FIELD GUIDE • UPDATED SEPTEMBER 2026
Transformer Sizing Guide
Calculate primary and secondary current, then apply protection and grounding rules.
Transformer kVA describes apparent-power capacity. Full-load current depends on kVA, voltage and phase. For single phase, divide volt-amperes by voltage. For balanced three phase, divide by voltage multiplied by the square root of three.
Field method
Calculate both primary and secondary current because conductor sizes and protective devices are evaluated on each side. A 75 kVA, 480-to-208-volt, three-phase transformer has very different currents on the two sides.
Current is only the beginning. Transformer overcurrent protection, conductor protection, secondary-conductor length and routing, disconnecting means, ventilation, working space, grounding and bonding all require separate review.
Common mistakes
Common mistakes include omitting the square root of three, using phase-to-neutral voltage in a line-to-line formula, treating calculated current as the final breaker size, and overlooking inrush or manufacturer instructions.
Start with NEC Article 450 for transformer protection. Also review Articles 240, 250 and 310, plus applicable secondary-conductor rules. Use the WireWise transformer planner to calculate full-load currents, then verify all final selections.
Never energize a transformer installation based only on a web calculation. Confirm nameplate connections, taps, system grounding, phase rotation, available fault current and equipment ratings.