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Wire material selection

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kscessnadriver

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Dec 13, 2023
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Working on buying wire to run from my ground mounted array to the house where the inverter is going to be located. It's going to be contained in conduit, so basically any wet rated wire should work. Is there any practical reason to select copper over aluminum on a run of about 300 ft? Yes, you have to go higher gauge with AL wire, but I think I found a deal on XHHW-2 that would make it cheaper than the copper THWN-2 I could use. So practically, is there a reason to avoid aluminum wire?
 
The old growatt has a lower voltage mppt. Combing that with a large'ish panel array ( low voltage, high amperage) and long run necessitates a larger gauge wire. I used 2/2/4 AL URD service entrance wire, more commonly called triplex, for the main run. In many areas, it does not have to be in conduit and was much cheaper than anything else.
 
Warpspeed said:
Three hundred feet is a fair distance, six hundred feet of wire.
It might be a good idea to work out the minimum acceptable wire gauge versus voltage drop required, if you have not already done so.
https://www.engineeringtoolbox.com/copper-aluminum-conductor-resistance-d_1877.html
Click to expand...

It's not a minimum voltage drop required issue. It's more of an issue with derating in said conduit, and then cost. Why spend $0.60+/ft for copper if I can find suitable aluminum for nearly half that. I'm looking at running a string with 21A, with 4 strings in a single conduit, so 8 conductors. Plus I'd like to derate a bit beyond what is legally required, just for peace of mind. So minimum of 8AWG copper (probably 6 to be honest). Aluminum, probably moving up to 4AWG. It depends if the local AHJ wants me to derate for both # of conductors in the conduit and a temperature derate.
 
Warpspeed said:
Not going to argue with you.
Its entirely possible to run higher temperature rated cable well within its thermal rating, and end up losing half your voltage along the way.
Click to expand...

Then educate me on why I’m wrong? I understand the voltage drop concept, I also understand derating is required by the NEC
 
teal95 said:
Most of us run a heavier gauge of wire for the PV connections than required by NEC simply to reduce the power loss.
Click to expand...

Exactly. Thats why I was planning on maybe going a size higher that I absolutely have to. My biggest question is just simply AL vs CU, but I'm not sure it really makes much of a difference, as long as they're properly installed
 
kscessnadriver said:
It's not a minimum voltage drop required issue. It's more of an issue with derating in said conduit, and then cost. Why spend $0.60+/ft for copper if I can find suitable aluminum for nearly half that. I'm looking at running a string with 21A, with 4 strings in a single conduit, so 8 conductors. Plus I'd like to derate a bit beyond what is legally required, just for peace of mind. So minimum of 8AWG copper (probably 6 to be honest). Aluminum, probably moving up to 4AWG. It depends if the local AHJ wants me to derate for both # of conductors in the conduit and a temperature derate.
Click to expand...
21 amps, 300', at what voltage, what inverter?
 
Last edited:
kscessnadriver said:
It's not a minimum voltage drop required issue. It's more of an issue with derating in said conduit, and then cost. Why spend $0.60+/ft for copper if I can find suitable aluminum for nearly half that. I'm looking at running a string with 21A, with 4 strings in a single conduit, so 8 conductors. Plus I'd like to derate a bit beyond what is legally required, just for peace of mind. So minimum of 8AWG copper (probably 6 to be honest). Aluminum, probably moving up to 4AWG. It depends if the local AHJ wants me to derate for both # of conductors in the conduit and a temperature derate.
Click to expand...
8/2 UF wire is pretty cheap, all things considered.

Why not combine all the strings first, then make your 300ft run in 1 shot? Then it might make sense to run your heavy aluminum.
 
seneysolar said:
8/2 UF wire is pretty cheap, all things considered.

Why not combine all the strings first, then make your 300ft run in 1 shot? Then it might make sense to run your heavy aluminum.
Click to expand...

Because the inverter MPPT's don't support that kind of amperage. Each inverter has 3 MPPTs, each capable of 22A at most.
 
You will probably find your solar equipment terminals say copper only.

You could transition from AL to copper, but I would just stick with copper thhn.
 
I am team don't voltage drop calculate the solar run. If they're single strings 12 or 10 awg is fine as appropriate for the amperage If they're combined strings, it gets trickier.

Just always confirm what the terminals accept, and don't assume they accept AL or large gauge wire.
 
hwy17 said:
I am team don't voltage drop calculate the solar run. If they're single strings 12 or 10 awg is fine as appropriate for the amperage If they're combined strings, it gets trickier.

Just always confirm what the terminals accept, and don't assume they accept AL or large gauge wire.
Click to expand...

Yeah, it would be easier to just run them all as single string 10AWG. But I'm hopefully future-proofing myself by combining 2 strings into 1, should I need to add more panels later.

I would only use the AL from the disconnect at the array to the external DC disconnect at the residence. Beyond that, I'll for sure be using THWN CU.
 
Voltage drop rises as voltage decreases but still around the 3% mark and the array may, or may not be operating continuously at max. Not a lot of room to grow, unless upping the mppt voltage. Most new inverters are pushing towards higher voltages anyway so yeah, the smaller wire may be fine and have room to grow.
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