3ph & 1ph Combination Transfer Switch

[Moosifer]

Customer has 2qty SINGLE PHASE panels and input will either be 100a 208v 3ph or 100a 240v 1ph.
They want to use a 4 pole double throw switch to put A&B legs to panel 1 and A&C legs to panels 2 while on 3phase.
Then A&B and A&B legs to both panels while on 1phase.
4 pole switch to switch neutrals also.
This will normally be connected to 3ph but they want then single phase for testing / backup plan.
It seems like it will work but I want to verify that I am not missing anything. Any thoughts are appreciated.
Substandard drawing attached.

 

[kwired]

4 pole switch to switch neutrals also.

How you going to do that when all four poles already have an ungrounded conductor on them? Would need a 5 pole switch.

 

[wwhitney]

How you going to do that when all four poles already have an ungrounded conductor on them? Would need a 5 pole switch.

Since you are disinclined to download the diagram and see, only 3 poles are required for ungrounded conductors in the proposed arrangement. One source has 3 ungrounded conductors, the other source has 2 ungrounded conductors.

Cheers, Wayne

 

[Elect117]

This is a manual switch correct? No electronics controlling it?

Electronics might have some kind of voltage protection that might not work with double landing the same phase. Though I have never tried it.

An issue will be abiding the code on taps. You are technicallying using one feeder to feed two panels. Without knowing what that is rated for I can not tell if you will have a tap rule.

I also can not tell if it would be considered parallel for B phase if it is not 3ph. You might need to use 1/0 Cu.

 

[wwhitney]

An issue will be abiding the code on taps.

If both supplies are protected at 100A, and all the conductors shown have an ampacity of 100A, there are no taps.

I also can not tell if it would be considered parallel for B phase if it is not 3ph. You might need to use 1/0 Cu.

There are no parallel conductors shown in the diagram in the OP. That would require two or more wires which are "electrically joined at both ends."
[Although not applicable to the OP's diagram, when switches are involved "electrically joined" precludes connected through a switch.]

Cheers, Wayne

 

[jaggedben]

Seems kinda nutty to me but I agree the basic idea in the diagram should work.

 

[jaggedben]

Don't see why you couldn't do it in a non-separately derived systems and not switch the neutral, and either use the 4th to completely separate the panels on the load side, or use a 3 pole switch.

 

[wwhitney]

Seems kinda nutty to me

Seems elegant enough to me. Possible thought process: "3 pole TS for each panel. We'd like them to be mechanically interlocked, so one 6 pole TS instead. Oh wait, 2 of those poles in the 6 pole are just transferring the same conductor as two of the other poles, so 4 pole TS instead."

Cheers, Wayne

 

[Elect117]

If both supplies are protected at 100A, and all the conductors shown have an ampacity of 100A, there are no taps.

If the feeder to the switch is rated 100A there is no taps. If the feeder is rated 200A then the circuits to the panelboards will be taps if the conductor is rated less than 200A. Since they are not protected at their ampacity. That is why I said it. The diagram doesn't show the feeder rating.



The red and blue are two different feeders to supply the switch? Is this supposed to be some kind of distributed generation thing?

I am missing the reason why you wouldn't just tap phases b, a and the neutral after the switch for both panel boards rather than even try to feed it with C or 3ph power.

You would also have two different voltage systems feeding the panel boards which is pretty wonky. That means different amps drawn for the same loads. Between the 3ph amps and 1 ph 240V amps for the feeders. The panel boards worst leg for the 3ph and the reusing of "B" phase. It would take some considerations.

 

[wwhitney]

If the feeder to the switch is rated 100A there is no taps. If the feeder is rated 200A then the circuits to the panelboards will be taps if the conductor is rated less than 200A.

Sure, but that's true of any diagram. There's nothing about the weird topology shown that would cause taps to be inherent or more likely than typical. So there's no particular reason to bring up taps just because the OCPD on the two supplies are not shown.

The wiring from B to B (top of switch - feed direction) and the A to A (load side of feed direction) on the switch was leading me to think that they were parallel feeds in a sense. Two B phase landings feeding two B phase landings.

I'm not following your comments. The transfer switch in the diagram is connecting either top terminals to bottom terminals, or middle terminals to bottom terminals. Maybe that's a non-standard way to draw it, and so it is confusing? I'm not familiar with standard ways to draw them, so I am going off the labels and the text of the OP.

Cheers, Wayne

 

[Moosifer]

This is a manual switch correct? No electronics controlling it?

Electronics might have some kind of voltage protection that might not work with double landing the same phase. Though I have never tried it.

