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How to transition from 10awg to 8awg wire

WaltAllen

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Feb 2, 2022
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The 4 panels I want to wire in parallel are 35 volts and about 10 amps output. My run to the MPPT controller is about 20 feet, so I would like to combine two of the panels and transition from 10awg to 8awg. But I can't find any mc4 connectors that make this size change. How should this be done?
 
Thanks, that's what I may do. It just seems like some of those 2 into one splitters should have the larger wire size mc4 connector on the single side.
 
The 4 panels I want to wire in parallel are 35 volts and about 10 amps output. My run to the MPPT controller is about 20 feet, so I would like to combine two of the panels and transition from 10awg to 8awg. But I can't find any mc4 connectors that make this size change. How should this be done?
Why not run 2s2p?
Or even 4s?
Does that minimise the voltage drop enough to use 10 awg for the home run?
 
For greater than 2p you need fuses so you might as well have a combiner box.
 
Thanks, that's what I may do. It just seems like some of those 2 into one splitters should have the larger wire size mc4 connector on the single side.
So long as the final wire guage is suitable for the array, it's probably fine.
 
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Why not run 2s2p?
Or even 4s?
Does that minimise the voltage drop enough to use 10 awg for the home run?
I have lots of trees nearby so shading happens a lot. I've read that wired in parallel loses less power
For greater than 2p you need fuses so you might as well have a combiner box.
I see references to combiner boxes, but where can I find these. Amazon?
.
 
I think that 4s might lose the least power.
(Only suitable if voltage remains within limits.)

Each panel is a diode-bypassed 2s or 3s configuration.
Shadow over part of panel drops its voltage by 1/2 or 1/3.
In parallel, that may cause loss of entire panel's output.
In series, only 1/2 or 1/3 of panel's output is lost.

Due to physical layout of panel, if you can select having shadow start covering one long edge rather than one short end, panel output voltage would only drop 50% or 33% instead of 100%. For a while until shadow covers more.

For a small array, MC "Y" cables may be more cost effective than combiner boxes.
You can get MC fuse holders, if doing 4p. But 4x Isc is probably over MC rating, so after two sets of 2:1 Y in parallel, maybe make last splice to larger gauge wire with something other than another "Y". I run MC pigtails into an electrical box and use wire nuts, set-screw nuts, or other splices there.
 
I have lots of trees nearby so shading happens a lot. I've read that wired in parallel loses less power
How about 2s2p?
If you really need 4p because of shading, that is a tough environment for solar.
Lets say you have 4 100 watt panel with a open circuit voltage of 20 volts and an short circuit current of 10 amps.
4s = 10 amps @ 80 volts
4p = 40 amps @ 20 volts
2s2p = 20 amps @ 40 volts

This calculator will allow you explore voltage drop in your environment.

I see references to combiner boxes, but where can I find these. Amazon?
.
I have no experience with them but this is an example of a combiner box.
 
Alternatively you can post the info on the sticker on the back of the panel and the round trip distance from your array to your solar charge controller and I will do it for you.
 
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Calculator references 105 degree insulation. More applicable to some vehicle/boat hookup wire. That's why it allows up to 60A in 10 awg.
Wire used for home construction usually 90 degree. Would be better if this was an input parameter to the calculator.

10 awg 90 degree wire has ampacity 40A, but where NEC applies, 30A limit. One could get away with the higher figure if not attached to real estate.
If designing according to NEC, required ampacity is Isc of panels x number of strings in parallel x 1.56, which may be slightly above 30A.
I figure 10 awg will meet OP's need, may not quite comply with NEC.

> 10% voltage drop "Don't even think about it"
That's following the conventional wisdom, good for AC and battery circuits. For PV, I'll think about anything I want. In OP's case, won't apply to this 20' run. If 200' or more, sure, I'd let voltage drop go way over 10% at peak current. Just a cost tradeoff between Cu and Si.
 
How about 2s2p?
If you really need 4p because of shading, that is a tough environment for solar.
Lets say you have 4 100 watt panel with a open circuit voltage of 20 volts and an short circuit current of 10 amps.
4s = 10 amps @ 80 volts
4p = 40 amps @ 20 volts
2s2p = 20 amps @ 40 volts

This calculator will allow you explore voltage drop in your environment.


I have no experience with them but this is an example of a combiner box.
Thanks, I'm liking the combiner box idea, then just two maybe 6AWG wires in the run.
 
Thanks, I'm liking the combiner box idea, then just two maybe 6AWG wires in the run.
Yes, my site is far from ideal. Lots of oaks and maples all around that I am not allowed to remove. I get maybe 4 hours of full sun. But the solar & batteries gives me a silent backup during outages, plus I can power some of my 120V circuits, too. Anyway, it's such a fun hobby for a retired engineer.
 
Yes, my site is far from ideal. Lots of oaks and maples all around that I am not allowed to remove. I get maybe 4 hours of full sun. But the solar & batteries gives me a silent backup during outages, plus I can power some of my 120V circuits, too. Anyway, it's such a fun hobby for a retired engineer.
You don't need a combiner box for 2s2p you can very likely get away with mc4 branch connectors.
That means just one positive and one negative lead between the array and the solar charge controller.
Combiner boxes are for arrays with greater than 2 strings.
 
You don't need a combiner box for 2s2p you can very likely get away with mc4 branch connectors.
That means just one positive and one negative lead between the array and the solar charge controller.
Combiner boxes are for arrays with greater than 2 strings.
Yes, that is a good compromise and much less expensive than the combiner box. And I could still go with 8awg for those two final wires going to the controller by using pollenface's idea of solder and shrink wrap.
 
Yes, that is a good compromise and much less expensive than the combiner box. And I could still go with 8awg for those two final wires going to the controller by using pollenface's idea of solder and shrink wrap.
Most engineers retired or not don't shy away from the math.
You can work out the wire gauge to get the acceptable voltage drop using the calculator I linked.
Or... as I said, if you provide the requisite data I will do it for you.
 
Most engineers retired or not don't shy away from the math.
You can work out the wire gauge to get the acceptable voltage drop using the calculator I linked.
Or... as I said, I will do it for you.
I'm still in early thinking stages, but I will definitely do the math! My first step is buying those panels before they get more expensive or hard to get. My existing 4 panels are 325 watt and they are closer to the controller, so I put them in parallel (2P2P) and used 10awg with no problem. The panels I'm looking at buying are 370 watt (new from SanTan) and about 10 feet farther away, so putting all 4 in parallel would be too much for 10awg, I think. So 2S2P seems like the way to go. Thanks again for your generous help.
 
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