Solar install turning out really bad - looking for some help/advice please.

[HX_Guy]

Actually #6 should fly at least between the combiner and the inverter, so really the only change on the permit there should be changing the #2 to #6.
The #6 on the AC side between the inverter and the service panel though does need to be #4, but that's easy to fix and I believe I can still run 3/4" conduit. I got a calculation of 33% conduit fill using 2x4AWG and 1x10AWG ground (NEC says up to 40% is ok).

As for the temperature adjustment calculation, all the conduit is in the attic, there is literally none on the roof...unless you count the 4" or so coming out of the roof and into the combiner box. I was told by the designer that anything under 24" does not need to be calculated for.

[inetdog]

Actually #6 should fly at least between the combiner and the inverter, so really the only change on the permit there should be changing the #2 to #6.
The #6 on the AC side between the inverter and the service panel though does need to be #4, but that's easy to fix and I believe I can still run 3/4" conduit. I got a calculation of 33% conduit fill using 2x4AWG and 1x10AWG ground (NEC says up to 40% is ok).

As for the temperature adjustment calculation, all the conduit is in the attic, there is literally none on the roof...unless you count the 4" or so coming out of the roof and into the combiner box. I was told by the designer that anything under 24" does not need to be calculated for.

It is even better than that: In addition to the 24" exception, you can ignore up to 10' or 10% of the length of the adjacent section (whichever is smaller) if the wire size is correct for the adjacent section.

[HX_Guy]

Man I keep flip flopping on this. I wonder if I'm not just opening a whole other can of worms by resubmitting the plans for yet a whole new review.

Maybe I should just have the wire changed to match the permit and swap out the terminals inside the combiner box with ones that will accept 2AWG conductors. That would probably be the quickest way to get this done and over with.

Changing the permit will also require still changing out some wires, because we do in fact need #4 wire where it says we do and he installed #6, so really the only thing being saved is not having to redo the wires in the conduit through the attic...but again resubmitting the permit who knows what that could open up.

[sensij]

Even your designer's first attempt was wrong, and the city was right to make him another submission.

Based on the SolarEdge code compliance document I attached in a different post, the following is how you would calculate the inverter input current:

Maximum Inverter Input Circuit Current
The maximum inverter input circuit current is limited by the lower of two factors:

1) The output current of the power optimizers is internally limited to 15 Amps continuous. In systems with multiple parallel strings the maximum continuous inverter input current will be limited to the lesser of:
a) 15A*n where n = the number of strings in parallel
b) The total array power divided by the fixed inverter input voltage of 350 Vdc (240 Vac) or 305 Vdc (208 Vac).

2) The maximum recommended inverter input current is proportional to the inverter power rating divided by the fixed input voltage. Recommended input limits for each inverter can be found in the inverter datasheet.

Calculate it out:
1a) 15 A * 3 strings = 45 A
1b) 12400 W / 350 V = 35.4 A
2) DC power rating for the 11400A = 15350 W / 350 = 43.9 A

Based on this, 35.4 A would be justifiable. Like everything else though, the AHJ is always right.

[HX_Guy]

Even your designer's first attempt was wrong, and the city was right to make him another submission.

Based on the SolarEdge code compliance document I attached in a different post, the following is how you would calculate the inverter input current:



Calculate it out:
1a) 15 A * 3 strings = 45 A
1b) 12400 W / 350 V = 35.4 A
2) DC power rating for the 11400A = 15350 W / 350 = 43.9 A

Based on this, 35.4 A would be justifiable. Like everything else though, the AHJ is always right.

This ****'s confusing.

So if my system was say 14,000W DC, you're saying the rating would be 40A? Because according to the inverter spec sheet, the maximum DC input is 34.5A so I don't see how it could ever be higher.

[sensij]

Changing the permit will also require still changing out some wires, because we do in fact need #4 wire where it says we do and he installed #6, so really the only thing being saved is not having to redo the wires in the conduit through the attic...but again resubmitting the permit who knows what that could open up.

You do not need #4 wire when you do the calculation correctly. Temperature corrections do not apply to the load, they apply to the base ampacity of the conductor. Every time you do something like load * 1.25 / (temp correction) you will get the wrong answer. Load gets multiplied by 1.25. Temp correction gets applied to the base ampacity of the conductor. Compare the two.

The termination ampacity must be greater than (1.25 * the load), no temp corrections are required.

[sensij]

This ****'s confusing.

So if my system was say 14,000W DC, you're saying the rating would be 40A? Because according to the inverter spec sheet, the maximum DC input is 34.5A so I don't see how it could ever be higher.

Yes, correct.

It could be higher if the inverter fails to limit the current properly. That is why code looks at the current that the source is rated to produce.

[HX_Guy]

You do not need #4 wire when you do the calculation correctly. Temperature corrections do not apply to the load, they apply to the base ampacity of the conductor. Every time you do something like load * 1.25 / (temp correction) you will get the wrong answer. Load gets multiplied by 1.25. Temp correction gets applied to the base ampacity of the conductor. Compare the two.

