45 kva to 75 kva transformer - keep the same primary feeders

[kwired]

I beg to disagree! You will let the primary circuit breaker protect the transformer for faults and let the secondary protection protect the whole transformer from overcurrent (that includes the protection of the primary lines as there will be no overcurrent in the primary if the secondary breaker trips!

Kind of depends on what scenario within T450.3(B) applies or can apply doesn't it?

Transformer that is primary topic of this thread should be allowed to have up to 250% primary protection, presuming secondary has 125% protection or up to next standard size higher. But if you do provide 250% OCPD, don't you also need minimum conductor ampacity that is sized per 310 and 240? Overcurrent protection rules that are in 450 apply to the transformer, conductor protection is not mentioned in 450.

 

[winnie]

I beg to disagree! You will let the primary circuit breaker protect the transformer for faults and let the secondary protection protect the whole transformer from overcurrent (that includes the protection of the primary lines as there will be no overcurrent in the primary if the secondary breaker trips!

I believe the above is often but not always correct.

What you describe above is correct for the transformer, but is considered a tap with respect to the conductors, and is only permitted if the appropriate tap rule is followed.

As I recall: the length of primary plus secondary conductors is limited to 25 feet, primary conductors must have an ampacity of 1/3 primary OCPD and must be protected by the secondary OCPD.

Jon

Jon