DougfromdaUP
Solar Enthusiast
That chart is wrong. It’s for 12 volts.
Go to the south wire (or any other calculator) and run it using the actual voltage.
Go to the south wire (or any other calculator) and run it using the actual voltage.
Need to buy/make thicker gauge wires for outside, but problem
- Thread starter whereitsat42
- Start date
seneysolar
Solar Addict
Voltage Drop Calculator | Southwire
www.southwire.com
Assuming your Vmpp will be somewhere around 90v insread of 120voc , it still comes out at 8 awg to carry 40A
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DougfromdaUP
Solar Enthusiast
i just ran 13.3 amps, 72 volts for a 65 foot circuit using #10 and got 2.5% voltage drop.
Are we talking about the same thing?
Again, that chart is for 12 volt wires exposed in engine rooms. It is useless for anything else.
Are we talking about the same thing?
Again, that chart is for 12 volt wires exposed in engine rooms. It is useless for anything else.
seneysolar
Solar Addict
We are probably using different numbers.
We dont have his exact Vmpp where the charge controller will keep that line voltage at. With Voc of 40.3, i would guess its around 32-33v??? So I just use 100v to keep things simple.
Now we have to determine acceptable voltage drop. The OP was using the 10% column as its a non critical system (it really doesn't care if voltage drops). I used 10% in the calculator as well. NEC max is 5% for most circuits but they dont have a max drop for PV because it doesn't pose any hazzard if voltage drops.
NEC does care about Ampacity of the wire as it is the ability to carry the current without overheating, no matter voltage or length.
Ampacity rating of 8awg is 40A.
So a run of 8awg will carry 40a with 7% voltage drop.
Same run with 6awg will have 4.6% voltage drop.
whereitsat42
New Member
See, I thought that chart was voltage-agnostic, hence why wiring in series is advantageous because it keeps the current down and allows you to use thinner/cheaper wire.
Brucey
Solar Wizard
Solar pv cable is most frequently available in 10 and 12AWG. 10AWG should be sufficient for anything up to 30A of current, keeping in mind that most common mc4 connectors also max out at 30A.
DougfromdaUP
Solar Enthusiast
Many people think that. It’s a shame that that chart is so often replicated without titles clarifying what it is.
As soon as voltage drop is expressed as a percentage you know that the chart is voltage specific.
Of course higher voltages and lower current leading to less loss doesn’t depend on the numbers on that chart being close to reality.
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OzSolar
Whatever you did, that's what you planned.
Size to the specs on your panel.
I bet that panel is pretty close to 10 Amps Imp and 33v Vmp
I get 10 Amps @ 99V in PERFECT conditions for each string.
Imp at NOC (normal operating conditions) will be closer to 8 amps. #10 is overkill in my book.
From Southwire.
I bet that panel is pretty close to 10 Amps Imp and 33v Vmp
I get 10 Amps @ 99V in PERFECT conditions for each string.
Imp at NOC (normal operating conditions) will be closer to 8 amps. #10 is overkill in my book.
From Southwire.
seneysolar
Solar Addict
Correction: The Southwire calc is already figuring twice the length for the circuit vs run length. So enter 65 instead of 130. The voltage drops listed above are overtstated. Thanks @DougfromdaUP .
I would still run 8awg in this scenario being the 4 strings combined out at the array could have up to 40A on the run to the house.
Brucey
Solar Wizard
I thought we were discussing the runs from the strings to the combiner box, not the home run from combiner box to house?
"Hi all, need some help: my panels are installed on ground mounts between 45-65 feet from my junction box and, like a moron, I made the classic mistake of not doubling the length of my cable runs from the panels to the box when sizing my wires, and need to upgrade from the 10 awg wires I have now. I have four inputs on my junction box, each connected to three 320w/24v panels in series (so roughly 13.3 amps/72v max apiece), and according to my consultation of THE CHART, and accepting a moderate but not horrible voltage drop, 6 awg seems to be the way to go. Only problem is I can't seem to find 50-65 foot 6 awg cables anywhere with the mc4 connectors already on them..."
seneysolar
Solar Addict
Ahh i misunderstood it as the 65ft run was with the strings combined. If the OP did 4 separate 65ft runs of 10awg theres nothing to do, its fine. Thats alot of wire... but its fine.
Brucey
Solar Wizard
Yes certainly no need to rip out the existing 10 and put 8 AWG in.
seneysolar
Solar Addict
Indeed
whereitsat42
New Member
We are indeed talking about panels to combiner box, and if the gurus here are telling me I don't need to rip all that cable out and replace it with a few hundred dollars' worth of slightly thicker wire, that's fine with me! Thanks again, all.
