Subpanel w/ separate service

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I have been reading the forum for a while in the background but now I have a question that I can't find the answer too.

We are wiring a small guest house that is going to be fed from its own 200A service. The existing house has its own 200A service that is maxed out. The guest house is for the owners son who has some disabilities, and he wants to install a generator that will backup the guest house. My question is, can I install a sub panel in the existing house, fed from the guest house (backup power)? The owner is not overly concerned about having his whole house up during an outage, he just wants some lights and the fridge etc. I am thinking it is OK, as long as it has its own disconnect, all neutrals are separate, and it is correctly labeled? I want to make sure I am not missing or overlooking anything.

Thank you
 
Welcome to the forum.

It sounds like you just want to supply two houses from one generator.

Shouldn't be a problem.

Just don't try to use the same conductors to feed power both ways.
 
LarryFine said:
Welcome to the forum.

It sounds like you just want to supply two houses from one generator.

Shouldn't be a problem.

Just don't try to use the same conductors to feed power both ways.
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Thank you

Yes, I am trying to use one smaller generator (~22kw) and a single transfer switch to supply two separate houses with two separate services. We were originally thinking of going with a larger liquid cooled unit with two separate 3 pole transfer switches, and having each house backed up 100%, but the owner did not think the extra cost was worth it for the amount of times they lose power.
 
pay close attention to where the various bonding jumpers are; you may want to use three -pole transfer switches and switch the neutral.

Also, would this run afoul of two services (or sources of energy) to one structure? I'd have to think about that.
 
LarryFine said:
You will need two transfer switches.
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Well, that was the original question, whether you have to run generator power from building 2 to building 1, necessitating a transfer switch at building 1. Or if you could just run a feeder from the load side of building one's transfer switch directly to a backed up panel at building 1.

If you say you need two transfer switches, what code section rules out the second option?

Cheers, Wayne
 
wwhitney said:
Well, that was the original question, whether you have to run generator power from building 2 to building 1, necessitating a transfer switch at building 1. Or if you could just run a feeder from the load side of building one's transfer switch directly to a backed up panel at building 1.

If you say you need two transfer switches, what code section rules out the second option?

Cheers, Wayne
Click to expand...


You still need some way to isolate the panel at Bldg 1 when on Gen power.
 
Seven-Delta-FortyOne said:
You still need some way to isolate the panel at Bldg 1 when on Gen power.
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The "backed-up" feeder from Bldg 2 to Bldg 1 would certainly need a disconnect at Bldg 1, just like any Article 225 application.

But when the grid is up, the backed up panel at Bldg 1 would be supplied by grid power going to Bldg 2, through the transfer switch, and through the feeder to the backed up panel in Bldg 1. The transfer switch at Bldg 2 would handle transition to generator power.

Cheers, Wayne
 
wwhitney said:
But when the grid is up, the backed up panel at Bldg 1 would be supplied by grid power going to Bldg 2, through the transfer switch, and through the feeder to the backed up panel in Bldg 1.
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There's the "two sources of utility power" issue.

Why not just run a generator feeder and use a second ATS?

(I hadn't realized you meant an automatic T/S.)
 
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LarryFine said:
There's the "two sources of utility power" issue.
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First, Article 230 and 225 aren't currently coordinated in their language; nothing prohibits a building from being supplied by one service and one feeder. I submitted a PI for 2026 to coordinate the relevant sections.

But even if you want to apply 230.2 to the situation, 230.2(A)(4) permits an additional service for an "optional standby system." Which certainly covers the situation under discussion. A building may have 2 sources of supply when one source is the normal grid, and one source is an optional standby system.

Cheers, Wayne
 
LarryFine said:
Is it still okay when the optional source is also normal power via the ATS's utility input?
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Well, 230.2(A)(4) says that "additional services shall be permitted to supply optional standby systems." Which means to me that when the grid up, you don't need to have all the grid-connected equipment connected to just one service. The equipment that is supplied via the optional standby system may be on a different service.

Is there some other way to interpret what 230.2(A)(4) means?

Cheers, Wayne
 
wwhitney said:
Well, 230.2(A)(4) says that "additional services shall be permitted to supply optional standby systems." Which means to me that when the grid up, you don't need to have all the grid-connected equipment connected to just one service. The equipment that is supplied via the optional standby system may be on a different service.

Is there some other way to interpret what 230.2(A)(4) means?

Cheers, Wayne
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I appreciate the response. I agree with your interpretation, I am going to have a conversation with the inspector just to make sure everyone is on the same page.

Thank you
 
I agree that you will need two transfer switches with the generator connected to both of them.
 
ggunn said:
I agree that you will need two transfer switches with the generator connected to both of them.
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Certainly you are going to need service conductors to each building, as well as an extra set of conductors to each building.

If you choose to run generator conductors to each building, then you will need transfer switches at each building.

But you could instead choose to run generator conductors to only one building, which has a transfer switch, and then run a backed up feeder from that transfer switch to the other building. Only one transfer switch in this scenario.

Cheers, Wayne
 
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