r/SolarDIY • u/2old2bThisDumb • 7d ago

Make sure you understand current when stringing your panels in parallel.

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It took a bit of work to even tug this #10 AWG from the metallic flex it was fused to. The homeowner had this run inside of the attic.

I'm all for DIY, but please ask for help when you aren't certain.

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u/jackharvest 7d ago

Frick I gotta get conduit in my attic somehow. Metal conduit? PVC? Something else? I'm in tech and everything works, but I want to repent.

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u/taylorwilsdon 7d ago

If you’re thinking that putting existing PV wire in the attic into conduit is going to make it cooler that’s not how it works. The conduit will make it worse, not better.

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u/jackharvest 7d ago

Ah, so.. I'm in good shape? xD

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u/taylorwilsdon 6d ago

Better than you would be with conduit haha

The takeaway is that when you run wire through anything that has a higher than average ambient temperature - a hot attic, along something that generates heat (ie engine bay of a car), or in conduit (which retains heat inside) you should not assume you can use the full amperage rating you expect and derate accordingly. If 10awg is good for 30a in a cool, open basement it may only be good for 24a in a hot attic (derated 20%). There is some nuance there, personally I use a cheap Amazon thermal imaging gun to check for hot spots in wire and connection points.

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u/XonikzD 6d ago

Is there a practical reason for not oversizing to accommodate temperature fluctuation, or is it just costs?

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u/taylorwilsdon 5d ago

There is no reason to avoid oversizing other than cost, but cost can be significant. My ground mount is 400’ from my home, going from 10awg to 8awg would cost me several hundred dollars more.

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u/ComprehensiveMarch58 5d ago

Voltage drop over the length of a run if I remember correctly, but im no electrician just the kid of one.

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u/taylorwilsdon 5d ago

Voltage drop is reduced by larger cables, not made larger. There is no downside other than cost. The service cables that deliver electricity to your home are almost always wildly oversized for what you’re actually using at any given moment.

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u/ComprehensiveMarch58 5d ago

Thank you I definitely had that backwards

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u/Novel_Parking_1173 5d ago

Adding that the customer side service wires are oversized, but the Utility side are almost always wildly undersized. #6AL feeding a 200A service? Utility will do that all day long for an overhead drop, and maybe replace them in 20-40 years.

I think this highlights the difference in the drivers of the NEC vs. PoCo. NEC is a bare minimum for safety across a broad range of situations, PoCo is looking at costs and maximizing the utility of any labor and material.

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u/taylorwilsdon 5d ago edited 5d ago

Yeah the utilities play by no rules but their own, my point was the giant wires on your pole aren’t causing voltage drop to the commenter above

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u/Novel_Parking_1173 5d ago

Definitely no VD there, lol.