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Long run 15kw off grid suggestion

Mecheng

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Joined
Aug 5, 2022
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Hello all,

I am in Northern California and living fully off grid currently. I have 2 dc solar trailers in parallel that keep up fine in the summer. In winter we run the generator quite a bit.

The trailers were always temporary and now it's time to install a big system. I calculated 15kw solar should keep my gene off in the winter if I continue to heat with wood which I have more than I can burn(100acres).

Problem is, I'm in a valley and the best panel location is 900ft from my house. I'm contemplating the best way to wire this?

Option #1 is ac coupled running 240v with inverters near the panels. Probably be 1/0 aluminum in 3" conduit.

Option #2 is dc coupled inverters by the batteries near the house. Using a newer sunny boy I get 100v mppt start voltage and up to 600vdc per string(sma max) at around 10a each using 144c double sided panels. Likely 6 or 8awg copper per string. Still running underground because we have fire danger.

Are there huge benefits or drawbacks? I'll be slightly below the 600v max, but I think thwn-2 is rated for that.

Looking at a mt solar pole mount rack top-32 likely as I have a lot of 6" sch40 pipe. Also like the thought of adjusting for winter production and maybe AC in the summer.

Thanks for your time and help.
 
Last edited:
Hello all,

I am in Northern California and living fully off grid currently. I have 2 dc solar trailers in parallel that keep up fine in the summer. In winter we run the generator quite a bit.

The trailers were always temporary and now it's time to install a big system. I calculated 15kw solar should keep my gene off in the winter if I continue to heat with wood which I have more than I can burn(100acres).

Problem is, I'm in a valley and the best panel location is 900ft from my house. I'm contemplating the best way to wire this?

Option #1 is ac coupled running 240v with inverters near the panels. Probably be 1/0 aluminum in 3" conduit.

Option #2 is dc coupled inverters by the batteries near the house. Using a newer sunny boy I get 100v mppt start voltage and up to 600vdc per string(sma max) at around 10a each using 144c double sided panels. Likely 6 or 8awg copper per string. Still running underground because we have fire danger.

Are there huge benefits or drawbacks? I'll be slightly below the 600v max, but I think thhn is rated for that.

Looking at a mt solar pole mount rack top-32 likely as I have a lot of 6" sch40 pipe. Also like the thought of adjusting for winter production and maybe AC in the summer.

Thanks for your time and help.
THHN is not rated for underground use, you would need thwn or better. Might as well use 1500V rated wire at that point…
 
AC coupled is the way I'd go IMHO.

I think you'll be hard pressed to find THHN that isn't dual rated as THHN/THWN-2 so it fine for underground as long it's in conduit.

If you haven't priced pvc conduit lately you could be stunned but then again you didn't mention budget constraints. We've done direct burial aluminum quadraplex URD (Dyke) on our last few jobs. That's what I did at my place over 20 years ago and it's still going.
 
Higher voltage means less voltage drop over the 900 feet. I didn't check your math/ampacity, but the small copper for DC sounds significantly less expensive.

You're fully off grid? If so, your solar is mostly charging a battery, so the most efficient method in that aspect would be to run DC coupled.
 
THHN is not rated for underground use, you would need thwn or better. Might as well use 1500V rated wire at that point…
Edited to thwn-2 for clarity, but yes. At this point 1500v wire wouldn't help as my inverters only allow max 600v.

I like the sunny boy inverters because the have the lowest turn on voltage for the mppt and still get to 600v per string, but I'm open to suggestions.
 
AC coupled is the way I'd go IMHO.

I think you'll be hard pressed to find THHN that isn't dual rated as THHN/THWN-2 so it fine for underground as long it's in conduit.

If you haven't priced pvc conduit lately you could be stunned but then again you didn't mention budget constraints. We've done direct burial aluminum quadraplex URD (Dyke) on our last few jobs. That's what I did at my place over 20 years ago and it's still going.
Should have mentioned that I have the 3" conduit already! Even Craigslist deal seemed pricey, but saved me 50%. Material cost is insane these days.
 
Higher voltage means less voltage drop over the 900 feet. I didn't check your math/ampacity, but the small copper for DC sounds significantly less expensive.

You're fully off grid? If so, your solar is mostly charging a battery, so the most efficient method in that aspect would be to run DC coupled.
For ~15kw I'll be making 3 parallel runs with dc coupled so at least 6 conductors. So yes, smaller wire, but more conductors which may be similar in the end.

Yes I am 100% off grid.
 
Should have mentioned that I have the 3" conduit already! Even Craigslist deal seemed pricey, but saved me 50%. Material cost is insane these days.
3" conduit it is then! ha

When the runs get long I try to compare the annual KWH losses of a few different sizes of wire to the cost of the wire. You'd have to pick your own metrics to use to decide on but remember that your array doesn't spend much time at it's full rated output which is never STC. EG: Your module may be rated to produce 10 amps at STC but that doesn't happen in reality. NOCT will be more like 8 amps and it's only doing that for a few hours day. Does it make $ense to size wire for that?

I'm doing this right now for a 300' run for a ~14kW ground mount with two Sunny Boy 6.0's at the array.

Using 2/0 aluminum vs 4 AWG aluminum results in somewhere around 300 kWH/year not lost to voltage drop. ~6000kWH total for 20 years but I'll have to spend an extra $700 on the 2/0. That's for the wire and since none of my terminals at either end might not be able to deal with that size of wire I'm going to have to get creative with polaris lugs and junction boxes to house those splices.
 
So I think I'm leaning toward DC coupled.

Using fronius primo 15kw I can run 970v DC string voltage 900ft with 2kv 6awg PV wire.

I can only get 550w panels so 20pcs gets me close to 12kw.

Should be plenty as I'm building 87kwh worth of lithium batteries. Eve lf304k in luyuan boxes with JK active 2a bms. As well as a 11kw backup diesel generator.
 
So I think I'm leaning toward DC coupled.

Using fronius primo 15kw I can run 970v DC string voltage 900ft with 2kv 6awg PV wire.

I can only get 550w panels so 20pcs gets me close to 12kw.

Should be plenty as I'm building 87kwh worth of lithium batteries. Eve lf304k in luyuan boxes with JK active 2a bms. As well as a 11kw backup diesel generator.
You are going to DIY a 970V run?
I am an electrician, and I do not carry the PPE needed to approach over 600V wiring. Electricity does funny things over 600V… please don’t become a statistic… IF you are going this route, hire qualified installers to handle the wiring…
 
Using fronius primo 15kw I can run 970v DC string voltage 900ft with 2kv 6awg PV wire.

I can only get 550w panels so 20pcs gets me close to 12kw.
So all 20 modules in series? That sound dangerously close to that inverters max Voc of 1000v. Have you checked that the Voc of the string won't exceed 1000v at lowest temps for your area?
 
I'll probably use 18 or 19 panels depending on the voltage at the mppt. No I don't want to over volt the mppt.

I'm in NorCal where temps are very rarely below freezing.
 
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