Smaller breaker feeding larger breaker??

[carl.arnsdorff]

We manufacture medical scanning equipment.

The equipment has an option for a Power Distribution Unit (PDU) that can feed several different configurations of scanners which also includes adjustable circuit breakers and isolation transformers, etc.

The main circuit breaker for the PDU is a 250 amp breaker. Is it a code violation to specify the feeder circuit breaker from the building supply to have a lower ampacity than the PDU (e.g. 175 amp)?

The end equipment after the PDU does not require the higher amperage and the equipment itself has circuit breakers rated for the load it will pull.

I am aware that the industrial equipment section does not allow a smaller breaker feeding a larger breaker but I'm not thinking this is applicable here. Can anyone hlep me with this?

 

[acrwc10]

It is fine to supply the larger circuit breaker from a smaller breaker as long as the smaller breaker is sufficient to supply the load. The second breaker is no more then a disconnect switch at that point. It is done all the time.

 

[carl.arnsdorff]

Transformer load with

Transformer load with

One more item comes to mind in this scenario. The larger breaker is in place which feeds a three phase delta to wye transformer where the secondary side has the smaller breakers to protect the load.

Does this change anything in terms of the main feed breaker being smaller than the 250 amp breaker supplying the transformer?

 

[don_resqcapt19]

Does this change anything in terms of the main feed breaker being smaller than the 250 amp breaker supplying the transformer?

Maybe...transformers have a high inrush current and a smaller breaker may trip when the circuit is energized. What is the kVA rating and primary voltage of the transformer?