Transformer Conductor and Overcurrent Protection
Primary/secondary FLA and the NEC 450.3(B) overcurrent maxima plus the 240.21(C) secondary-conductor minimum for a transformer.
Formula and source
FLA = kVA*1000/V (1-phase) or kVA*1000/(sqrt(3)*V) (3-phase). Primary/secondary OCPD per NEC Table 450.3(B) (125% / 167% / 250% / 300% bands keyed to current and whether secondary protection exists; Note 1 next-higher standard size for the 125% case). Secondary conductor ampacity >= secondary FLA per NEC 240.21(C).
Transformer primary/secondary full-load current and overcurrent-protection maxima per NEC Table 450.3(B), with the secondary-conductor tap rules of NEC 240.21(C), by name. A computational aid for the <= 1000 V case; the AHJ-adopted NEC edition and the design engineer govern.
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