Transformer Conductor and Overcurrent Protection
Transformer primary and secondary FLA with the NEC 450.3(B) overcurrent maxima.
Example
You enter
- Transformer kVA 45
- Primary voltage (V) 480
- Secondary voltage (V) 208
- Phase 3
- Protection provided false
You get
- Primary FLA (A) 54.13
- Secondary FLA (A) 124.91
- Max primary device (A) 70
Checked against published worked examples from: NEC Table 450.3(B) and 240.21(C) (by name).
Details, formula, and sources
ScopePrimary/secondary FLA and the NEC 450.3(B) overcurrent maxima plus the 240.21(C) secondary-conductor minimum for a transformer.
Formula or tableFLA = 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).
Source and editionTransformer 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.
Free accessNEC is free to read at nfpa.org/freeaccess. Inrush/point-of-supply coordination is not modeled.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field nameskva, 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)