90c, THWN-2. Alsoi 2017 NEC.
Ah, 2017 NEC.
But 310.15(B)(3)(c) has the temperature adder for less than 7/8 off the roof, so that is a difference. Is anything accomplished by having essentially the same table repeated in 690 without the 7/8 roof adder?
Correct, and I don't take the instruction to do temperature correction "in accordance with Table 690.31(A)" is an amendment to 310.15(B)(3)(c)'s discussion of "temperature adder", so that still applies.
No, which is why the 2020 NEC amended 690.31(A) to remove the overlap.
Ok glad they got rid of the temp correction in 690. Just wanted to make sure i wasnt misunderstanding something.
Don't know the actual temp but I know a roofer that got a second degree burn on a hot roof in summer. It was a metal roof, sunny side, he slipped and fell on it and in the time it took to recover his footing his leg was burned, (he had shorts on, bare legs). After that they decided it was just to hot to continue that day.
It really depends on the color and material of the roof. I use 10AWG for all PV strings, and the wiring generally never is within 3/4inch or less of the roof, so I never worry about any kind of derating.
So is it pretty standard in the industry to just use the 86 degrees as the temperature and just add in the 60 degrees if there is less than 7/8 between the roof and the conduit? Or do you pick something higher, say 130 degrees, and add the 60 to that if you have less than the 7/8 in?
The latter, where "something higher" is some sufficiently unlikely peak outdoor temperature for the location. For example, ASHRAE says the 20 year maximum extreme high temperature in Seattle is 35C, so that would be a pretty safe choice for outdoors in Seattle.
With a 35C outdoor temperature, a 33C roof adder, and 6 CCCs in the conduit, #10 Cu with 90C insulation has an ampacity of 40 * 0.8 * sqrt((90-68)/60) = 19A. Edit: even at a 46C outdoor temperature with a 33C roof adder, the ampacity would be 40 * 0.8 * sqrt((90-79)/60) = 13.7A.
In real life, 99.8% of the time for non PV electrical work, electricians dont adjust for temperature, they just use that standard ampacity table, which assumes 86F. I actually cant remember a single case of making a temp ampacity adjustment or someone making one. This includes roofs, but typically they are white TPO or PVC. PV is kinda its own thing, and often have conductors running on dark colored pitched roofs which likely is the most extreme case. just wondering if PV installers typically make a temp adjustment
The answer I got was "what's a temp adjustment, and how do you do that?"
I would say it is highly unusual for me (and I would think most other installers) to ever install wiring on a roof that is within 3/4 inch of the roof surface and exposed to sunlight. If it was unavoidable, I would add 33C to the 0.4% max local temperature, which in the Cleveland area is 34C, so would use 33 + 34 = 67C in the Temp correction table based on 30C, giving a derating factor of 0.58 for THWN-2.
When I was new to the industry I summered as an installer on residential PV systems, and once we were installing a system on a corrugated metal roof where the metal had a mirror finish. I was wearing shorts, and at the end of the day I had a weird sunburn from the knees down.
Ok so we can ignore that the temperature will be higher than ambient if the raceway has 7/8 of clearance off the roof?
Yes.
