40a on a short run of 10awg?

slipperysam

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Feb 2, 2021
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I just need to verify some fundamentals here. I've currently got a 2s3p array of 400w panels, each 48.9 voc and 10.39 isc. The distance between the combiner box and the SCC is about 6'. I'm using 10awg for that run. With my 24V battery bank charging begins at about 35V which happens quite early in the morning around 7:30am. Most of the day (here in sunny Florida at least) I'm above 80V. The voltage drop IMO is negligible.

I realized I can fit another two panels after I completed my build. I went ahead and bought and mounted them and plan to wire them to the combiner box this week. However this will mean that my total possible current after the combiner box is 40a (if we forego the technicality that my 100a charge controller will now be over paneled and hence likely won't see that full theoretical amperage). The voltage drop is still negligible, at 0.57V. It might mean I start charging a few seconds later in the day or something.

But what about safety? I buy my wire from Greg's Marine and it says max continuous amperage of 60a. Does this mean that as long as the voltage drop is acceptable, I can run 60a at "safe" temperatures, but those might be a bit more than room temp?

In a similar vein, I'm using 10awg from my lynx distributor to my 24v fuse block. It's fused at 60a, and it's a distance of about 1'. Again at that distance the voltage drop calculator is fine, but I want to make sure that seems reasonable. I see most setups using larger gauge wires for such things, but maybe that's because their components are farther apart.
 

sunshine_eggo

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But what about safety? I buy my wire from Greg's Marine and it says max continuous amperage of 60a. Does this mean that as long as the voltage drop is acceptable, I can run 60a at "safe" temperatures, but those might be a bit more than room temp?

Yes. The copper in the wire can handle crazy current. The rating is based on the temperature the insulation can take. Those are rated for 105°C, so they can boil water but still be safe. You'll get unpleasant voltage drop before you hit 105°C.

Great place. I've bought a fair amount of pre-made cable from them.

In a similar vein, I'm using 10awg from my lynx distributor to my 24v fuse block. It's fused at 60a, and it's a distance of about 1'. Again at that distance the voltage drop calculator is fine, but I want to make sure that seems reasonable. I see most setups using larger gauge wires for such things, but maybe that's because their components are farther apart.

Sounds good.

If you're using wire rated for the current, it's fused/breakered appropriately, and the voltage drop is acceptable to you, that's it.
 

Hedges

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Ampacity of single wire in free air, 90 degree insulation, is 55A for 10 awg


Bundle of up to 3 wires, 40A


However, for NEC compliance, 30A is the max allowed.
In other words, it works from a technical perspective but is not allowed in wiring of buildings.
I think it might be found in appliances, vehicles, etc.

PV panels can exceed Isc, for instance if in full sun on a cool day and nearby clouds reflect additional light.
Wiring ampacity is supposed to be designed for 1.56x Isc.
That's 1.25x for higher current, times the usual 1.25x margin used for fuses/breakers to avoid nuisance tripping.

For my home system I would probably use 6 awg.

2s3p --> 2s4p?
Not all have to be same orientation. With some series strings of 2 oriented differently, you can reduce peak current to about 0.7x as much and extend hours of production.
 

slipperysam

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Feb 2, 2021
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Interesting, I didn't know about the possibility of exceeding ISC. So then fuse/ampacity should be 1.56*4*10.39A = 64.83A. I didn't mention this but between the combiner box and my SCC is a disconnect switch that happens to use a 2-pole 63A breaker. Seems serendipitous! I would think rating for 60A continuous at 105 °C with possible rare peaks between 60A - 63A is quite safe for now. I can monitor with a laser temperature gun for a bit. If I feel it's too hot, the wire would actually be quite easy to replace with a bigger gauge.
 
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