primary and secondary protection for 30 kVA transformer



the previous designer specified 50 for primary and 100 for secondary. i came up with 150 for primary and 175 for secondary.

here's how i got there from Table 450.3(B) of the 2014 NEC:

Primary – 480 V single phase
Full load amps: 30000/480=62.5
Protection can’t be more than 62.5*2.5=156.25
Next size down – 150 A CB

Secondary – 120/240 V single phase
Full load amps: 30000/240=125
Protection can go to next standard size up from 125*1.25=156.25
Use – 175 A CB

why would one go lower? is there some benefit or rule i'm not aware of? thanks in advance.
Their previous design isn't necessary wrong. You're able to undersize the breakers depending on the applied load. Undersizing breakers just doesn't allow the transformer to be utilized to it's till potential. Obv feeders still have to be sized appropriately. Sounds like your on the right track though. Only advantages I could see would be maybe feeder/breaker price savings of there was overstock or sales.


Electron manager
NE Nebraska
First double check transformer data, thread title said 30kVA and I assumed right away a three phase transformer because that is common size for three phase, single phase typically goes from 25 to 37.5 kVA.

Otherwise like mentioned there is nothing that says you must use max allowed protection for the secondary, you could put a 30 amp secondary device on there if you wanted, kind of waste of $$ spent on an oversized transformer if you do so - but sometimes for temporary applications and using what you might already have on hand you may do something like that.
The disadvantage of going large is you then need the equipment and conductors to serve it. That be the difference between using say a 100 amp safety switch or breaker and a 200. That is not an insignificant cost and can be a real hassle depending on the serving equipment.