The termination ampacity must be greater than (1.25 * the load), no temp corrections are required.

Well, that may be the case but I have a feeling my city doesn't know or won't accept that. Apparently my designer did have to account for temp corrections and as I posted in the other thread, I have a copy of a permit a friend of mine submitted for his solar and it was also accounted for on there too.

[HX_Guy]

Yes, correct.

It could be higher if the inverter fails to limit the current properly. That is why code looks at the current that the source is rated to produce.

I guess I can see how the city could be struggling to accept this then. I mean the current that the source is rated to produce is what the inverter tell is to. If the inverter isn't working properly, then the optimizers will put out 1V. They won't work unless the inverter is working properly. And if they speculate a what if scenario where they think the optimizers will put out power without the inverter..then the calculation would be insane (they will put out up to 85V! and 15A). And if they further think what if the inverter and the optimizers fail, and somehow power direct from the panels makes its way through the conductors to the inverter, then I guess you would use the panels specs for all the number...but that is impossible to ever happen.

[inetdog]

I guess I can see how the city could be struggling to accept this then. I mean the current that the source is rated to produce is what the inverter tell is to. If the inverter isn't working properly, then the optimizers will put out 1V. They won't work unless the inverter is working properly.

Yes and no....

They will not work unless the inverter keeps giving them command updates. But it does not have to be working properly.
Also, if there is a medium resistance short circuit along the wire, the inverter will command the optimizers based on the current it is receiving, not the optimizer output. Whether that scenario is actually possible depends on the details of the interaction between inverter and optimizers.

[sensij]

Well, that may be the case but I have a feeling my city doesn't know or won't accept that. Apparently my designer did have to account for temp corrections and as I posted in the other thread, I have a copy of a permit a friend of mine submitted for his solar and it was also accounted for on there too.

Seriously? The example is exactly the calculation I just told you to do.

the base ampacity of 8 AWG THWN-2 is 55 A. The temp correction they used is 82%, which was looked up from the table cited. 55 * 0.82 = 45.1 A

The load *1.25 must be less than that 45.1 A.

With respect to the termination, 8 AWG @ 75 deg is 50 A. Because the termination ampacity is higher than the corrected conductor ampacity, it is OK.

[sensij]

I guess I can see how the city could be struggling to accept this then. I mean the current that the source is rated to produce is what the inverter tell is to. If the inverter isn't working properly, then the optimizers will put out 1V. They won't work unless the inverter is working properly. And if they speculate a what if scenario where they think the optimizers will put out power without the inverter..then the calculation would be insane (they will put out up to 85V! and 15A). And if they further think what if the inverter and the optimizers fail, and somehow power direct from the panels makes its way through the conductors to the inverter, then I guess you would use the panels specs for all the number...but that is impossible to ever happen.

The SolarEdge code compliance document is very clear about how to calculate the inverter input current. Presumably, they are not making up wild numbers, but are giving realistic failure scenarios.

[HX_Guy]

Seriously? The example is exactly the calculation I just told you to do.

the base ampacity of 8 AWG THWN-2 is 55 A. The temp correction they used is 82%, which was looked up from the table cited. 55 * 0.82 = 45.1 A

The load *1.25 must be less than that 45.1 A.

With respect to the termination, 8 AWG @ 75 deg is 50 A. Because the termination ampacity is higher than the corrected conductor ampacity, it is OK.

Either I'm too tired here or not sure why I'm not grasping it.

So you take the inverter max output, 47.5A in this case, and multiply by 1.25 which give you 59.38A.
Then separately calculate the derated wire capacity, in this case 65A x .82 = 53.3A.

What am I missing?

EDIT: Wait, I think I caught my own mistake. When calculating the conductor size, you go by the maximum amp rating of the wire? Not by what the terminations are (which would be 75º)?

If that's the case, then it would be...

47.5A x 1.25 = 59.38A
75A x .82 = 61.5A

61.5A > 59.38A so all is good?

[sensij]

Either I'm too tired here or not sure why I'm not grasping it.

So you take the inverter max output, 47.5A in this case, and multiply by 1.25 which give you 59.38A.
Then separately calculate the derated wire capacity, in this case 65A x .82 = 53.3A.

What am I missing?

Base ampacity for 6 AWG THWN-2 is 75 A, not 65 Amp. 75 A * 0.82 = 61.5 A

When you are look at the terminations at 75 deg, you do not temp correct, you just make sure it is higher than the conductor. Termination ampacity is 65 A, which is higher than 61.5 A and is OK.

[HX_Guy]

Yeah, I caught it too and edited my last post.

So looks like what's already in place is fine, now the question is what is the easiest route to get it resolved...try to teach the city or just comply with what they want?

By the way, the inspector also said we need a bond bushing for the DC disconnect. Is that accurate? (See arrow)

She also didn't like the the double lugged ground wire I ran in the AC disconnect, I think I remember you guys warning me about that. She said she needs paperwork that that's allowed or we need to loop the ground wire that's already there around that ground screw, which she also mentioned needs to have more paint scraped off (but she was ok with the green ground screw).