Read the transformer’s kVA rating and rated primary and secondary voltages. Select its phase system to calculate the theoretical full-load line current on each side.

Formula: single phase: line A = kVA × 1,000 ÷ V; balanced three phase: line A = kVA × 1,000 ÷ (√3 × line-to-line V); calculate each side at its own rated voltage

Worked example

A 25 kVA single-phase transformer rated 480 V primary and 240 V secondary has theoretical full-load currents of 52.083 A and 104.167 A respectively. Its rated voltage ratio is 2:1.

Use line-to-line voltage for three phase

Enter the line-to-line voltage on each side. For a 208Y/120 V secondary, enter 208 V for this total three-phase calculation, not 120 V. It assumes balanced loading and reports line current; winding current depends on the transformer connection.

kVA already expresses apparent power

The nameplate kVA rating is apparent power, so these rated-current equations do not use a load power factor. They do not convert input watts, model magnetizing current, or predict real operating current from load efficiency.

Full-load current is one nameplate calculation

This tool does not select breakers, conductor sizes, taps, overload settings or transformer capacity. Inrush, losses, harmonics, cooling, load imbalance and manufacturer instructions require separate consideration. For split-phase equipment, the result is at the entered full secondary voltage; it is not an allowance to draw that entire kVA from one leg.

Explore the related calculation

For a different starting point, try the line-to-line and line-to-neutral voltage guide or return to the main calculator. Browse the complete tool directory for this site.

Reference and corrections

The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.

Found a problem? Email contact@wattstoamps.app.

Transformer current calculator

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