Protecting a Feeder Supplied by Two Sources

[JuliaTruchsess]

I have two 120kVA 208V UPSs, which can be paralleled to act as one 240kVA UPS. Each UPS has its own integrated 450A output breaker.

The conductors from the tie point will be sized appropriately for 240kVA under the usage conditions.

My question is whether or not I need to add a separate breaker to protect the 240kVA feeder, or will the fact that each of the two sources already has an OCPD be sufficient.

 

[don_resqcapt19]

It appears you have 240 kVA of conductors on a circuit that can supply 480 kVA of current into those conductors.
How long are the conductors and do they terminate into an OCPD that has a rating that does not exceed the conductor ampacity?

 

[JuliaTruchsess]

It appears you have 240 kVA of conductors on a circuit that can supply 480 kVA of current into those conductors.
How long are the conductors and do they terminate into an OCPD that has a rating that does not exceed the conductor ampacity?

The combined sources can supply 240kVA (two paralleled 120kVA UPSs), not 480kVA.
The 240kVA feeder could be over 25ft, but is TBD at present.
The 240kVA feeder will terminate on busbars in a distribution panel. The busbars will have short (< 3ft) taps terminating on OCPDs in the same DP that will supply smaller feeders to multiple loads of ≈ 10kVA each.

 

[jaggedben]

Perhaps there's code section that specifically addreses UPSs that I'm unaware of. But if not, I do not think you need additional overcurrent protection so long as all conductors are sized for 900A (not just those on the load side of the tie point). That would be a similar approach to paralleled circuits from solar panels or batteries that do not directly connect to other sources that can also provide current.

That said, most of these things need a single disconnecting means for the aggregated output even when they don’t need a single overcurrent device.

 

[JuliaTruchsess]

most of these things need a single disconnecting means for the aggregated output even when they don’t need a single overcurrent device

Interesting point, thanks. Do you have a code reference for this requirement? Given that the raison d'etre of the UPS is to ensure that the load is powered 24/7/365, I don't think my client will be keen to add this disconnect.

 

[JuliaTruchsess]

Do you have a code reference for this requirement?

Answering my own question: 645.10 and 645.11 seem to address this.