Single or multiple transfer switches

[hillbilly1]

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.

Apparently the inspector thought it was code, and your AHJ may too, so that’s why you should care. You may want to check with the AHJ for your answer.

 

[wwhitney]

Seems like a bit of a blind spot in the NEC: for a building supplied by multiple services (or sets of service entrance conductors), the NEC generally requires the disconnects to be grouped. And for a building supplied by multiple feeders, the NEC generally requires the disconnects to be grouped. But for a building supplied by one feeder and one service, I don't see anything requiring them to be grouped.

I guess the CMPs behind Articles 225 and 230 didn't consider that case?

Cheers, Wayne

 

[electrofelon]

Seems like a bit of a blind spot in the NEC: for a building supplied by multiple services (or sets of service entrance conductors), the NEC generally requires the disconnects to be grouped. And for a building supplied by multiple feeders, the NEC generally requires the disconnects to be grouped. But for a building supplied by one feeder and one service, I don't see anything requiring them to be grouped.

I guess the CMPs behind Articles 225 and 230 didn't consider that case?

Cheers, Wayne

This is commonly understood, but for multiple services, the service disconnects do not need to be grouped. Even for a single service, it is only the disconnects for/within EACH set of service entrance conductors allowed by 230.40 exception #1,3,4,5 that need to be grouped.

 

[electrofelon]

Can anyone comment on the practical implications of setting up multiple transfer switches in an arrangements such as Wayne's (2) and (3) post #10? Do typical transfer switches allow this flexibility? The equipment has not been purchased yet but likely will be kohler RXT series due to familiarity of this product by client and myself.

 

[shortcircuit2]

For simplicity, I would install 1 generator at Building A with feeders to the important loads in B & C

The generator is going to run when power is out at A... so mine-as-well pick up the loads in B & C when power is out in A thereby having the utility cash register off while burning fuel money.

But if the owner were to sublet the campus to different tenants in A,B & C, you would want to keep the separate cash registers. Backup optional power could be inclusive in the lease.

 

[ggunn]

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!

I haven't read all the comments, but I don't see how you can have selected loads in buildings B and C backed up by the generator while normally powered from the utility without having transfer switches in buildings B and C. With only a single transfer switch at building A, backup of buildings B and C are all or nothing. I have seen similar setups before and they all used multiple transfer switches.

 

[hillbilly1]

I haven't read all the comments, but I don't see how you can have selected loads in buildings B and C backed up by the generator while normally powered from the utility without having transfer switches in buildings B and C. With only a single transfer switch at building A, backup of buildings B and C are all or nothing. I have seen similar setups before and they all used multiple transfer switches.

The OP is setting a sub panel fed from the load side of the transferswitch in building A, so essentially having two services at buildings B &C, so no transferswitch necessary. His alternate plan is a transferswitch at B&C using the generator output only for source two on the switch, with source one being the existing service on the building.

 

[ggunn]

The OP is setting a sub panel fed from the load side of the transferswitch in building A, so essentially having two services at buildings B &C, so no transferswitch necessary. His alternate plan is a transferswitch at B&C using the generator output only for source two on the switch, with source one being the existing service on the building.

I see; two feeders to each B and C, each feeding a panel with one backed up and one not. Are the two feeders to each building an issue?

 

[electrofelon]

Are the two feeders to each building an issue?

225.30 doesn't seem to object to one service and one feeder, it seems to only object to multiple feeders or multiple Branch circuits. But irregardless (not a word), 225.30(A) has exceptions for this application.

 

[electrofelon]

For simplicity, I would install 1 generator at Building A with feeders to the important loads in B & C

The generator is going to run when power is out at A... so mine-as-well pick up the loads in B & C when power is out in A thereby having the utility cash register off while burning fuel money.

But if the owner were to sublet the campus to different tenants in A,B & C, you would want to keep the separate cash registers. Backup optional power could be inclusive in the lease.

Thinking about it more, the control logic could be interesting when you have multiple transfers switches that are on different services. Not only would you want any transfer switch to be able to start the generator, you may want the ones that still have normal power to not transfer - or like you say, maybe you would want them all to transfer since you are running the generator anyway. Do transfer switches Typically have this capability?

 

[hillbilly1]

If all three buildings were fed from the same poco line, or transformers, it really would be a mute point, since loss of power would affect all three, unless a fault trips one of the building mains, which in case due you want to transfer into a fault? The 2000 amp motorized breaker type transferswitchs, would not transfer if the main tripped.

 

[hillbilly1]

Thinking about it more, the control logic could be interesting when you have multiple transfers switches that are on different services. Not only would you want any transfer switch to be able to start the generator, you may want the ones that still have normal power to not transfer - or like you say, maybe you would want them all to transfer since you are running the generator anyway. Do transfer switches Typically have this capability?

Depends on the transferswitch, if it is a smart switch that controls the generator start signal and transfer, it will go to the preferred source, which is utility if it is still available. With the “dumb” switches Generac uses, the generator decides the start and transfer.

