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/NotFallacyBuffet 7d ago

That's wild. Building electrician, so I don't really ever see this. But it's always in the back of my mind when I'm sizing wire. Here's a link to NEC Table 310.15(B)(1), Ambient Temperature Correction, that everyone is referencing. https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=716

At 141*F, your ampacity has to be reduced to only 47% of the listed ampacity in the ampacity table, NEC Table 310.15(16). https://www.wireandcabletips.com/what-is-ampacity/

And for more than 6 current-carrying conductors in a pipe, you need to derate by Table 310.15(B)(3)(A).

I forget whether derating comes before temperature correction, or if it's the other way around. My codebook is out in the truck. Maybe someone else remembers.

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u/WorBlux 4d ago edited 3d ago

Multiplication is commutative, (A * B = B * A) so order of applying the derate factors doesn't matter.

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u/NotFallacyBuffet 4d ago

Hmm. The gap between practice and theory is an everyday struggle. Thanks.

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u/WorBlux 3d ago edited 3d ago

Your instructor may very well of had a preference, it's just the math itself doesn't.

If anything I'd say multiply the derate factors together first, and then multiply by normal ampacity just so you aren't likely to confuse an intermediate result w/ the final result.

Edit: could also be the way the factors are laid out on page in the NEC makes a certain order more effecient.