Single or multiple transfer switches

[electrofelon]

I have a group of three buildings, call them A, B, and C, where the owner wants an options standby generator for certain loads (mostly refrigeration). Services are 120/208 3 phase, and generator would be about 150KW. Most of the load is in building A. I am strongly leaning toward a single 400 amp transfer switch in building A and running a feeders from A to B and A to C to pick up the generator loads. I just wanted to check and see if there is anything I might be missing or not thinking about that would point to instead having a separate transfer switch in each building? The service in building A is adequate to run the generator loads in the other two buildings. Thanks for any thoughts!

 

[hillbilly1]

You would have to group the service disconnects together and hope it doesn’t exceed six.

 

[hillbilly1]

Just read it again, your running the entire load in b&c? So a single disconnecting means would be sufficient. I originally thought you were only serving partial power to b&c.

 

[electrofelon]

Just read it again, your running the entire load in b&c? So a single disconnecting means would be sufficient. I originally thought you were only serving partial power to b&c.

So the backed up loads in buildings b and c would be served by feeders from building a

 

[wwhitney]

So the backed up loads in buildings b and c would be served by feeders from building a

Rather than running those loads in building b (for instance) all the time on the feeder from building a, what are the pros and cons installing an additional cascaded transfer switch that would supply those loads either from building b's service, or from the backed-up feeder from building a, as necessary?

Cheers, Wayne

 

[electrofelon]

Rather than running those loads in building b (for instance) all the time on the feeder from building a, what are the pros and cons installing an additional cascaded transfer switch that would supply those loads either from building b's service, or from the backed-up feeder from building a, as necessary?

Cheers, Wayne

Yes that is exactly the question, I cannot think of any good reason code or otherwise but just wanted to check with the group. I could understand keeping the backed up loads off their respective building's service if putting all the loads off building A's service exceeded A's capacity, but that is not the case. In the case of running everything off A, granted you have to run a feeder to the other two buildings versus just a small control wire, but then of course you have to buy multiple transfer switches...

 

[hillbilly1]

So the backed up loads in buildings b and c would be served by feeders from building a

All of the loads in b&c? Or just select loads? I don’t think I’m understanding you correctly?

 

[electrofelon]

All of the loads in b&c? Or just select loads? I don’t think I’m understanding you correctly?

Select loads in b&c. Most of A will be backed up.

 

[hillbilly1]

Ok, then you will need a disconnecting means grouped with the non-backed up feed, but other than that, you should be good to go. Setting a sub panel for the backed up loads?

 

[wwhitney]

Yes that is exactly the question

Well, I was envisioning three scenarios:

(1) Generator near A, generator feeders to A, B and C, transfer switches at A, B, C
(2) Generator near A, generator feeder to A, transfer switch at A, backed-up feeders from A to B and C for backed up loads all the time.
(3) Generator near A, generator feeder to A, transfer switch at A, backed-up feeders from A to B and C, transfer switches at B and C to switch backed up loads from building B/C services to backed up feeders from A during outages (cascaded transfer switches).

So I guess I'm wondering about 1 vs 3 (both use 3 transfer switches). Then your question is about 2 vs (1 or 3).

One difference I see is that the feeders from A to B or C carry only generator current in scenario (1) and only during an outage; carry grid current during normal operation in (2); and only carry grid current in (3) if B or C has an outage but A does not. Available fault current would vary between generator and grid. Does that matter?

Also, (1) requires that the generator start during an outage from any of A, B, or C (and hence require extra control wires), while (2) and (3) only start the generator during an outage at A.

Cheers, Wayne

 

[electrofelon]

Ok, then you will need a disconnecting means grouped with the non-backed up feed, but other than that, you should be good to go. Setting a sub panel for the backed up loads?

So are you saying the optional standby system feeder disconnect needs to be grouped with the building service disconnect? I am not sure I agree with that.

Yeah it would be a small sub panel fed by a feeder from building A to serve the optional standby loads.

 

[hillbilly1]

So are you saying the optional standby system feeder disconnect needs to be grouped with the building service disconnect? I am not sure I agree with that.

Yeah it would be a small sub panel fed by a feeder from building A to serve the optional standby loads.

Yes, because it is a service disconnect also. In order to remove all power to the building, code requires the disconnecting means to be grouped in one location, or signage where the others are located along with the total number of disconnect locations.

 

[electrofelon]

Yes, because it is a service disconnect also. In order to remove all power to the building, code requires the disconnecting means to be grouped in one location, or signage where the others are located along with the total number of disconnect locations.

Can you cite a code reference for that? I admit there is a article 225 feeder disconnect requirement but that is not a "service disconnect" and I am not seeing a requirement that that be grouped with the normal service disconnect serving the building.

