NEC Transformer OCPD Protection


Tempe, AZ
Per NEC 450.3 the primary and secondary OCPD can be sized higher than the FLA rating of the transformer. For example, for a 2500kVA transformer 12.47kV primary-4.16kV secondary, the primary can be sized up to 600% and the secondary circuit breaker can be sized up to 300%. My question is why does the NEC allow setting up the primary and secondary way higher than the nominal rating of the transformer FLA?

jim dungar

Staff member
Retired Engineer
Partly because medium voltage protective devices are often applied slightly differently than their LV counterparts. For the most part, at medium voltage 'distribution', the industry is often more concerned with short circuits and ground faults than with overloads. Medium voltage protective devices often have different time current curves than LV devices.
One example is 450.3, where 480V fuses and breakers are sized to the same maximum value , but using the values from your transformer, MV fuse and breakers have different sizes.
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