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

[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.

[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.

[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]

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]

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.

[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]

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]

Now I'm not sure how the city would calculate it but the way the permit designer calculated it was:

35.8A (maximum output amps) X 125% (NEC safety buffer) = 44.75A per conductor
44.75A x 2 conductors = 89.5A
89.5A / 85% (temperature derate factor) = 105.29A

#2 wire is rated to 115A @ 75º C so there you go. The big mistake here is the PER CONDUCTOR which is completely wrong because if if they are both "hot", the inverter will never have more than 34.5A sent through the wires (that's a different spec, but it's close enough to the 35.8A on the permit that the 35.8A could be left alone on there).

Ouch! I suspect that the same designer would say that I have two 100A 120V loads, so I need a 200A 120/240 service.

Well, this will certainly be a good time to amend that part of the submission.

Although as you say, # 6 would probably not fly either.
I wonder just how the temperature adjustment calculation was done? Clearly only two current carrying conductors in the raceway, so it must all be pure temperature.
Some use the highest high ambient temperature and then take the conduit above roof adder on top of that. Did he figure in the correct height in inches for your conduit run? What temperature rating did he use for the insulation? You can probably justify using 75C, as long as the terminations at both ends are rated for 75C.
And if the run in conduit above the roof is short enough you may be able to ignore the whole calculation. The other place where you need the temperature calculation is inside the attic.
Quaint tip: If you have 90C wire and only 75C terminations at some point you can put in another junction box and go from large wire at the termination to smaller wire calculated at 90C for the body of the run. Just make sure you use 90C wire nuts or terminal strips to make the transition and run at least a foot of wire at the larger size.

[yocane]

Seams like quite the issue

haha get it?

http://www.howto.solar

[HX_Guy]

I spoke with the permit design company and they gave me that 85% temperature derate number, they said that's what my city (Peoria, AZ) requires to be used for conductors ran through metal conduit.

As for doing the calculation wrong, they actually said the city forced them to do that. In their first draft (on the right side), they actually showed the SolarEdge figures which were accurate and the city didn't accept them. They had to revise (the cloud bubble) to satisfy what the city asked for.



Your interpretation of SolarEdge though is also not right I don't think, at least not how it was explained by SolarEdge. While each string can have a maximum output of 15A, all strings combined will never have an output of more than 34.5A. This is regulated by the inverter and the communication back and forth with the optimizers. The optimizers will raise their voltage to drop their amps so that the total of all strings in parallel does not exceed 34.5ADC, so the correct figure to always use is 34.5A...which is why the inverter only allows max 6AWG wire. There isn't a scenario ever where you would need more than that because 34.5A x 1.25 x whatever temp derate you use is not going to exceed 65A.

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

[sensij]

Also back on the AC side, it looks like #6 wire will not work. Well, it would work I'm sure, but I don't think it'll pass with the city.

The inverter is rated at 47.5A max AC output. You take that, multiply it by 125% and you get 59.38A...so far so good (it's under 65A for #6 wire) but then once you derate for temperature, 59.38A / 85% = 69.85A...oops.

Now I'm confident it would be fine because A) the inverter will never pump out 47.5A, that would mean all the panels are putting out 100% output and that won't happen because of different orientations (some facing south, some facing east) and B) that's already taking into consideration a 125% buffer, which is overkill anyway. The city though isn't going to buy any of that though, so looks like we do need to run #4 between the inverter and the meter.

Now to figure out if 1" conduit is really needed or if we can use the 3/4" already in place. Anyone know for sure what the conduit fill specs are? I seem to find different answers online.

85% is not a valid temp adjustment. Where did you get that number? 82% is the correction for up to 50 deg.

See this post. You are doing the math wrong.

[sensij]

Yes, your designer really screwed things up. In a SolarEdge system, the PV Circuit is only between the solar panel and the optimizer. Voc, Vmp, etc for the entire array is meaningless, and should not have been listed in the bubble the way that was done. From the optimizers forward, what matters is that the rated continuous amperage for each optimizer string is 15 A. After the combiner, with three strings, 45 A is the rated amps, at 350 V. 45 A * 1.25 = 56.25 A. Your THWN-2 base ampacity is 75 A. If 85% is the temp correction is correct, then 75 * .85 = 63.75 A. 63.75 A > 56.25 A, so you are ok. 85% can't be the right correction though, because that number is not in the 2011 NEC table.

If the temp correction is 76%, which cover you up to 55 deg C, #6 THWN-2 would just barely be acceptable.

It could be that in the original 4 string design, the combined amperage was 60 A (four strings * 15 A). 60 A * 1.25 = 75 A. With a 76% temp correction, 4 AWG THWN-2 would be too small (95*.76 = 72.2 A). Therefore, 2 AWG THWN-2 would have been required.

If the number of strings is what is screwing things up, a permit revision showing only three instead of four may solve the problem.

[Alisobob]

Can you post up a photo of your "installer"...

I really need to see what this asswipe looks like....

[HX_Guy]

Also back on the AC side, it looks like #6 wire will not work. Well, it would work I'm sure, but I don't think it'll pass with the city.

The inverter is rated at 47.5A max AC output. You take that, multiply it by 125% and you get 59.38A...so far so good (it's under 65A for #6 wire) but then once you derate for temperature, 59.38A / 85% = 69.85A...oops.

Now I'm confident it would be fine because A) the inverter will never pump out 47.5A, that would mean all the panels are putting out 100% output and that won't happen because of different orientations (some facing south, some facing east) and B) that's already taking into consideration a 125% buffer, which is overkill anyway. The city though isn't going to buy any of that though, so looks like we do need to run #4 between the inverter and the meter.

Now to figure out if 1" conduit is really needed or if we can use the 3/4" already in place. Anyone know for sure what the conduit fill specs are? I seem to find different answers online.

[HX_Guy]

Now that I look at everything again more closely, I'm not 100% sure if the city needs to be educated, if it even matters. I think the problem may be in how the permit was designed.

The whole issue stems from this page of the permit..



Zooming in specifically on this area here...



Now I'm not sure how the city would calculate it but the way the permit designer calculated it was:

35.8A (maximum output amps) X 125% (NEC safety buffer) = 44.75A per conductor
44.75A x 2 conductors = 89.5A
89.5A / 85% (temperature derate factor) = 105.29A

#2 wire is rated to 115A @ 75º C so there you go. The big mistake here is the PER CONDUCTOR which is completely wrong because if if they are both "hot", the inverter will never have more than 34.5A sent through the wires (that's a different spec, but it's close enough to the 35.8A on the permit that the 35.8A could be left alone on there).

Now that's something the permit designer did wrong, not that the city asked him for I don't think. The two "hot" wires are no wired in parallel so I don't know why they doubled the amperage for the calculation...each wire should be carrying at most 35.8A and even after all the NEC safety and temperature derating, the total would be 52A which is way below the 65A rating of #6 wire.