When to use 75º C wire rating vs 90º C?

[HX_Guy]

So would you be in agreement that it would probably be best to not mess with the approved permit at this point and just do what's needed, and safe, to meet what the permit asks for? (would mean changing out the terminal blocks in the disconnect to accept 2AWG).

[sensij]

If my calculation are correct, then 35.4ADC x 1.25 x 1.25 = 55.3A.
6AWG conductor x .65 derate = 48.75A which is below the above number. Seems we would need 4AWG in that case, if the conduit were on the roof.

This portion of the calculation may be where the city is misunderstanding. 2014 NEC made clearer that the optimizers are DC utilization equipment, and therefore only the connection from the module to the optimizer is the PV source circuit. There is no PV output circuit, because modules are not connected to each other. The extra 1.25 correction only applies to PV source and output circuits, and should not apply to the inverter input circuit in this case. However, this point was not clear in NEC 2011, and may be contributing to the requirement for 2 AWG (along with the mixed up temp corrections, and miscalculated amperage).

[Amy@altE]

Wow. Guys, I am learning so much from this project. HX_Guy, I am sorry you're having so much trouble, but man oh man has this been a great learning experience. I'm so glad I joined this forum last year.

[russ]

Wow. Guys, I am learning so much from this project. HX_Guy, I am sorry you're having so much trouble, but man oh man has this been a great learning experience. I'm so glad I joined this forum last year.

Amy - We are glad you are here as well - your contribution has been great!

[sensij]

Wow. Guys, I am learning so much from this project. HX_Guy, I am sorry you're having so much trouble, but man oh man has this been a great learning experience. I'm so glad I joined this forum last year.

Yeah, no kidding. With regard to temperature correction, the designer of this system also referred to Table 310.15(B)(3)(c). That is created below, and explains how they might have tried to calculate the temp for the rooftop portion of the circuit relative to ambient:

Adders.JPG

It still doesn't quite add up... if 46 deg C was used as ambient, and the conduit in direct sunlight is 3.5" off the roof, the temp for the conduit should be 46 + 17 = 63. I'm not sure how to got two temps to land in the 46-50 range and 56-60 range using the code based corrections.