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.

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.

NEC is free to read at nfpa.org/freeaccess. Inrush/point-of-supply coordination is not modeled.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: kva, primary_v, secondary_v, phase, secondary_protection, primary_fla_A, secondary_fla_A, primary_ocpd_max_A

• Protection method primary-only vs primary-and-secondary changes the permitted percentage; a labeled toggle, never silently mixedNEC 450.3(B)