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

[HX_Guy]

Definitely spots for 2 strings on all Solaredge dc disconnects. I don't combine on the roof unless absolutely necessary. Not sure on the conduit fill and temp corrections off hand though.

Probably better, seems to be pretty tricky when going over 2 strings as I'm finding out.

I emailed SE back explaining the original permit with 4 strings, which would require a 75A conductor after NEC calc, and with a 65% derate here in Phoenix, it would need 115A or a 2AWG conductor to which he replied back and said yes, that is correct.

[sensij]

Probably better, seems to be pretty tricky when going over 2 strings as I'm finding out.

I emailed SE back explaining the original permit with 4 strings, which would require a 75A conductor after NEC calc, and with a 65% derate here in Phoenix, it would need 115A or a 2AWG conductor to which he replied back and said yes, that is correct.

As predicted by this post, although the temp correction I used was not quite enough.

Sorry about the misinformation on connecting multiple strings.

[HX_Guy]

As predicted by this post, although the temp correction I used was not quite enough.

Sorry about the misinformation on connecting multiple strings.

You've been very spot on Sensij.

I do believe that with a resubmitted permit, we could still use the existing wiring.

The way I figure it...

15A x 3 strings = 45A x 1.25 = 56.25A
6AWG is rated at 75A x temp correction of .82 (since it's in the attic, not on the rooftop) = 61.5A

61.25A > 56.25A so it would be ok, no? Did I leave out any figure there in the calculation?

[foo1bar]

Probably better, seems to be pretty tricky when going over 2 strings as I'm finding out.

I emailed SE back explaining the original permit with 4 strings, which would require a 75A conductor after NEC calc, and with a 65% derate here in Phoenix, it would need 115A or a 2AWG conductor to which he replied back and said yes, that is correct.

So I think for your current design you need
3 * 15A * 1.25 == 56.25A
which means #6 might work.
(I think in the attic you might still need a temp correction factor. I'm not sure.)

But at $150 for a permit amendment, it's probably cheaper to replace it with #2 and do a pair of butt splice reducers at each end.
(At least that's what I'd do if I were you at this point. Splices are ~$10 each, 50 feet of wire will be <$100.)

I'd probably go with http://www.amazon.com/Gardner-Bender.../dp/B000FPDIHM for the splice. (UL listed, rated for copper, looks easy enough to tape or put a heat-shrink tube over.) But I don't have the experience so if others have better suggestions based on their experience, listen to them.

[HX_Guy]

So I think for your current design you need
3 * 15A * 1.25 == 56.25A
which means #6 might work.
(I think in the attic you might still need a temp correction factor. I'm not sure.)

But at $150 for a permit amendment, it's probably cheaper to replace it with #2 and do a pair of butt splice reducers at each end.
(At least that's what I'd do if I were you at this point. Splices are ~$10 each, 50 feet of wire will be <$100.)

I'd probably go with http://www.amazon.com/Gardner-Bender.../dp/B000FPDIHM for the splice. (UL listed, rated for copper, looks easy enough to tape or put a heat-shrink tube over.) But I don't have the experience so if others have better suggestions based on their experience, listen to them.

I don't know if the inspector would allow that since the splices aren't shown on the permit, and the permit shows the 2AWG conductor gong all the way to the combiner inputs. If I were to do that, I'd probably be better off replacing the terminal blocks with ones that accept 2AWG.

[sensij]

I do believe that with a resubmitted permit, we could still use the existing wiring.

The way I figure it...

15A x 3 strings = 45A x 1.25 = 56.25A
6AWG is rated at 75A x temp correction of .82 (since it's in the attic, not on the rooftop) = 61.5A

61.25A > 56.25A so it would be ok, no? Did I leave out any figure there in the calculation?

Calculation looks right to me, but maybe not the conclusion. You could even tolerate the next higher temp range (0.76 correction, for 51-55 deg C) and still be OK. Does the temp in your attic exceed 131 deg F? Honestly, I bet it does, but am not sure there are hard and fast rules on what temperature to use. Here are a couple threads on MikeHolt forums, but I don't think there is a definite answer. Whatever the city approves, must be right.

Edit: Setting code aside, I would be a little concerned that the time of day when the attic is hottest is also when the most current will be going through those wires.

[foo1bar]

I don't know if the inspector would allow that since the splices aren't shown on the permit, and the permit shows the 2AWG conductor gong all the way to the combiner inputs. If I were to do that, I'd probably be better off replacing the terminal blocks with ones that accept 2AWG.

Ask.

My guess is that the inspector would allow it.

"The connector on the equipment isn't sized to for #2 wire. Any problem with a 5" pigtail of 6 gauge to connect to a reducer splice?"

I did guess wrong about the permit revision being a reasonable price, so it's quite possible I'm wrong on this too - but ask and see.
(although $150 for a major revision might be reasonable - after all, you would be changing the permit from having 4 to 3 strings.)

If it were me, I wouldn't replace a portion of the inverter - even something that logically shouldn't be a problem like a terminal block.

[HX_Guy]

Heard back from the manufacturer. They said we could either run two sets of #6 conductor (which would require new permits and rewiring) or we could use a short piece of #6 spliced to #2 inside the combiner box (would require rewiring and also not sure if the inspector would accept that if it's not shown on the permit?).

