Feeder CB

[nkalghatgi]

I am feeding a mini power center via 22.5 KVA 408-208/120 V transformer. The transformer manufacturer is providing both primary and secondary over current protection but I also want to include a feeder protection. How should I size this feeder protection CB.

 

[petersonra]

I am feeding a mini power center via 22.5 KVA 408-208/120 V transformer. The transformer manufacturer is providing both primary and secondary over current protection but I also want to include a feeder protection. How should I size this feeder protection CB.

It is a feeder. Size it according the the standard feeder calcs.

 

[nkalghatgi]

So I should calculate the CB according to the rated primary current of the transformer ?

 

[petersonra]

So I should calculate the CB according to the rated primary current of the transformer ?

given the transformer is already protected at the power center, all you are protecting is the wiring going there. you can make it as big as you want as long as the OCPD protects the wires.

 

[kingpb]

Start in article 450.

 

[petersonra]

Start in article 450.

why would article 450 apply to the feeder circuit?

that would apply out at the power center, but not the feeder to it.

 

[don_resqcapt19]

You need to supply the transformer via an OCPD that will not trip on transformer inrush and size the feeder conductors to the OCPD. What size is the primary OCPD that is supplied with the unit?

I think a 40 amp OCPD and #8 would work.

 

[nkalghatgi]

Thank you Bob

 

[nkalghatgi]

the manufacturer said, we can vary the ampere rating of the primary OCPD. I calculated to be 50A OCPD and 6 AWG.

 

[petersonra]

You need to supply the transformer via an OCPD that will not trip on transformer inrush and size the feeder conductors to the OCPD. What size is the primary OCPD that is supplied with the unit?

I think a 40 amp OCPD and #8 would work.

I might be inclined to use a larger OCPD and wire size to limit any chance of tripping.

22.5 kva @ 480 3phase is about 27 A. 250% of that is 68A. I don't think there is any limit to the feeder breaker as the transformer is already protected.

 

[kingpb]

why would article 450 apply to the feeder circuit?

that would apply out at the power center, but not the feeder to it.

Because if you don't understand why the transformer protection is sized they way it is, how are you going to size the upstream feeder?

Besides, you can size it to allow full load, or you can size it to limit output.

 

[don_resqcapt19]

...
Besides, you can size it to allow full load, or you can size it to limit output.

You have to be careful with sizing it to limit the output...too small and it will trip on inrush.

 

[petersonra]

Because if you don't understand why the transformer protection is sized they way it is, how are you going to size the upstream feeder?

Besides, you can size it to allow full load, or you can size it to limit output.

or you can make it as big as you want since the power center already has appropriate sized protection built in for the transformer.

I just do not see there is any maximum size feeder for this. there may be some practical considerations, but I do not see that there is a code enforced maximum size breaker (assuming the feeder wires are protected).