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

[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.

[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!

[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.

[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).

[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]

Caught me in another edit. The mistake was simpler than that, and wrong table was even called out on the permit.

[HX_Guy]

The designer was looking at the wrong table. 0.65 and 0.85 match up with those temperatures in table 310.20 in the 2008 NEC. Massive fail on that one.

Is it possible that my city is still going off of 2008 NEC?

Nevermind, according to this PDF they are using 2011 NEC. Guess the designer did not know that.

http://www.peoriaaz.gov/uploadedFile..._Standards.pdf

[sensij]

Good eye! Here is a clearer picture...

The designer was looking at the wrong table.

Edit: Table 310.15(B)(2)(b) cited in the permit in 2011 NEC has those corrections with those temperatures. The problem is that isn't the right table to use with THWN-2 wire.

310.15(B)(2)(a) is the table I posted earlier, and the right one to have used.

[HX_Guy]

Good eye! Here is a clearer picture...



What...did he use .65 adjustment for? I guess for the DC wire from the combiner to the inverter, which was supposed to be in conduit on the roof?
That would make sense because according to ASHREA, 3.5" about the roof is 60º C.

Seems the .85 is a typo though? But the city accepted it, so I don't know.

The designer has actually been super cool throughout all of this though and has tried to be as helpful as they can, they will provide new plans if I need them at no charge (which is expected seeing at it seems their calculation were wrong). This is their first time ever dealing with a SolarEdge system though.

EDIT: Hmm, if that's the case though, that the conductor has that big of a derate for temperature, then 6AWG wouldn't be good based on conduit on the roof, so if we resubmit the permit, we also need to specify that the conduit is not run outside on the roof?

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.

[sensij]

It may be academic at this point, but the temperatures used by your designer were listed on the permit. I can't read them because of the quality of the picture, but it is in the lower right of the image attached.

permit.JPG

Also, if you would, can you share the name of who did your permit? They should go on the "stay away" list too.

[sensij]

Ok quick follow up, I looked at this site here: http://www.solarabcs.org/about/publi...map/index.html

For my location, the closest "station" shows...


So it looks like the 46ºC is what should be used to calculate the temperature correction derate? That's 114.8º F, which falls just inside the 82% derate correction.

In this post, showing that 6 AWG THWN-2 is OK, I used the 0.82 correction. That was not a coincidence.

[sensij]

Hmm, so then where did the city come up with the 85% derate? Seems odd, as the cooler the outside temps, the lower the derate right? If the 82% derate is for ambient temps of 114-122F, that seems pretty accurate for the climate here on the worst days of summer, which is what I assume you have to account for?

Attached is the applicable temp correction table from 2011 NEC. Maybe this will help steer the conversation more productively.

Temp correction.JPG

[HX_Guy]

Ok quick follow up, I looked at this site here: http://www.solarabcs.org/about/publi...map/index.html

For my location, the closest "station" shows...



So it looks like the 46ºC is what should be used to calculate the temperature correction derate? That's 114.8º F, which falls just inside the 82% derate correction.

[HX_Guy]

Hmm, so then where did the city come up with the 85% derate? Seems odd, as the cooler the outside temps, the lower the derate right? If the 82% derate is for ambient temps of 114-122F, that seems pretty accurate for the climate here on the worst days of summer, which is what I assume you have to account for?

[inetdog]

Ha that word! The designer mentioned that ASHRAE word when talking to him, he said that's where the temp calculations come from. I'll have to read up on it.

That may be where the temperatures come from, but the wire ampacity calculations come entirely from the NEC.
ASHRAE may also give calculations for the increase in attic temperature over ambient temperature. But that part depends strongly on the configuration of the attic. In particular, putting solar panels over the south part of the attic roof will decrease the attic temperature.