PV wire size sanity check

hamburgerman

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Aug 9, 2021
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My county requires me to have a 3rd party provide engineering docs for our solar. We're doing a Sol-Ark 15k, and and it will be connected to 3 arrays, each consisting of 16 390w panels, in a 2(p)8(s) config.

I've engaged a company to do it, and provided detailed drawing and specs based on all my research and (admittedly far from expert) electrical knowledge. I've had to have them correct a few things, but one point still stands, and I am curious if I'm wrong, or they are, so a 2nd opinion would be great!

These are going to be ground mounted, with underground conduit with a total max length of about 200'. When the original drawings came back, they had messed up, and drawn out 6 separate arrays, each with 8 panels in series. No biggie - but they also called for 8 AWG cable for each, back to the inverter.

I called out the wiring issue, but also questioned the wire size. I referenced the spec sheet (attached) showing that even under 100% absolutely perfect conditions, a 2(p)8(s) config here should only ever see a max of 28 amps... so realistically, never even that much. So I asked why on earth they were using 8 AWG for the original drawing, when it was mistakenly ignoring the paralleled strings, thus meaning that would have only ever seen a theoretical max of 14 amps or so.

I asked them to fix the wiring, but then asked why I can't use 10 AWG (and, especially why they hadn't in the first version). They said that the first one had to be 8 AWG because "all of the wires in a single conduit", which I have no idea if that's true or not so I'm willing to accept that. For the revised version, they still said it had to be 8 AWG because "with the parallel config, current goes up." Well yea, I know that, I explained all that and showed my calculations in my preceding message.

I get that 8 AWG won't hurt anything other than costing ~$600 in wire. But, do I really NEED 8 AWG? Am I just misunderstanding or doing my own math wrong in thinking 10 AWG is sufficient?
 

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timselectric

If I can do it, you can do it.
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With more conductors in a conduit, you do have to de-rate their current carrying capacity. So, they are correct in that regard.
Going on your figures (200', 28a, and guessing 222v)
To keep the voltage drop below the recommended 3%.the recommended wire size should be #6.
#8 would have a 3.7% voltage drop. (Which isn't horrible)
 

hamburgerman

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Aug 9, 2021
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With more conductors in a conduit, you do have to de-rate their current carrying capacity. So, they are correct in that regard.
Going on your figures (200', 28a, and guessing 222v)
To keep the voltage drop below the recommended 3%.the recommended wire size should be #6.
#8 would have a 3.7% voltage drop. (Which isn't horrible)

The de-rate makes sense, to keep total resistance and temperature down inside the enclosed space. I don't know why that didn't dawn on me before.

It'll be ~384v (8 panels at ~48v in series). I swore even at #8 I was below 3% loss, but I'm not too proud to admit I messed something up :)
 

rmaddy

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First, your panels will hopefully be 8S2P, not 2P8S. 8S2P means you take 8 panels and put them in series. Do that again. Now combine those two long string in parallel. This requires 1 pair of Y connectors. 2P8S means take two panels and put them in parallel. Repeat this 7 more times. Now take those 8 pairs of parallel panels and connect them all in series. That requires 8 pairs of Y connectors.

8S means a Vmp of about 307V. 2P means a max Isc of about 28A. 400' roundtrip of wire. 3 arrays means 6 wires in a conduit.

The following calculator shows that 10AWG will result in just under 3% voltage drop:

 

hamburgerman

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Joined
Aug 9, 2021
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16
First, your panels will hopefully be 8S2P, not 2P8S. 8S2P means you take 8 panels and put them in series. Do that again. Now combine those two long string in parallel. This requires 1 pair of Y connectors. 2P8S means take two panels and put them in parallel. Repeat this 7 more times. Now take those 8 pairs of parallel panels and connect them all in series. That requires 8 pairs of Y connectors.

8S means a Vmp of about 307V. 2P means a max Isc of about 28A. 400' roundtrip of wire. 3 arrays means 6 wires in a conduit.

The following calculator shows that 10AWG will result in just under 3% voltage drop:


Yes sorry, 8S2P, that's indeed what I meant!
 

timselectric

If I can do it, you can do it.
Joined
Feb 5, 2022
Messages
5,675
The de-rate makes sense, to keep total resistance and temperature down inside the enclosed space. I don't know why that didn't dawn on me before.

It'll be ~384v (8 panels at ~48v in series). I swore even at #8 I was below 3% loss, but I'm not too proud to admit I messed something up :)
With 384v , #8 would be 2.14% VD.
#10 , would be 3.14% VD.
 
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