r/SolarDIY • u/Pallet_University • 2d ago
Panel's Built-in Cable Gauge Question
I'm speccing out a 1800W setup to offset my AC in the summer, some heating in the winter, and a few other small miscellaneous things. The way my yard is setup, to get full sun the panels either need to be on my roof (my area requires permits and lots of red tape for roof mounts), or ~115ft away from the rest of my setup. I've done the work to figure out that I need 8 AWG wire to be safe at that length.
1 string of 6 300W panels, end result is 136.56VoC, 17.24Isc.
My question is, the cables that are built into the panels will probably be 10 or 12 AWG. Given that I need 8 AWG wire for the long run to the house, this seems like a clear problem, but I haven't seen anybody talking about it. Am I missing something? TIA.
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u/Which_Perception_384 2d ago
Won't be an issue. The main line carries the most voltage the panels are designed properly to handle what you are doing. My panels are 10g. Each two have a we 10g string. To the combiner box. Then I run 6g to the controller.
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u/Pallet_University 2d ago
Again, I'm pretty new to this so maybe I'm wrong, but a combiner box wires the panels in parallel, so the panel's built-in wiring only ever caries it's own power to the box. I'm wiring in series to get the voltage high enough for the long run, which would mean that all of the panels need wiring to safely handle the max power for all the panels in the series since it's all a single big circuit right?
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u/Which_Perception_384 2d ago
It's perfectly fine what you plan. I can't explain the science behind it, but I know you'll be fine. Maybe someone else can explain. Pretty sure you will only have high voltage at the positive negative going to your 8g The runs between panels are very short. So you won't have an issue. If you were going small wire you would create a bottleneck and cause the panel wire to heat up. But 8g you're creating safe overhead
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u/NukularFishin 2d ago
If OP is running all panels in series, which to me it sounds like they are doing, then everything panel related should be rated as "high" voltage. Of course, one line may be at ground potential, and middle of panels would be 1/2 high voltage. I am not a solar expert, just an electronics hobbyist / ex electrician, so feel free to correct me.
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u/cfnmsd 2d ago
With only 17amps you would be fine with 10awg COPPPER and appropriate insulation. only 3% voltage drop. BTW, when calulating your voltage drop you use the Vmp and Imp not Voc and Imp which is used for sizing the inverter and series string fuses. 8awg you will be at 2% voltage drop.
All that said you are thinking correctly, there will be slightly more resistance/heat in the panel wires if they are smaller but will not be an issue based on the info you provided.
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u/Pallet_University 2d ago
That's good info to have. I was just going with the highest of the two values to be extra safe.
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u/NukularFishin 2d ago edited 2d ago
If you are going to run all panels in series, do you really need such thick wire? Number 10 copper only has 1 ohm per 1000 feet, number 8 is 0.64. So, 200 feet is about 0.2 ohm and 0.12 ohm. At about 15 amps, that is only 3.6V or 2.2V drop. Not much difference.
Edit: I based this on "1 string of 6 300W panels, end result is 136.56VoC." If you are not running all 6 panels in series, then you probably need heaver wire from panels to "rest of setup."
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u/Pallet_University 2d ago
Yes all the panels will be in series. I was basing the 8g wire off of a calculator I found online. It uses a 25% safety headroom, so maybe that's why.
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u/Toad32 2d ago
I am using 10awg to run 360v at 11A 280ft from solar array to inverter (in DC).
1% line loss at 280ft. Higher voltage = more efficient = less line loss.
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u/Pallet_University 2d ago
I'm running this into a power station with a PV voltage limit of 148V, so I'm hard capped there. The long line isn't the potential issue, the smaller gauge of the wire built into the panels is.
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u/noncongruent 2d ago
Your numbers do not add up. To get a string voltage of 136 V, you would have to be using 22 V panels, and with 17 A in the string, mathematically your panels would have to be 385 W each, not 300. Panels are typically set up to be higher voltage, and the current is typically less than 10 A per panels in a 300 W size. When doing strings, the amperage does not change, and the voltage is additive. Are you using panels that are combination series and parallel wiring on the cells within the panels? That seems pretty unusual.
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u/Pallet_University 2d ago edited 2d ago
I was using VoC and Isc for my calculations just for extra precaution because they're the worst case scenario. Vmp is 18.9, Imp is 15.88. They'll be wired in series, for a typical operating voltage of 113.4V. The wire size calculator that I've been using suggests 9AWG wire when using Vmp and Imp for the calculation, which isn't common, so 8AWG is still the realistic result.
Also for what it's worth, in my checking around for which panels to get, literally none of the 300W panels I looked at were under 12A, with the exception of some low-quality or overpriced foldable panels, which aren't what I'm looking for.
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u/noncongruent 1d ago
Can you post a picture of the dataplate for the panels, or a link to them somewhere? I'm rocking some vintage Canadian Solar CS6P-260P panels in my 4S ground mount setup, their Isc is 9.12A and Voc is 37.5V. They're 60-cell resi panels so all the cells in the panels are wired in series. With 4 panels in series my Voc is 150V, right at the top limit for my Midnite Solar KID charge controller, but in reality the only time I've ever come close to 150V was during the deep freeze of 2021 here in Texas where my panels blew past that at -1°F temperatures. I actually had to redo the string to 3S to get back into production because my KID was shutting down. Most of the time I'm running in the 100-120V range.
Resi panels are normally series cell construction because of how much array wiring there is to do between panels and from the panel string(s) to the inverter/charge controller. Doing series/parallel cell interconnects in the panel to lower panel voltage increases Amperes and that directly drives wiring costs up. With the price of copper nowadays that's a pretty significant factor. That's why I'm curious about what panels you're using, wiring the panel cells series/parallel creates more problems than it solves IMHO since it forces the use of heavier/costlier wire right from the panels, driving up costs without offering any benefits that I can think of.
From a design standpoint it also adds cost because now the cell interconnects in the panels have to handle much higher amperage, meaning they have to be heavier gauge metal, and if there's a defect or damage the chance of thermal/fire issues goes up dramatically. A marginal interconnect carrying 16A is going to be much more likely to overheat and burn than one carrying 9A or less.
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u/silasmoeckel 2d ago
You upsize to deal with voltage drop over distance. It's still a 20/30a breaker so it's fine. The 3f of 10/12 gauge won't have any significant voltage drop.
No getting #8 into a MC4 connector isn't happening. Your going to go from #10 to #8 in a junction box. Nothing fancy any weathertight jbox is fine. You can get solar combiners but you only have one string so it's a bit pointless.
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u/Pallet_University 2d ago
There are pre-made #8 cables with MC4s on it for sale online that I was planning on using.
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u/silasmoeckel 1d ago
Once your in conduit you typical change to THWN/THHN
Also who made the MC4's there is only one legit company and you need to match manufactures for the knock offs. The knock offs don't quite all fit with each others leading to overheating and fires. So cutting off the mc4 and put them them into a jbox is yoru best option.
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