[sensij]

Heard back from the manufacturer. They said we could either run two sets of #6 conductor (which would require new permits and rewiring) or we could use a short piece of #6 spliced to #2 inside the combiner box (would require rewiring and also not sure if the inspector would accept that if it's not shown on the permit?).

The #6 spliced to #2 should be allowed by the exception in 310.15(A)(2). I could see this being approved during inspection even if it isn't in the permit, as long is the splice is done with a listed connector and properly crimped.

(2) Selection of Ampacity.
Where more than one ampacity applies for a given circuit length, the lowest value shall be used.

Exception:
Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.

Informational Note: See 110.14(C) for conductor temperature limitations due to termination provisions.

[sensij]

Heard back from the manufacturer. They said we could either run two sets of #6 conductor (which would require new permits and rewiring)

This agrees with Ben25's suggestion.

Pair 1: 30 A * 1.25 = 37.5 A
Pair 2: 15 A * 1.25 = 18.75 A

6 AWG THWN-2 base ampacity = 75 A
4 conductor correction of 0.80 [310.15(B)(3)(a)]
75 A * 0.80 = 60 A is the CCC corrected ampacity
37.5 A / 60 A = 0.625, which means a temp correction of 0.65 (61-65 deg C) should be OK.

[HX_Guy]

This agrees with Ben25's suggestion.

Pair 1: 30 A * 1.25 = 37.5 A
Pair 2: 15 A * 1.25 = 18.75 A

6 AWG THWN-2 base ampacity = 75 A
4 conductor correction of 0.80 [310.15(B)(3)(a)]
75 A * 0.80 = 60 A is the CCC corrected ampacity
37.5 A / 60 A = 0.625, which means a temp correction of 0.65 (61-65 deg C) should be OK.

That would require both a new permit AND rewiring, I don't see the advantage to that plan.

If we were to submit a new permit, then as we talked earlier, should the 3 strings @ 45A be fine since they are running through the attic?

[sensij]

That would require both a new permit AND rewiring, I don't see the advantage to that plan.

If we were to submit a new permit, then as we talked earlier, should the 3 strings @ 45A be fine since they are running through the attic?

OK, last comment from me on this particular train of thought.

I would be uncomfortable with the 6 AWG that is currently installed, since I think a higher temp correction (at least 60 deg C, maybe higher) would be appropriate for the attic run. I realize this is a slightly different opinion than much earlier in the thread, but the temperature discussion has changed my mind. (edit: yes, even considering the fact that your "actual" current is unlikely to ever be 45 A, and is more likely peaking in the low-mid 30's). To me, the path of least pain would be to re-run from the combiner box to the inverter with 2 AWG per the existing permit. Replace the white wire with black, while you are at it. It sounds like the AHJ would have been ok with that, and not objected to the missing 4th string. By going with 2 AWG, you've got wire sized that will be safe under any reasonable projection of your attic temp. Use a splice or replace the terminal block at the DC disconnect of the inverter, I don't have a strong opinion.

[PVAndy]

Thanks Volusiano, the electrical stuff is so over my head, I love when it's explained well like that so thanks for that.

The 12,400 is DC STC (40 x 310w panels). For AC the permit says 11.4kW AC though that seems like the maximum amount, not for my specific system.
I say that because looking at the SolarEdge spec sheet, it shows a max DC STC of 15,350 with a nominal AC output of 11,400.

http://www.solaredge.com/files/pdfs/products/inverters/se-single-phase-us-inverter-datasheet.pdf

Going by what you said, 11.4kw (AC) / 240V = 47.5A...with the 80% rule, that's 59.4A...but again that's the max the inverter will put out if I maxed out the DC input I assume, right?

So how do I figure out the AC system rating for my actual system if the 11.4kW seems to just be something they plugged in from the spec sheet?

I assume I'm late to the game and this has already been answered. When you calculate overcurrent protection, disconnects, and wire sizes you go by the inverter AC rating. It has nothing to do with the size of the DC system.

Andy
PV Design Engineer
NABCEP Certified Installation Professional

[HX_Guy]

OK, last comment from me on this particular train of thought.

I would be uncomfortable with the 6 AWG that is currently installed, since I think a higher temp correction (at least 60 deg C, maybe higher) would be appropriate for the attic run. I realize this is a slightly different opinion than much earlier in the thread, but the temperature discussion has changed my mind. (edit: yes, even considering the fact that your "actual" current is unlikely to ever be 45 A, and is more likely peaking in the low-mid 30's). To me, the path of least pain would be to re-run from the combiner box to the inverter with 2 AWG per the existing permit. Replace the white wire with black, while you are at it. It sounds like the AHJ would have been ok with that, and not objected to the missing 4th string. By going with 2 AWG, you've got wire sized that will be safe under any reasonable projection of your attic temp. Use a splice or replace the terminal block at the DC disconnect of the inverter, I don't have a strong opinion.

I hear you Sensij and I agree that's probably the path of least pain..if I ever hear back from the installer after the whole failure. I haven't heard back yet since the permit failure.

[HX_Guy]

Are you kidding me??

I was up on the roof making a change to the DC combiner box and while working on it, by accident I notice...WTF did that wire just move? Oh yea...a freaking wire that's supposed to be crimped tight just slid right out of the connector! A wire that has up to 500V going through it. This is a connector that the genius installer crimped together with a plier, instead of the proper MC4 connector crimper that costs a whopping $35 on Amazon! How the hell is this guy licensed and in business?!?