 

[electrofelon]

If all three buildings were fed from the same poco line, or transformers, it really would be a mute point, since loss of power would affect all three, unless a fault trips one of the building mains, which in case due you want to transfer into a fault? The 2000 amp motorized breaker type transferswitchs, would not transfer if the main tripped.

Actually, at least two of these buildings are on separate utility feeds, as we recently had an outage where one building was out and the other was still on. The third building may very well be on yet another feed because it comes from the next street over.

 

[hillbilly1]

Actually, at least two of these buildings are on separate utility feeds, as we recently had an outage where one building was out and the other was still on. The third building may very well be on yet another feed because it comes from the next street over.

You would probably be better off then, putting in separate transferswitches, unless you install a set of signal wires and relay to force a transfer to all. Would be much cheaper that way using the relay if you don’t mind transferring the full load. Kohler has an input to force a remote transfer.

 

[tortuga]

I had a similar project, it involved a large site and 5 services, the utility had offered to make the site a 'primary customer' so one generator could be installed.
I was proposing something along your option 2 a large underground feeder and using tap rules, but the site was too large.
What ended up happening was 5 separate generators, because of redundancy and fuel tank sizing.

 

[wwhitney]

(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 if a "smart" transfer switch signals the generator to start, and a "dumb" transfer switch just switches to the alternate source of power when it is available but the grid is not, then the above 3 scenarios could be better described as:

(1) Generator feeders to A, B, and C, transfer switches at A, B, and C, and control wires from B and C back to the generator. The transfer switches could be "smart" and able to work together to start the generator when any one of them needs it. Or they could be dumb, and the control wires from B and C just provide a "grid is present or not" signal to additional control circuitry to start the generator when required.

(2) Generator feeder to A, transfer switch at A, backed-up feeders from A to B/C power the backed up loads all the time from service A or the generator.

(3) Generator feeder to A, transfer switch at A, backed-up feeders from A to B/C, dumb transfer switches at B/C so the backed up loads normally run from the local service.

(2) and (3) have the advantage of not requiring the additional control wires.

Cheers, Wayne

 

[electrofelon]

You would probably be better off then, putting in separate transferswitches, unless you install a set of signal wires and relay to force a transfer to all. Would be much cheaper that way using the relay if you don’t mind transferring the full load. Kohler has an input to force a remote transfer.

So if a "smart" transfer switch signals the generator to start, and a "dumb" transfer switch just switches to the alternate source of power when it is available but the grid is not, then the above 3 scenarios could be better described as:

(1) Generator feeders to A, B, and C, transfer switches at A, B, and C, and control wires from B and C back to the generator. The transfer switches could be "smart" and able to work together to start the generator when any one of them needs it. Or they could be dumb, and the control wires from B and C just provide a "grid is present or not" signal to additional control circuitry to start the generator when required.

(2) Generator feeder to A, transfer switch at A, backed-up feeders from A to B/C power the backed up loads all the time from service A or the generator.

(3) Generator feeder to A, transfer switch at A, backed-up feeders from A to B/C, dumb transfer switches at B/C so the backed up loads normally run from the local service.

(2) and (3) have the advantage of not requiring the additional control wires.

Cheers, Wayne

I am thinking #2 for this application. There is really no disadvantage in supplying the backed up loads in B and C from A during normal operation. In fact, A has a larger service and more excess capacity than B or C so it is somewhat advantageous to pull load from B and C and put it on A. Additionally, it seems that providing multiple transfer switches is more cost.

The only disadvantage I can think of is if A losses power but B does not, then the backed up B loads get run off the generator un-necessarily. However for the amount of load we are talking about, its probably splitting hairs in terms of gas cost vs utility power cost.

 

[RadioShack]

There are a couple Issues that i see here. This is not an exhaustive list. There may be other issues.

Grounding.
If multiple transfer switches are installed at each building the Generator would need to be installed as a Separately Derived System and each transfer switch would need to switch the neutral. Otherwise you could have objectionable current on the EGC for the generator because the neutrals are tied together and the N-G bond at each building would create a parallel path.

Disconnect.
225.31 Requires a disconnect for this generator feeder where conductors enter building. There will need to be a disconnect on each building

Directory

230.2(D) and 225.37 require a directory indicating where other sources are. This is critical for emergency personnel and should be easy to understand.
Example:
Label at service: Building Disconnect 1 of 2 Critical Feeder disconnect located on N side of building.
Label at Critical Feeder Disconnect: Building Disconnect 2 of 2 Service Disconnect of East side of building.

 

[ggunn]

Can anyone comment on the practical implications of setting up multiple transfer switches in an arrangements such as Wayne's (2) and (3) post #10? Do typical transfer switches allow this flexibility? The equipment has not been purchased yet but likely will be kohler RXT series due to familiarity of this product by client and myself.

I didn't think (correct me if I am wrong) you can power a panel in building B by a feeder from building A if building B has a service.

 

[wwhitney]

I didn't think (correct me if I am wrong) you can power a panel in building B by a feeder from building A if building B has a service.

A plausible prohibition, but one I don't find in the NEC. Do you have a reference?

Cheers, Wayne