MBE / HUB-Certified Texas MEP
Transformer FLA Calculator
Quick lookup for full-load amps on a three-phase or single-phase dry-type transformer per NEC Article 450. Pick the kVA, voltage, and phase — we return the FLA on both primary and secondary, plus a code-minimum OCPD recommendation.
Common nameplate ratings: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750.
Three-phase formula uses √3 factor.
Most TX commercial work is 208Y/120V or 480Y/277V three-phase.
Step-down primary is most often 480V on commercial gear.
Secondary FLA
208A
Basis: 75 kVA × 1000 / (208 × √3) = 208.2 A
Primary FLA: 90.2 A · Recommended OCPD: 125% × FLA per NEC 450.3(B)
125%
Per NEC 450.3(B), primary OCPD is sized at up to 125% of the calculated primary FLA (with rules for higher % when no secondary protection is provided). We always show both the calc and the rounded-up standard breaker on our submittals.
Need this stamped on your one-line?
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Important. This tool returns the formula-based full-load amps and standard OCPD multiplier. It does not account for inrush, harmonic-rich loads (K-factor transformer derate), short-circuit available fault current, or AHJ-specific amendments. Sealed one-line drawings require a licensed engineer.