To my knowledge there is nothing in the NEC that prohibits oversizing a dry type transformer for a given load as long as the transformer is lager than the calculated load? For instance if there is a calculated load of 45kvA but there is a 225kVA spare transformer laying around there is nothing that I know of that prohibits the use of the 225kVA transformer?
With this oversized transformer the secondary protection does not necessarily have to be based on the 225kvA transformer value but rather has to be based on the actual load. So if I have a 225kvA 480-208/120V transformer with a 45kVA load then I only need to size my secondary protection and secondary feeders off of the 45kVA value and can use a 150A secondary OCPD instead of a 800A (and associated cable) should I use the full 225kVA capacity of the transformer?
I think the only issue that comes into play here is sizing the OCPD so that it does not nuisance trip for the larger kVA size. If I had a 45kVA transformer this would dictate a primary protective device of 80-100A. Although it would not be an NEC violation to use the 100A OCPD on the primary of a 225kVA unit (primary rating of 270A) there would likely be nuisance tripping of this breaker upon inrush of this unit, and you would likely never be able to energize the transformer.
Looking at the transformer inrush at about 10x FLA it appears I can use a 250A breaker at a minimum in order to get above the transformer inrush. Although this 250A breaker is below the primary FLA is there any issue with using this breaker on this transformer? This would obviously also mean that the primary transformer feeder cable would need to be increased to match the 250A breaker size.
I'm not saying this is a practical application, just curious if I understand the situation correctly.
With this oversized transformer the secondary protection does not necessarily have to be based on the 225kvA transformer value but rather has to be based on the actual load. So if I have a 225kvA 480-208/120V transformer with a 45kVA load then I only need to size my secondary protection and secondary feeders off of the 45kVA value and can use a 150A secondary OCPD instead of a 800A (and associated cable) should I use the full 225kVA capacity of the transformer?
I think the only issue that comes into play here is sizing the OCPD so that it does not nuisance trip for the larger kVA size. If I had a 45kVA transformer this would dictate a primary protective device of 80-100A. Although it would not be an NEC violation to use the 100A OCPD on the primary of a 225kVA unit (primary rating of 270A) there would likely be nuisance tripping of this breaker upon inrush of this unit, and you would likely never be able to energize the transformer.
Looking at the transformer inrush at about 10x FLA it appears I can use a 250A breaker at a minimum in order to get above the transformer inrush. Although this 250A breaker is below the primary FLA is there any issue with using this breaker on this transformer? This would obviously also mean that the primary transformer feeder cable would need to be increased to match the 250A breaker size.
I'm not saying this is a practical application, just curious if I understand the situation correctly.
