Overcurrent protection for PDU (transformer)

[rich000]

I am drawing a blank on this. I have a 300kVA PDU (480V to 208V/120V). It is being fed from a 450A breaker (not 100% rated).

The calcs come to 451A.

I am typically used to going up a size.

I cannot remember what part of the code allows me to go down a breaker size. This may not be a code issue, but an engineering issue. i.e. the load is still protected, it just cannot get to 100%.

 

[pete m.]

You are probably looking for 450.3(B). If you are going with primary and secondary protection for the transformer you can (per table 450.3(B)) size the primary OCPD up to 250% of the primary FLC and the secondary at 125% of the secondary FLC.

Pete

 

[rich000]

Thank you, I understand. But my calculations at 125% come to 451 amps. Again, I am pretty sure this is an engineers call to drop to the lower size breaker.

 

[ron]

Thank you, I understand. But my calculations at 125% come to 451 amps. Again, I am pretty sure this is an engineers call to drop to the lower size breaker.

It is, but be sure it is a low inrush xfmr, or the primary breaker will trip.

 

[jim dungar]

I cannot remember what part of the code allows me to go down a breaker size. This may not be a code issue, but an engineering issue. i.e. the load is still protected, it just cannot get to 100%.

You are slightly misinterpreting the code, Table 450.3(B) deals with maximum rating or setting, you are always allowed to go down in size. There is no code requirement that transformers be able to 'provide' 100% output.

The inrush of the transformer should not be ignored.

 

[rich000]

You are slightly misinterpreting the code, Table 450.3(B) deals with maximum rating or setting, you are always allowed to go down in size. There is no code requirement that transformers be able to 'provide' 100% output.

The inrush of the transformer should not be ignored.

Thanks. My inspections typically entail making sure the breakers are sized correctly. I thought the breaker being undersized was a code violation.

I had a "brain fart" and totally overlooked the fact that of course you can undersize a breaker.

The inrush is being taken care of the programable trip unit on the breaker.

Thanks again.