When you have primary and secondary protection for a transformer,art.450.3(B) allows you to size the primary ocpd to 250%. If you do this how do you size the conductors? Do you use the nameplate or do you size at the 250% amprage.
This is the way I understand it. Take a look at the 25' tap rule and apply it as if it were a 25' tap. Otherwise you would have to size the conductor based upon that 250%.
I've always considered that the sec. OCPD protects the transformer form overload. Then the primary cable which feeds the transformer would be protected from overload as if it were a tap conductor.
The 250% rating for the primary OCPD then protects the upstream distribution system from a transformer failure. If one is able to apply the 250% rule it provides extra head room to prevent nuisance tripping of the pri. OCPD when the transformer is energized with its higher magnetic (instantaneous) trip value. If this is not a problem I don't see any advantage and stay with the 125%. You may have to anyway if you're dealing with more than 25'.
