Transfer Switch Sizing

[Little Bill]

I don't have much experience with transfer switches but I think I know what would determine the size of one. If my understanding of this is wrong, please correct me.

Among others, I think the size would be determined by either the load being served or if in the case where the transfer switch is service rated and is being used as the first means of disconnect w/OCP.

What I've run into is a customer that has multiple houses on his property. He has a house, his Mother has a house, and his sister has a house.
He has a 20KW Generac on his house with a 200A ATS. His setup is he has a sub panel with all the circuits in it that are to be backed up. The panel is fed like this:

the POCO (line) side of the ATS is fed from a 100A breaker from one of his main panels
of course the other side from the 20KW generator
the ATS is outside and is fed from the generator and from the panel inside, then back into the house where the backed-up sub panel is.

Since the ATS is not being used as the service and only being supplied 100A from the utility side, and the generator is only a 20KW or 83.33A, I say he only needed a 100A transfer switch.
This discussion came about because he has ask me to connect another generator and ATS to his mother's house. He had mentioned the high cost of the transfer switch on his and I told him he could have used a 100A one. But he said the electrician that installed it told him he had to use a 200A ATS. I told him I was pretty sure he could have just used a 100A.

I think the electrician was confused because the panel feeding it was a 200A panel. But since the ATS wasn't being used as service equipment, or between the meter and his MDP, the 200A was unnecessary.

Am I correct on this?

 

[ceb58]

Am I correct on this?

Yes

 

[Smart $]

Yes.

 

[Little Bill]

Yes

Thanks!

Yes.

Thanks!!

 

[augie47]

You might save him another $ by picking an ATS that is not service rated. Since you have OCP ahead of the ATS there is no need for it to have internal OCP or be service rated.

 

[Little Bill]

You might save him another $ by picking an ATS that is not service rated. Since you have OCP ahead of the ATS there is no need for it to have internal OCP or be service rated.

Thanks Gus! That is if his other generator/backup can be set up can be done like his first one. Or if he wants it set up that way. The 2nd house is very similar to his house with the genny in that it is a 400A service with two 200A panels inside. I don't know yet what loads he wants backed up, or if there might be an easier way to do the next one.

Everything looked fairly easy to do with the exception of getting the desired circuits over to the sub panel to be backed up. I posted some pics of it a few weeks ago and it looked like it was a nightmare to get the desired circuits moved over.
It was this:

 

[AllCircuitsElectrical]

I don't have much experience with transfer switches but I think I know what would determine the size of one. If my understanding of this is wrong, please correct me.

Among others, I think the size would be determined by either the load being served or if in the case where the transfer switch is service rated and is being used as the first means of disconnect w/OCP.

What I've run into is a customer that has multiple houses on his property. He has a house, his Mother has a house, and his sister has a house.
He has a 20KW Generac on his house with a 200A ATS. His setup is he has a sub panel with all the circuits in it that are to be backed up. The panel is fed like this:

the POCO (line) side of the ATS is fed from a 100A breaker from one of his main panels
of course the other side from the 20KW generator
the ATS is outside and is fed from the generator and from the panel inside, then back into the house where the backed-up sub panel is.

Since the ATS is not being used as the service and only being supplied 100A from the utility side, and the generator is only a 20KW or 83.33A, I say he only needed a 100A transfer switch.
This discussion came about because he has ask me to connect another generator and ATS to his mother's house. He had mentioned the high cost of the transfer switch on his and I told him he could have used a 100A one. But he said the electrician that installed it told him he had to use a 200A ATS. I told him I was pretty sure he could have just used a 100A.

I think the electrician was confused because the panel feeding it was a 200A panel. But since the ATS wasn't being used as service equipment, or between the meter and his MDP, the 200A was unnecessary.

Am I correct on this?

what we have been told is that if the Service is XXX then the ATS must be the same ..ie 200 amp service = 200 ATS ...
next
if the ATS is outside .... then you must have a Disconnect outside .... the " all disconnects being grouped together" ....


and the workaround for this is the 12, 14, and 16 circuit prewired ATS panels located INSIDE the house.

 

[ceb58]

what we have been told is that if the Service is XXX then the ATS must be the same ..ie 200 amp service = 200 ATS ...
next
if the ATS is outside .... then you must have a Disconnect outside .... the " all disconnects being grouped together" ....


and the workaround for this is the 12, 14, and 16 circuit prewired ATS panels located INSIDE the house.

Please reread the OP. The home could have had a 600 amp service but in this case the ATS was/is fed from a 100 amp breaker, therefore the ATS's rating would not have to be greater. The service disconnect doesn't have to be out side. Read 230.70 (A)(1). Don't always rely on what you have been told. As the late great Ronald Reagan said "Trust but verify"