Backfeeding SubPanels from Solar Arrays

[Belldiver]

Hi folks,

A question about backfeeding a subpanel vs. main service panel. I've searched the indexes and found a few similar threads, but not quite the same. Thought I'd throw this out there for opinions.

I'm taking bids on a ~9.1 Kw Grid-Tie (only) solar array that will be mounted on the backside of my shop, not my actual house roofline. The panels will be approximately 180' from my main service meter (and main service panel). The main service panel is a 200 amp service, with a 60 amp breaker that feeds a 200 amp sub-panel in the shop near where the solar panels will be installed. I believe the copper feeding the subpanel is #4 or #6, but I'll have to verify for sure. It is definately larger than #8 though.

My question is... Can I backfeed the subpanel with the 9.1 Kw array? Or do I need to bury another run of dedicated copper all the way up to the main panel/meter? Is there some reason I would NOT want to do this? Code? Safety?

One more consideration, I plan on installing a transfer switch somewhere in the system for a (future) ~12-14Kw propane generator. Where would this fit into the system so the generator and panels don't fry each other? (I know they can't work together)

I can mount that generator either up by the house or back by the shop, but the bulk propane tank will have to be back by the shop due to some physical space limitations of keeping the propane tank away from electrical sources and buildings. I may have to bury a LP fuel line for that run. Those are some logisitics I can work out later.

Thanks for any input.

Jim

[Belldiver]

Bump

Just a bump for any opinions...

[Mike90250]

It's possible. It all depends on the local electrical code, and what they allow in your area.

backfeeding a sub panel can work.

Long 240VAC AC lines can work, but it's better to run the HV DC that distance, it will cause less problems with the inverter and false tripping off-line.

Only inverter style generators must be isolated from the PV inverter. Conventional generators will never be stable enough to qualify the AC for the inverter to power up. But all generators should have their own transfer switch anyway.

[Belldiver]

AC versus DC

I've discussed that 180' run with both conractors submitting bids, and they both want to run it 240v AC. One with a single inverter, and the other with Enphase micros. I've specifically brought the long higher voltage DC run up and they don't seem very enthused. Contractor #1 does say they'll design the system for ~1% voltage drop though, and mentions the high voltage DC run being more dangerous.

If they size the copper runs correctly (#6?), is there a difference between 240v AC versus 400-500v DC, other than having to oversize the wire? I know the amperage drops, but you mention false tripping of the inverter?

As far as location of the (future) generator. I've pretty well answered my own question with a little more research. I know where it has to tie in to be legal, just a question of burying more copper to keep the generator back by the shop near the propane tank, or burying LP gas line of equal length to fuel the generator up by the house. Either way, the fuel source is back by the shop.

Thanks,

Jim

[Naptown]

On long runs I tend to run the DC high voltage and place the inverter next to the house service. This reduces the false tripping due to the voltage rise at the inverter and going out of voltage range.
Now if the wires are large enough (They will most likely need to be #4 copper to account for voltage drop) you can tap into the sub panel BEFORE the main breaker with a fused disconnect there. You will also need to set a disconnect by the meter and perhaps one inside at the panel. If you tie into the panel you are limited to 120% of the bus bar rating on the panel. This means that a 200A panel you can only feed 40A into it maximum.

[ppreceipt]

Yep, you'll need to get that 200amp breaker downsized to a 175amp one
Siemens makes one, as well as GE and Square D.

they are hard as heck to find, most electricians have never even heard of them. Just went through this headache myself.

[inetdog]

Yep, you'll need to get that 200amp breaker downsized to a 175amp one
Siemens makes one, as well as GE and Square D.

they are hard as heck to find, most electricians have never even heard of them. Just went through this headache myself.

The other thing to check (which is almost as frustrating to get an authoritative answer) is what the BUS rating on the panel you have now is.

Some manufacturers furnish 225 amp or higher rated bus bars in a panel supplied with a 200 amp main breaker. If that is the case, then you could avoid getting a smaller main breaker. For example, if the bus rating is 225, then 120% is 270, and you could back-feed up to 70 amps while keeping the 200 amp main breaker.

[green]

If you tie into the panel you are limited to 120% of the bus bar rating on the panel. This means that a 200A panel you can only feed 40A into it maximum.

Could you guys explain this one for me. I'm gonna need to know this. Do you mean all the used breaker ratings added up? Or is there a limit for the rated service for backfeeding? Help me out here it's just not clicking.

Thanks,
Green

[inetdog]

Could you guys explain this one for me. I'm gonna need to know this. Do you mean all the used breaker ratings added up? Or is there a limit for the rated service for backfeeding? Help me out here it's just not clicking.

Thanks,
Green

The bus bar (hunk of metal that the breakers all connect to, that carries current from one to another) can handle a certain maximum amount of current continuously. In a normal situation, that current will come from the main breaker only, and will go to whatever loads are currently turned on. Regardless of what size and how many load breakers there are, they cannot pull more total current from the bus than what the main breaker puts in.

Now look at a grid tied inverter backfeeding: In the worst case, the loads can pull all of the current that the main breaker can supply PLUS all of the current that the GT can supply through its breaker(s). To avoid overloading the bus, the NEC says that the sum of the main and the GT cannot exceed 120% of the bus bar rating, and also that the GT breaker should be located at the opposite end of the bus from the main.

It has nothing to do with how much power you ever draw, or what the sum of the load breakers is.

This same limit also applies to the main breaker and bus bar rating of any sub-panel that has any loads other than the GT breaker connected to it as well as the backfeed breaker.

[NorthwestPV]

The 120% rule not only applies to the busbars of both panels but also the conductors from the service to the sub-panel. 705.12(D)(2)

[Naptown]

/\
To the post above this is almost correct
this is because the load (or in this case feed) can last longer than 3 hours. This makes it a continuous load which will require conductors rated at 125% of the maximum load or feed.

[green]

So if I have 100 Amp service I can only backfeed through a single 20 A breaker. Or is it 20A on each leg in the box or would it be 10A each leg.

My guess (and I don't want to be guessing with this) it is 20A total on 100A service. Just want to be sure.

Thank you for your patience,
Green

[Naptown]

With a 100A 240 volt service you could backfeed with a maximum of a 20A 2 pole or double breaker.
However check with the manufacturer it generally costs them more to produce different rated buss assemblies than to use the same buss bars in say a 100 amp panel and a 200A panel.

[green]

Ok that makes sence. Thank you.

Green

[NorthwestPV]

/\
To the post above this is almost correct
this is because the load (or in this case feed) can last longer than 3 hours. This makes it a continuous load which will require conductors rated at 125% of the maximum load or feed.

Have you read 705.12(D)(2)