Help me make sure I understand running DC high voltage wires buried in conduit back to inverter from remote (125ft) array

[cdherman]

I am contemplating putting some underground wires to get additional panels on a shed that would be around 125ft away from my inverter. This was suggested in a couple other threads. Another suggestion was to run the wires in conduit. Allowing for upgrades in the future, if need be.

I am looking at some 405w 34.73v panels. 14 in a row gives me 486v, 5670w. That calculates to 11.66A

I threw those numbers into a DC wiring calculator online. Using 125', 1% voltage loss, it says 11AWG is enough.

Do I need to over rate to 125% for safety? If panels are rated as above, they don't go over much, right?

What kind of wire? THWN rated to 90c? Or could I even get by with say 10/2 Romax? But that's not supposed to be buried, even in conduit, I think....... Something about not wet tolerant. Would be the cheapest.. I think I know that answer, just musing.....

As noted by some other helpful person, running the DC wires back to the inverter seems like it might be much cheaper than putting the inverter in the "wrong" place. In my case, I have a great place for an AIO inverter, batteries, new entrance pole, disconnects etc. Just not a great place there for the panels.

 

[#40Fan]

Any chance in the future you would need a larger wire?

It would suck if you had to:

A- Rewire everything

B- Pay more again

 

[Bluedog225]

Following. Considering something similar.

That voltage concerns me but it seems a valid way to go.

Also, I wonder what type of pulling rope or strap you would leave in the conduit for future pulls. I’m thinking something that would not melt or catch fire.

 

[cdherman]

Following. Considering something similar.

That voltage concerns me but it seems a valid way to go.

Also, I wonder what type of pulling rope or strap you would leave in the conduit for future pulls. I’m thinking something that would not melt or catch fire.

Yeah, I think some good poly rope or nylon in the conduit as well. And probably 10AWG cables. I've been reading about conduit. Does not need to be proper conduit if I use the right wire -- some people use unperforated drainage tile (stuff you can get in 250' roles). Makes pulling wires easier.

Mostly, I am curious about rules/de-rating of HV DC. I just don't know much about that. It makes sense that higher voltage lines can carry more watts at lower amperage -- that's a given. But DC over a greater distance -- I guess we were all taught that Edison had problems with his DC idea's because you cannot do DC over distances. New to this idea. Looking for re-assurance.......

 

[robby]

Yep use good conduit and 10 AWG wire and I highly suggest that you put concrete around the pipe as your laying it. Your dealing with nearly 600V and it would not be good if someone dug down and made contact with that wire. Also make the pipe big enough that you can double the amount of wires in case you decide to expand later on. Pulling through some Dacron cord is also a must.

 

[MajicDiver]

Mostly, I am curious about rules/de-rating of HV DC. I just don't know much about that. It makes sense that higher voltage lines can carry more watts at lower amperage -- that's a given.

I’m using awg10 THHN/ type TC/TC-ER cable rated for direct burial - 4 conductors in one cable, then two cables with 2 conductors each. It’s direct buried and a distance of 240 feet one way. It’s rated 600 vdc, my VOC gets to about 525vdc on 10 degree day which almost never happens here. IMO, and I’m a mechanical engineer, as long as your VOC is within the voltage rating of the cable you need not worry about the high voltage. I thought I would only need 3 pair so I buried 4 pair, and used the 4th 6 months after my initial install. That was 18 months ago. I’ll likely add another at some point and will use a separate trench - so yes, as others stated, if you can afford to, either put in spares or make it easy to do in future. I’m getting upwards of 13 amps most of the year in full sun.

 

[Bluedog225]

Covering with some concrete is a good idea. And definitely metal marker tape. Hitting 600 volts direct current with a shovel would suck.

 

[Skypower]

Well I just deleted most because others stole my thunder LOL. THHN 10 AWG 90c? Warning tape ? I’d use 3/4” gray pvc because bigger is easier to pull and the cost difference is insignificant. Always pull into the flared end and if you use a coupling make sure you break the edges. Pulling more than a total of 90 degree bends can be difficult so plan in sections or entrance boxes.

 

[Bluedog225]

Pull into the fair?

And break the edges?=ease them with a file?

Thanks

 

[cdherman]

Couple of you folks keep saying THHN. But its outdoor conduit and my understanding is that you need THWN. W in there is for water resistent. That's really what make basic Romax (MN-B) no go in the ground anywhere, even in a conduit. And THHN is not water resistent rated.

