From a transformer’s kVA rating and its voltages, work out the full-load current on each side and the turns ratio — the numbers behind sizing cables and protection.
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Start free trialA transformer’s kVA rating and its voltage fix the full-load current on each side: current is the VA divided by the voltage (and by √3 for three phase).
Because power in equals power out (ignoring losses), stepping voltage down raises the current in proportion — and vice versa.
The ratio of primary to secondary voltage is the transformer’s turns ratio, which also tells you how the currents relate between the two windings.
Divide the rating in VA by the voltage. For a single-phase transformer, I = VA ÷ V. For three phase, I = VA ÷ (√3 × V line). Do it separately for the primary and secondary voltages to get the full-load current on each side.
The turns ratio is the ratio of primary to secondary voltage (and turns). An 11,000 V to 400 V transformer has a ratio of about 27.5:1. It steps voltage down by that factor and steps current up by roughly the same factor.
kVA is apparent power — the total power the transformer can deliver, combining real power (kW) and reactive power (kVAr). Transformers are rated in kVA because their limits are set by voltage and current (heating), independent of the load’s power factor.
No — it gives ideal full-load currents assuming no losses. Real transformers have copper and iron losses and an efficiency typically in the high 90s percent, and inrush current at switch-on is much higher than full-load current. Use manufacturer data for detailed design.
This calculator gives ideal full-load figures and is a general engineering aid, not a substitute for design to AS/NZS 3000 and the relevant standards. It ignores losses, impedance and inrush. Installation and protection must be designed and verified by a licensed electrician.
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