonged period to get an accurate assessment, I think.
(But rest assured, I'm sitting on the fence on this one!
)