Ground mount 650 ft. wire run - planning for future expansion

System · October 2010

I'm a noob. Having a 4kw ground mount system installed. It is 650 ft. from
Panels (2 strings of 9 panels) and inverter to the house. My understanding from installer is that if I go to 8kw or 12 kw in future, I just run more wire (assuming I ensure conduit size allows for more wire). Other possibility is to put extra wire in the conduit now - extra expense now but I'm thinking wire cost may soar in near future. Appreciate any comments.

mike95490 · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

It's tough to pull wire in a condouit with wire in it allready. I'd lay an extra conduit in the ground (if it's not too late).

Or size wire NOW to handle future amps, or will these be split to different inverters?

Check into the max size you can gridtie, my area it's 10KW limit per meter drop.

solarvic · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

How many feet is your inverter from the dc source solar panels, and how far are you running the AC from inverter to grid tie in? I hope someone gives you some good answers as I am thinking of adding another small inverter and 1 string of 8 180 watt panels. Presently my inverters are about 75 feet from my solar combiner box and about 100 feet run of 220v from inverters to meter. I know my ac line is big enough #8 copper but would need to add the dc wire from combiner box for the extra panels. :DSolarvic:D

dwh · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

Always easier to do it now.

20 years ago I did complete re-wire on a storefront owned by a buddy's dad. There was no sign on the building (it was painted on the plate glass). He said he'd never had a sign and never would. I was pulling 6 wires to the ceiling box anyway, so what the hey, I ran an extra for a sign - just in case.

Last week he calls me up. Guess what? Yup, he's having a sign installed and wants to know how much trouble it would be to run power for it.

I went over there, hopped up on the ladder, opened up the 4-0 pullbox and pulled down the capped off spare wire. Told him, "This wire - it's for the sign. That switch there - that's for the sign."

He said, "I always wondered what that switch did..."

Pulling more wire 650' through a conduit that already has wire in it could be a nightmare. I would do what Mike suggested and lay in a spare conduit if possible.

If not, then pull oversize wire now that can handle the present and future needs. Even if you don't end up using it, it won't hurt and it will minimize voltage drop on such a long run.

But, if the installer put in pullboxes every 100' or so, then pulling more wire later probably won't be a big deal.

System2 · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

Thanks all. Inverter is 30 ft. from panels; 650 ft. from inverter to grid. "or size wire now to handle future amps"; "pull oversize wire". Hmmm, I must have some wrong data, or I misunderstood - I got impression from installer that running wire for 8kw system would not work well with 4kw system (??), hence his idea of running more wire in the future if I double the panels. I was NOT planning on a split to different inverters. I just figured I'd upgrade the inverter if I went to 8kw. Maybe he's worried I'll try to squeeze him for more expensive wire (I won't). No, if I can run wire NOW which will work fine now AND in the future if I double the panels then that's what I'll probably do (I'm excited about prospect of solar charging of electric car - some are doing it now, but I think in the next few years it might be a practical option for me). Or, as Mike says, at least run a second conduit (trenching not yet started).

System2 · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

Oh, and if I might ask: if I did have an 8kw system (current installation calls for Sunpower 230 panels), what would be an acceptable wire size for a 650 ft. run?

dwh · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

Well, 8kw is 33.3 amps @ 240 volts.

You have to multiply that by 1.25 to get the minimum breaker size. That would be 41a.

The next common size up from that would be 50a.

(Can your main panel handle a 50a feed breaker from the solar? You can only go 120% of the panel's rating on feed breakers, so a 200a main with a 200a breaker from the utility could only accept up to a 40a additional feed breaker from the PV and you would have to downsize your inverter to 30a max output.)

50a rated wire would require a minimum wire size of #8 if it was THHN wire.

However, #8 would have a 33v drop over 650' and would only supply 207v at the main panel. #3 would carry it with a voltage drop of 5% to 230v.

http://www.electrical-designer-guide.com/electrical-wire-size.html
http://www.csgnetwork.com/voltagedropcalc.html

Cut that in half for a 4kw system.

