Transformer Question

[Jimmy7]

I was wondering if someone could help me with a transformer question that is attached to this message. If the Primary protection is 169A, how could you use a 150A, or a 175 overcurrent device. I do get the next size up for the 175 A device, but why would you use a 150A?

 

[EC Dan]

Using the next size up is only permitted, not required. If you did not want to go with next size up, then 150 A would be your selection.

 

[david luchini]

It's a poor question. A 125A c/b could be used if the load was say 100kVA.

Or a 200A, 225A, 250A, or 300A c/b could be used.

 

[augie47]

As to "why".... you must size the conductor in relation to the over-current device. The use of a lower ampere breaker still adequate for the transformer current, you might be able to use a smaller conductor saving $$

 

[Jraef]

But... the lower the trip rating, the more likely it is to trip on energization from the transformer inrush. The NEC rules here address the MAXIMUM ratings you can use. Minimums are up to you and your tolerance for risk of nuisance tripping.