When to "upsize" feeder and ocpd for xformer

[YZERMAN2010]

Ok, I think I understand that:
When you have primary and secondary ocpd, Primary ocpd can be up to 250% of the transformers ampacity. Also, the secondary ocpd can be no more than 125% of the load being served. Also, I understand that what ever ocpd you install on the primary, the primary conductors must match. This being said and hopefully correct then....What determines if you upsize the primary fuse and feeders? I understand the inrush of the transformer when energized can blow the fuses. So at the sake of getting the job with the smallest conductors/pipe/fuses, how do I know if I am going to have an issue on start-up? Experience?


Thanks all!

 

[augie47]

If you know all the transformer particulars (impedance, etc), an engineer can give you some guidance and often a factory rep can help, but, for the most part, you have the most common method, experience.
For common transformers I don't recall any problems with nuisance trips using the 125% rule.
The only times I recall a problem at all is when someone is using a much larger transformer than needed (because they had it on hand) and they have OCP far less than the primary allowed.