Just a question about table 450.3(B)...When can the 250% of primary rated current be used for overcurrent protection? Is it only when the transformer is equipped with manufacturer-supplied thermal overload protection, or can it be used for a "standard" transformer with 125% max rating for secondary protection? Just to clarify, can I protect a "standard" 45kva transformer (480 delta to 208y/120) with a 125 amp breaker (45,000/480X1.732)X250% =approx. 135 amps...and then downsize to next standard size (125)? And then have a 175 amp breaker on the secondary side (45,000/208X1.732)X125%=approx. 156 amps...and then upsize to the next standard size (175)? The table doesn't spell things out very well :?
250% Transformer Primary Protection
- Thread starter A-1Sparky
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- Connecticut
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It can be used for a "standard" transformer. A 45kVA transformer with a 125A primary OCPD and a 175A secondary OCPD would comply with 450.3(B) "Primary and Secondary" Protection.
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- Illinois
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- retired electrician
Looks like you have it correct.
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