Another 4 pole question

[e57]

So I got a panel with two feeders in, one parallel, and two loads out. One feeder is from a 480-208/120 transformer - the other from a 480-208/120 UPS. Do the breakers between the two need to be 4-pole?

 

[Smart $]

So I got a panel with two feeders in, one parallel, and two loads out. One feeder is from a 480-208/120 transformer - the other from a 480-208/120 UPS. Do the breakers between the two need to be 4-pole?

Not enough info to say...

First question needing asked is, with a solidly-connected neutral configuration, is there a parallel path for neutral current? If yes, you'll either have to interrupt the parallel path somewhere and in a compliant manner, or use 4-pole disconnecting means.

 

[e57]

Well it's yet to be built - and as spec'ed it would/could not switch the neutral.

The transformer is bonded I believe, and the UPS is rectified DC. As drawn it would have a solid connection on the neutral as well.

 

[ron]

e57,
I will guess that with the step down of the voltage at the UPS, that it is a SDS so, suggest to the designer that 480V, 3ph, 3W into the transfer for both sources and serve a transformer on the output of the transfer.

 

[Smart $]

e57,
I will guess that with the step down of the voltage at the UPS, that it is a SDS so, suggest to the designer that 480V, 3ph, 3W into the transfer for both sources and serve a transformer on the output of the transfer.

I'm getting the impression both the transformer and UPS serve the same loads... just not at the same time. Perhaps there already is a transfer switch upstream for the feeders, but to service either will require disconnection on the downstream side also.

 

[Smart $]

Well it's yet to be built - and as spec'ed it would/could not switch the neutral.

The transformer is bonded I believe, and the UPS is rectified DC. As drawn it would have a solid connection on the neutral as well.

If both the transformer and UPS have N-G bonding you have a parallel path for neutral current. One or the other will need to be unbonded or a 4-pole transfer means will be required.

The best approach is to remove the N-G[EC] bonds at each source and provide a single N-GEC bond at the panel under discussion... provided it is the first ocepd/disconnecting means downstream of the sources, and of course, meets the tap rules for transormer secondaries.

 

[e57]

I'm getting the impression both the transformer and UPS serve the same loads... just not at the same time. Perhaps there already is a transfer switch upstream for the feeders, but to service either will require disconnection on the downstream side also.

My - you are intuitive.... To further complicate issues up-stream there is also a transfer sw for the XFR, and another transfer swtich for the proposed UPS, so both have the option of the same normal 480 buss - or the other generator fed one. These would then RECONNECT in the panel in question as well..... If we nail the job - there'll be a storm of RFI's on the grounding of the whole kitNcaBBootle. Since scope is limited to a UPS in a room...

 

[Smart $]

My - you are intuitive.... To further complicate issues up-stream there is also a transfer sw for the XFR, and another transfer swtich for the proposed UPS, so both have the option of the same normal 480 buss - or the other generator fed one. These would then RECONNECT in the panel in question as well..... If we nail the job - there'll be a storm of RFI's on the grounding of the whole kitNcaBBootle. Since scope is limited to a UPS in a room...

Sounds like the project needs a cluster of interlocked transfer switches!