Transformer Sizing Calculator

How transformer sizing works per NEC 450

Full-load amps (FLA) for a transformer are calculated from the kVA rating and the system voltage. For single-phase circuits, divide kVA × 1000 by the voltage. For three-phase circuits, divide kVA × 1000 by the voltage multiplied by √3 (1.732). These FLA values determine conductor sizing and overcurrent protection requirements on both the primary and secondary sides.

NEC 450.3(B) governs overcurrent protection for transformers 600V and under. Under primary-only protection — when the secondary has no separate OCPD — the primary overcurrent device must not exceed 125% of the primary full-load current for currents at or above 9A. The calculator rounds up to the next standard OCPD size per NEC 240.6(A). Always verify secondary protection requirements, transformer impedance, and the full table at NEC 450.3(B) for your specific installation.