What wire was your installer planning to use?

And yes, you can use oversized wire - and for a very long run like yours you have to to avoid excessive voltage drop.

Another alternative would be to put a 240v-480v transformer at each end, and step it up to 480v for the long run and step back down at the other end. Then you could cut the wire size in half.

EDIT: Oh, and if you were to use wire now sized for a 4kw system and then double the size of the PV and inverter at a late date - you would not be pulling in additional wire at that time...you would be pulling out the existing wire and replacing it with bigger wire.

mike95490 · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

Or run the DC high voltage thru the 650'

8KW @ 500V = 16A which 4 ga alum (6ga copper) gives 2.2% drop

Wire voltage drop Calculators :
http://www.nooutage.com/vdrop.htm
http://www.electrician2.com/vd_calculator.htm
http://www.elec-toolbox.com/calculators/voltdrop.htm

And I found these nifty splice blocks for alum/copper
Power Distribution Blocks with covers, make painless transitions al/cu (and use the anti-ox goop too)

System2 · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

Thanks so much dwh and Mike. This is very helpful. I've checked out the calculators. No, my service is 200 so I would have to limit inverter to 30a (or a 7.5kw system could use a 40a breaker if I follow the math correctly.) I think I could live with that-I'm in the mountains and heat/cooking is wood/propane. 4kw may end up being plenty, but my thinking in these times is to plan for abundance, not scarcity. I will look into both the AC and DC solutions you've recommended, and price out the wire and probably get the lower gauge now (and Uncle Sam's discount on it). Thanks again. Bob

solarvic · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

ltlredwagon wrote: »

Thanks so much dwh and Mike. This is very helpful. I've checked out the calculators. No, my service is 200 so I would have to limit inverter to 30a (or a 7.5kw system could use a 40a breaker if I follow the math correctly.) I think I could live with that-I'm in the mountains and heat/cooking is wood/propane. 4kw may end up being plenty, but my thinking in these times is to plan for abundance, not scarcity. I will look into both the AC and DC solutions you've recommended, and price out the wire and probably get the lower gauge now (and Uncle Sam's discount on it). Thanks again. Bob

With my system I had to get 2 inverters because of having 2 different brands of solar panels. You could have the same problem if when you want to grow your system you might have to get a 2nd inverter because the panels you now have might be discontinued and you can,t get anymore of the same. Like right now I wish I could find 4 more Kyocera kc 158 and 2 or 3 more sharp 167 watt panels.
:Dsolarvic:D

System2 · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

Gentleman, I wanted to ask if any of you have looked at any new solar mounting or racking solutions? I was at the California conference last week, and we received information on a more user friendly product and it was almost half the cost of my Unirack product line. Did anyone hear of this company if you were at the event?

mike95490 · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

solarvic wrote: »

With my system I had to get 2 inverters because of having 2 different brands of solar panels.

No, it's because you had voltage mismatches, or current limits. Only the pos ground Panels need a special wired inverter. The brand label on the panel does not need to have a special brand label on the inverter.

solarvic · October 2010

Re: Ground mount 650 ft. wire run - planning for future expansion

mike90045 wrote: »

No, it's because you had voltage mismatches, or current limits. Only the pos ground Panels need a special wired inverter. The brand label on the panel does not need to have a special brand label on the inverter.

Mike it is irevelant what brands my solar panels are. I was trying ro get the point accross to get the smaller inverter for present unless he plans on growing the size soon. I first bought one brand and model of solar panels and later on they were no longer made and I couldn,t get any more of the same so had to buy different brand and size of solar panels that didn,t match then. In my personal case I think it was about the same cash outlay for 2 smaller inverters as one bigger one would be. :cool:Solarvic:cool:

Ground mount 650 ft. wire run - planning for future expansion

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