Panel wiring questions.

[NothingInCommon]

panel_wiring.PNG
Hi guys!

Does this wiring setup of 30 - 370w panels make sense to you? If so:

1) What AWG would I need to support 28.05 amps (400v)(~250ft)?

2) Does this mean only 2 wires are going to the string inverters' DC inputs?

3) Does this mean that if shade hits panels on 1 of the 3 in series the other 20 would still function uninterrupted ?

4) If I have an existing subpanel near the ground mount (as opposed to going into the main panel 250ft away) can the AC output of the inverter be simply wired to a 30amp circuit breaker in that subpanel where can back feed into the house? ( subpanel is in a shed 200ft away from the meter - but only about 40ft from where the panels would be)


Thanks in advance - yes I am a newbie

for context - to visualize exactly what Im talking about

layout_distance.PNG

[Mike90250]

400VDC transports on a smaller wire with less loss than 240VAC.

You need a 3 circuit combiner box at the panels, so that each string has a breaker of about 15 amps. The right amps will be listed on the sticker of the panel as a FUSE rating
make sure voltage spec is correct for your nearly 500V Voc (what ever the panel sticker says)

30A @ 400V would want at least 8 ga, maybe 6ga. there are wire distance/amp/volt loss calculators. Be sure you are using the right input conditions, round trip or one way. there would only be 2 wires in the conduit, and it should be well marked and protected. hit that with a shovel on a sunny day and you will wet your pants with excitement !

11Kw @ 240V = 46 amps AC Remember, your MPPT controller down converts 400v VOLTAGE into more amps at 240V

[NothingInCommon]

Excellent info brother!!!

Ok so forget using the subpanel in the shed and carry 400v@30amp DC all the way to the house(inverter) with two 6 awg cables?

[Mike90250]

I'll let you do the math. With #6 copper, for 500 feet of it, I'd look at the cost of making that run in #2 aluminum wire Going to need larger conduit but the expense is in digging the trench

[bcroe]

carry 400v@30amp DC all the way to the house(inverter) with two 6 awg cables?

With 30A through a 500 foot loop of 6 ga, you are looking at about 6V drop or 1.5%. That could be
considered OK for wire already in place, perhaps bumping to 4 gauge for new would be justified.
Bruce Roe

[Mike90250]

With 30A through a 500 foot loop of 6 ga, you are looking at about 6V drop or 1.5%. That could be
considered OK for wire already in place, perhaps bumping to 4 gauge for new would be justified.
Bruce Roe

Thanks for the calculations, I don't have good enough web at home to run the java rich ad loaded sites

[NothingInCommon]

I'll let you do the math. With #6 copper, for 500 feet of it, I'd look at the cost of making that run in #2 aluminum wire Going to need larger conduit but the expense is in digging the trench

Using the same 3" conduit that supplies the subpanel ( in shed) hopefully i can fish 2 more wires and save myself 200ft of trenching ( hopefully im not breaking any county electrical codes)

solar.png

[Mike90250]

Most areas, code prohibits mixing DC & AC in the same conduit. It's not a good idea anyway, too easy to cause a future mistake - I've turned off the AC, we're good to cut those unused wires out ZZZZZZZZZ pop

[NewBostonConst]

I don't think you can have 3 parallel strings coming in together to two wires, ran through 200 ft conduit and then separate back out to two inverters? I think you need to just pick one larger inverter.

[NothingInCommon]

I don't think you can have 3 parallel strings coming in together to two wires, ran through 200 ft conduit and then separate back out to two inverters? I think you need to just pick one larger inverter.

Didnt even think of that.. true!

[NothingInCommon]

Most areas, code prohibits mixing DC & AC in the same conduit. It's not a good idea anyway, too easy to cause a future mistake - I've turned off the AC, we're good to cut those unused wires out ZZZZZZZZZ pop

https://forum.solar-electric.com/dis...n-same-conduit

Looks like someone else had a similar idea. Cant really tell if NEC prohibits this practice. This will be a real bummer if I can't

[SunEagle]

https://forum.solar-electric.com/dis...n-same-conduit

Looks like someone else had a similar idea. Cant really tell if NEC prohibits this practice. This will be a real bummer if I can't

The NEC will prohibit the practice of running wires with AC & DC voltages in the same conduit. You can run them in the same wire tray if there is a barrier but never in a pipe where they can touch each other.

[bcroe]

The NEC will prohibit the practice of running wires with AC & DC voltages in the same conduit. You can run them
in the same wire tray if there is a barrier but never in a pipe where they can touch each other.

My wiring runs hundreds of feet underground too. The solar DC is in conduit, but I drop direct burial 12-2 into
the trench so there can be convenience 120VAC outlets on the array. Voltage regulation not great out there,
but V starts pretty high, and electric drills and lights are not fussy about it. Bruce Roe

[SunEagle]

My wiring runs hundreds of feet underground too. The solar DC is in conduit, but I drop direct burial 12-2 into
the trench so there can be convenience 120VAC outlets on the array. Voltage regulation not great out there,
but V starts pretty high, and electric drills and lights are not fussy about it. Bruce Roe

A direct burial AC wire run next to DC in conduit is acceptable since the conduit is the "barrier" between the AC & DC wires.

[JSchnee21]

Alternatively, if you put the inverters proximal to the array. You'd be looking at ~225 feet of 120V AC @ 47 amps (times 2 for 240V split phase) for which Southwire's tool recommends 3 AWG (4.13% voltage drop -- 5V). Or 2.62% drop (3.1V) with 1 AWG (both copper in this case).

This will cost you more in wire, but if you can re-use the existing conduit, it may be cheaper overall. You could also use a separate inverter per string (3) or split into two strings, and run multiple sets of smaller gauge wiring but this will likely cost even more (multiple inverters to buy, many more runs of wire).

I still agree running the DC would be best, but I don't know how much retrenching will cost. Can you rent a ditch witch?

One other aspect to consider (though electrical code won't cut you any slack) is that for 20+ hours per day, your currents (regardless of voltage) won't be anywhere near maximal. So the voltage drop will be pretty minimal during all but peak sun hours.

3 AWG copper is currently about ~$0.99 per foot (L1, L2 and ?neutral? = 675 feet, smaller or reuse existing ground) per inverter Not sure of these are 3-wire (HHG) or 4 wire (HHNG).
1 AWG is ~$1.65 per foot. 6 AWG is ~$0.57 / ft.

Pricing from www dot wireandcableyourway dot com

"4) If I have an existing subpanel near the ground mount (as opposed to going into the main panel 250ft away) can the AC output of the inverter be simply wired to a 30amp circuit breaker in that subpanel where can back feed into the house? ( subpanel is in a shed 200ft away from the meter - but only about 40ft from where the panels would be)"

Unlikely to pass inspection. But what size sub panel? How big is the breaker feeding it? Wire gauge from house? Could be feasible if panel, breaker, wire are big enough -- but unlikely for such a large PV array. I believe this would generally follow the same 120% rule for a load side tap -- I believe this is permissible on a branch, but I'm not an electrician.