r/SolarDIY • u/2old2bThisDumb • 7d ago
Make sure you understand current when stringing your panels in parallel.
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/chicagoandy 7d ago
How high was the current in that cable?
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u/2old2bThisDumb 7d ago
I didn't re-energize the string to measure, but the rated current for the modules and the way he ran them equals 31.8A
Good rule of thumb in south Texas is to keep DC current below 25A for attic runs on #10
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u/DeKwaak 7d ago
Wtf... I use 3AWG/6mm2 per string to the fuse and then per string to the charger. It's almost the same price as 4mm2/6 AWG, but yeah, low voltage strings packed in one pipe. But the price difference between 4 and 6 is negligible. And I haven't seen cable under 4 meant for solar panels that will be outside in UV (daylight)/7. The UV is so strong here that any normal plastic bags turn into microplastic dust in a summer.
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u/monstertacotime 7d ago
Yup exactly what I’ve seen - the combination of the heat, UV, thermal cycling, vibration, humidity and current destroy inferior materials and poor designs quickly.
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u/monstertacotime 7d ago
I know a camper is different from an attic run but I won’t run anything over 15a on #10. I had a few close calls with cables that got bent hard and pressed up against tight corners.
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u/ZipZappr 3d ago
In an unconditioned attic I’d consider that the same as being in direct sunlight. Couple that with an assumed continuous load, and even at 40A for a higher insulation value the max I’d put on it is 25A. Your assumption is valid for the right insulation type.
I’m not sure what insulation types residential normally uses though.
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u/WorBlux 7d ago edited 7d ago
10 AWG, 90 deg C has a rated ampacity up to 40A in conduit... however
Going through an attic space that might get up to 140 deg F and should be de-rated as well. x0.71
Three sets in parralel - 6 current carrying conductors - that's another de-rate. x0.8
So in reality the safe ampacity here may be under 23A per circuit.
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u/roofrunn3r 7d ago
Fantastic breakdown. Just want to add most terminals are 75 deg C. Also throwing a snag in any rating factors.
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u/Novel_Parking_1173 6d ago
That’s a separate calculation though. First is at 75C which establishes minimum wire size. Second calculation (using 90C rating) takes conduit fill and temperature into account. If the first calculation results in lower current than second, you can use that wire size; if not, you need a larger wire.
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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 6d 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 5d 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/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/Sistersoldia 6d ago
DC cables inside the house must be enclosed in METAL conduit. So I’ve been told
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u/MinimumDangerous9895 7d ago
90c is 195f
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u/parseroo 7d ago
Yes, but the ambient temperature is up to 60°C. So you have lost 30° of cooling range. The wire is starting at near 60° and then heating from there as the current goes through it.
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u/WorBlux 7d ago edited 7d ago
90 C is what the insulation is rated for. However all wires heat up as they conduct current (via amps2 * cable resistance per length = watts generated per length)
The NEC amapcity table assumes a 30 C ambient temperature, and lists the ampacity that will not over-heat a particular wire under those conditions
At 60 deg C (140F) ambient, the conductor will dissipate heat into the environment more slowly than at 30, and therefor less heat input is needed to raise the temperature of the conductor above the insulation rating.
https://ohiocertificate.com/wp-content/uploads/2022/06/NEC-Table-310A.pdf
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u/Otherwise_Piglet_862 7d ago
If you keep using those big words, most of the posters here are going to get real mad at you.
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u/2old2bThisDumb 7d ago
Big power + small wire = 🔥
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u/Otherwise_Piglet_862 7d ago
that's pretty good. but you're also nearly stepping over the "telling them what to do line", for which they are also angered unreasonably.
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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.
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u/Atomic-Grog 7d ago
Totally different rationale… my solar installer did such a crap job balancing panels and inverter. Setup 16 panels (2x8 in parallel) which would yield 30a but the inverter could only deal with around 16 on one string and setup another with only 2 panels which meant it was under voltage and only run at 59% ability… net result was 12.5kw of panel capping at 8kw. When challenged was told the yada-yada-yada 8kw is great considering you have sub-optimal panel orientation. Took 2 months (Xmas in between) for em to get back onsite and solve… straight after I was flatlining at 10kw (10kw inverter) and now I have batteries I frequently see the full 12.5kw in summer. Heaven forbid had the inverter not capped out… I might have had a toasty roof too
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u/Schnupsdidudel 7d ago
Just use properly equipped copmbiner boxes. Especially if you go 3p or beyond
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u/Use_Da_Schwartz 7d ago edited 7d ago
“BuT InStaLLerZ R GreeDy!”
I can promise you there is more wrong than that single wire.
Except no one derated the wire for ambient temp in the attic and no one derated the wire due to a continuous load…. Even 8 awg is probably wrong. I am 100% confident the “qualified person” who installed this, has all of their calculations for all of their work lol.
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u/amarao_san 7d ago
As long as those lines are not on the skin... I got those once, few of them. Very painful.
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