transformer overcurrent sizing

[michaelsharp]

sizing overcurrent protection for a 480volt primary, 208/120 secondary transformer when both the primary and secondary will have over current protection,
so based on 450.3(b) should I be using 250% for primary and 125% for secondary? or can I use 125% for both?

my transformer is a 30 kva 480 volt primary and a 208/120 volt secondary.

so based on 450.3(b)

I come up with 50 amp breaker for primary using 125%
and 90 amp breaker using 250% which one is correct?

 

[Ingenieur]

sizing overcurrent protection for a 480volt primary, 208/120 secondary transformer when both the primary and secondary will have over current protection,
so based on 450.3(b) should I be using 250% for primary and 125% for secondary? or can I use 125% for both?

my transformer is a 30 kva 480 volt primary and a 208/120 volt secondary.

so based on 450.3(b)

I come up with 50 amp breaker for primary using 125%
and 90 amp breaker using 250% which one is correct?

both are 'correct'
250% is the max
you can go lower, ie 125%, but not higher
too low may cause nuisance tripping when energizing

 

[augie47]

Keep in mind your conductor must be protected according to it's ampacity. If you elect to size at 250% you will incur higher cost than 125% due to wire size and possibly higher cost for your over-current device,.