 

[hillbilly1]

Can you cite a code reference for that? I admit there is a article 225 feeder disconnect requirement but that is not a "service disconnect" and I am not seeing a requirement that that be grouped with the normal service disconnect serving the building.

I’m on the beach this week, and don’t have my code book with me, but if these are separate buildings, each building can have no more than six disconnecting means.

 

[electrofelon]

Well, I was envisioning three scenarios:

(1) Generator near A, generator feeders to A, B and C, transfer switches at A, B, C
(2) Generator near A, generator feeder to A, transfer switch at A, backed-up feeders from A to B and C for backed up loads all the time.
(3) Generator near A, generator feeder to A, transfer switch at A, backed-up feeders from A to B and C, transfer switches at B and C to switch backed up loads from building B/C services to backed up feeders from A during outages (cascaded transfer switches).

So I guess I'm wondering about 1 vs 3 (both use 3 transfer switches). Then your question is about 2 vs (1 or 3).

One difference I see is that the feeders from A to B or C carry only generator current in scenario (1) and only during an outage; carry grid current during normal operation in (2); and only carry grid current in (3) if B or C has an outage but A does not. Available fault current would vary between generator and grid. Does that matter?

Also, (1) requires that the generator start during an outage from any of A, B, or C (and hence require extra control wires), while (2) and (3) only start the generator during an outage at A.

Cheers, Wayne

Ok it is all rather confusing.......So the difference between (2) and (3) is that in (2), the backed up loads are powered by building A during normal conditions, while in (3), the backed up loads in (say) building B are powered by building B's service during normal operation? My immediate thought is it doesnt really matter much, other than what the control logic is capable of. Interestingly, these buildings are supplied by a different POCO feeder despite being adjacent. Just today we had an outage with building A having power and building B not having power. I am not sure about the control logic or having one of the building's transfer switches not transfer if that building still has power, I would have to look into that. This will probably be a Kohler RXT transfer switch.

 

[hillbilly1]

Did a quick search, and Mike Holt posted that only applies to services, not buildings, which I find kinda stupid, because first responders will not know to turn power off at the service of the other building to kill power at that building, Another flaw in the NEC.

 

[electrofelon]

I’m on the beach this week, and don’t have my code book with me, but if these are separate buildings, each building can have no more than six disconnecting means.

Sweet! I am super jelly. Its snowy and cloudy here in central NY!.

I dont think that is correct. 225.33 says: "The disconnecting means for each supply permitted by 225.30, shall consist of not more than six switches or six circuit breakers...." So I could have 6 "normal" service disconnects AND 6 optional standby disconnects.

furthermore 225.34(B) states:

(B) Additional Disconnecting Means. The one or more additional
disconnecting means for fire pumps or for emergency,
legally required standby or optional standby system permitted
by 225.30 shall be installed sufficiently remote from the one to
six disconnecting means for normal supply to minimize the
possibility of simultaneous interruption of supply.

So in fact it appears that the optional stand by disconnect(s) serving the building actually MUST be non grouped with the normal supply.

 

[hillbilly1]

Sweet! I am super jelly. Its snowy and cloudy here in central NY!.

I dont think that is correct. 225.33 says: "The disconnecting means for each supply permitted by 225.30, shall consist of not more than six switches or six circuit breakers...." So I could have 6 "normal" service disconnects AND 6 optional standby disconnects.

furthermore 225.34(B) states:

So in fact it appears that the optional stand by disconnect(s) serving the building actually MUST be non grouped with the normal supply.

Dennis would probably know for sure, but an inspector on here a couple of years ago said they required a service disconnect for the generator be grouped with the utility service disconnect. Fire pump, yes they should be in different locations. With optional standby, I see no reason not to group with the utility disconnect.

 

[wwhitney]

So the difference between (2) and (3) is that in (2), the backed up loads are powered by building A during normal conditions, while in (3), the backed up loads in (say) building B are powered by building B's service during normal operation?

Yes, that was the distinction I was trying to make. In (2) you only need one transfer switch that can control the generator. In (1) you would need (3) transfer switches that can jointly control the generator (fire it up whenever any one of the transfer switches needs power). In (3) you'd need one transfer switch that can control the generator (at A), and (2) transfer switches that just have normal power and backup power, no generator control.

Cheers, Wayne

 

[electrofelon]

Dennis would probably know for sure, but an inspector on here a couple of years ago said they required a service disconnect for the generator be grouped with the utility service disconnect. Fire pump, yes they should be in different locations. With optional standby, I see no reason not to group with the utility disconnect.

Sorry to be blunt, but I dont care about "reason" or "what the inspector wants" or "should". I am looking for what the NEC says.