An issue will be abiding the code on taps. You are technicallying using one feeder to feed two panels. Without knowing what that is rated for I can not tell if you will have a tap rule.

I also can not tell if it would be considered parallel for B phase if it is not 3ph. You might need to use 1/0 Cu.

Thanks. No electronics. 100a feeding so I will make sure the panels can't do more than 50 each I suppose. Good point for me to ponder. Thank you.

 

[Moosifer]

Don't see why you couldn't do it in a non-separately derived systems and not switch the neutral, and either use the 4th to completely separate the panels on the load side, or use a 3 pole switch.

There will be a plug / male connector for each feed, so I am switching the neutrals to keep the unused plug completely disconnected. If that is what you are wondering.

 

[wwhitney]

Thanks. No electronics. 100a feeding so I will make sure the panels can't do more than 50 each I suppose. Good point for me to ponder. Thank you.

If you use 100A conductors to supply each panel, there's no issue with taps. The panels don't even need main breakers, as each source is protected at 100A.

But for each of the two possible sources, you do need to do a load calculation for the combination of both panels, and ensure that the load on each source conductor is at most 100A.

[That's one possible reason for the extra complexity of splitting the two panels A-N-B and B-N-C when on the 3 phase supply. E.g. suppose each panel supplies L-L constant power loads, for simplicity 12 kVA out of each. On 120/240V power, 24 kVA = 100A @ 240V, good. If both panels are supplied A-N-B on 3 phase power, then 24 kVA = 115A @ 208V, not good. Whereas with the panels spread across all 3 phases, 12 kVA @ 208V = 58A each, and if the loads are identical between the two panels (or just have the same power factor angle), on the common B supply conductor, the currents will add as 58A + 58A = 100A due to the 60 degree phase shift.]

Cheers, Wayne

 

[Moosifer]

Seems elegant enough to me. Possible thought process: "3 pole TS for each panel. We'd like them to be mechanically interlocked, so one 6 pole TS instead. Oh wait, 2 of those poles in the 6 pole are just transferring the same conductor as two of the other poles, so 4 pole TS instead."

Cheers, Wayne

If you use 100A conductors to supply each panel, there's no issue with taps. The panels don't even need main breakers, as each source is protected at 100A.

But for each of the two possible sources, you do need to do a load calculation for the combination of both panels, and ensure that the load on each source conductor is at most 100A.

[That's one possible reason for the extra complexity of splitting the two panels A-N-B and B-N-C when on the 3 phase supply. E.g. suppose each panel supplies L-L constant power loads, for simplicity 12 kVA out of each. On 120/240V power, 24 kVA = 100A @ 240V, good. If both panels are supplied A-N-B on 3 phase power, then 24 kVA = 115A @ 208V, not good. Whereas with the panels spread across all 3 phases, 12 kVA @ 208V = 58A each, and if the loads are identical between the two panels (or just have the same power factor angle), on the common B supply conductor, the currents will add as 58A + 58A = 100A due to the 60 degree phase shift.]

Cheers, Wayne

Wayne,
Both feeds will have 100a OCPD's and 100a wire throughout to the panels. Are you saying A-N-B and B-N-C is better than A-N-B and A-N-C? I am thinking you have surpassed my comprehension level; which is why I asked to begin with so thank you and forgive me for needing clarification.

Edit - if you are saying to use 2qty 3 phase panels, the reason I tried to avoid this is to prevent someone from moving things around later and causing issues when on single phase. I have seen this happen on similar setups when they move 20a breakers to a stinger phase, with this I suppose they would end up with a double pole on a dead leg on single phase if they move things...

 

[wwhitney]

Are you saying A-N-B and B-N-C is better than A-N-B and A-N-C?

No, those are the same, I just misremembered your diagram.

I was commenting on an earlier suggestion to just use a 3 pole switch and supply both panels A-N-B when on the 3 phase supply. Which would be simpler if you don't mind the reduction in available power for L-L loads when on 3-phase supply.

Cheers, Wayne

 

[Moosifer]

Wayne,
Both feeds will have 100a OCPD's and 100a wire throughout to the panels. Are you saying A-N-B and B-N-C is better than A-N-B and A-N-C? I am thinking you have surpassed my comprehension level; which is why I asked to begin with so thank you and forgive me for needing clarification.

No, those are the same, I just misremembered your diagram.

I was commenting on an earlier suggestion to just use a 3 pole switch and supply both panels A-N-B when on the 3 phase supply. Which would be simpler if you don't mind the reduction in available power for L-L loads when on 3-phase supply.

Cheers, Wayne

OK thank you.