Or do you know something I am missing? I *think* a LOT of THHN wire is also THWN rated, and perhaps THHN is just the common term. Wanting to be clear however.

 

[DIYrich]

But DC over a greater distance -- I guess we were all taught that Edison had problems with his DC idea's because you cannot do DC over distances. New to this idea. Looking for re-assurance.......

DC is more efficient than AC. The problem with DC in Edison's time is that they didn't know how to boost the DC voltage, so the higher AC voltage (transformers) won out. The higher the voltage, the lower the transmission loss.

 

[MajicDiver]

And THHN is not water resistent rated.

My referring to thhn can be confusing- thhn is not rated for direct burial, my cable is thhn with a pvc jacket and is rated for direct burial. At the time I was buying stuff that’s the most cost effective way I could find to do it. It’s also rated TC and TC-ER. And I was putting in a well at the same time, the PV cables are in same trench under a 1” PEX water pipe and pump power cable. The trench is marked with T posts that make it obvious- I agree with others about protecting it all, but I chose to just make it obvious to see it’s there.

 

[Skypower]

Pull into the fair?

And break the edges?=ease them with a file?

Thanks

Oops, flared
I hate auto correct

 

[DIYrich]

Does not need to be proper conduit if I use the right wire -- some people use unperforated drainage tile (stuff you can get in 250' roles). Makes pulling wires easier.

Mostly, I am curious about rules/de-rating of HV DC. I just don't know much about that.

If you don't use proper conduit, then it is considered direct burial, and must be deeper.

120% margin is also required.

Derating is optional as it refers to voltage drop.

 

[Zapper77]

Yeah, I think some good poly rope or nylon in the conduit as well. And probably 10AWG cables. I've been reading about conduit. Does not need to be proper conduit if I use the right wire -- some people use unperforated drainage tile (stuff you can get in 250' roles). Makes pulling wires easier.

Mostly, I am curious about rules/de-rating of HV DC. I just don't know much about that. It makes sense that higher voltage lines can carry more watts at lower amperage -- that's a given. But DC over a greater distance -- I guess we were all taught that Edison had problems with his DC idea's because you cannot do DC over distances. New to this idea. Looking for re-assurance.......

Long distance HVDC transmission lines are becoming more common.

High-voltage direct current - Wikipedia

en.m.wikipedia.org

Multiple 1000-3000 mile HVDC lines.

 

[Zwy]

Following. Considering something similar.

That voltage concerns me but it seems a valid way to go.

Also, I wonder what type of pulling rope or strap you would leave in the conduit for future pulls. I’m thinking something that would not melt or catch fire.

Long runs, you will be pulling any wire back out and adding new wires to pull it all thru together.

Wires inside conduit will twist together. The mule tape or rope can wear thru insulation as you pull leading to short circuits.

If you think you will need more circuits later, best to run the wires with the initial pull.

 

[CoolWill]

Yep use good conduit and 10 AWG wire and I highly suggest that you put concrete around the pipe as your laying it. Your dealing with nearly 600V and it would not be good if someone dug down and made contact with that wire. Also make the pipe big enough that you can double the amount of wires in case you decide to expand later on. Pulling through some Dacron cord is also a must.

Covering with some concrete is a good idea. And definitely metal marker tape. Hitting 600 volts direct current with a shovel would suck.

You guys are forgetting how all this works. It would be much more dangerous to hit a 120 volt line on a grounded system than a 600 volt line on an ungrounded system. Using concrete is an unnecessary expense unless the conduit is too shallow to meet code requirements. And then you need a minimum of 2" of concrete cover.

 

[Mike 134]

Use PVC conduit buried to the proper depth, THHN is almost always dual rated on the jacket thhn/thwn, you don't double derate. Your 11amp load on 10ga wire is more than enough to satisfy the 80% rule for continuous loads.
Never in 38 years of pulling conduit have I had trouble later adding wires in a 3/4 pipe with 2-#10s in it. Never use string in a PVC conduit unless you've put steel elbows at the 90s otherwise you'll burn through the PVC. If you think you'll want to add wires later put a 14 ga THHN in as your fish tape.

 

[CoolWill]

And any derating that you might have to do on THWN has to start at 75 degrees C instead of 90 degrees C. Because THHN is 90, and THWN is 75, even if it's the same wire. I don't think anything needs to be derated, I'm just stating that for educational purposes.

 

[rcrracer]

"I threw those numbers into a DC wiring calculator online. Using 125'"
Did the calculator calculate using round trip?