Sizing Transformer feeders???

[Mattula]

I am feeding a 45KVA D/Y Transformer with 480V and stepping the voltage down to 208.

My primary amps at full load would be 54A. Since I am using primary protection only (125%) my breaker would be set at 70A.

My question is.......do I set my conductors to match my breakers ampacity rating?

How about my secondary conductors? A 45 Kva transformers max output is 125Amps............so do i multiply 125A by 125% making it 156 Amps??

Please show me the math so I can follow your answers......thanks!! - Matt

 

[Mattula]

Wow.....no takers...huh. Was this a stupid question?????

 

[Mike01]

lets go

lets go

No such thing as a stupid question, the only stupid question is the one not asked. Ok enough pilosopihy.

If you look at section 450 you can find the transormer chart for overcurrent protection, you mention primary overcurrent protection only be careful this can only be used in certian cases (it sounds like you do not meet the critera) next look at 240 overcurrent protection, and then appliance panelboard this is what it sounds like you will be feeding and if that is the case you are required to have a main c.b. and now with that being the case you now can increase your primary as much as 250% you could use a 70A overcurrent protection on the primary but if you allow the secondary protection to get to large you will loose all coordination and might not be protecting you transformer within it's damage curve I guess it just depends how coordinated of a system you require.