Free Electrical calculator

Transformer current calculator

Calculate primary and secondary full-load current from transformer kVA rating and winding voltages.

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All values remain in your browser. Review the assumptions before using the result.

Your result will appear here.Calculate to see the outputs, formula and substituted working.

How the calculation works

Full-load current on both sides from the nameplate kVA rating.

Formula

I = S ÷ V (1Φ) or I = S ÷ (√3 × VLL) (3Φ)

Worked example

ArrangementThree-phase

Rating500 kVA

Primary11,000 V

Secondary400 V

Expected currents26.24 A / 721.69 A

Assumptions and limitations

Assumptions

The kVA rating applies to both windings.
Three-phase voltages are line-to-line.

What this does not check

Does not include losses, impedance, vector group, inrush, harmonics or tap position.
Does not size protection, conductors or switchgear.

Formula version: 1.0 · Last reviewed: 16 August 2026

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Engineering basis: first-principles electrical relationships using user-entered values. The page does not reproduce BS 7671 tables or establish compliance. Complete the project-specific design against the current BS 7671 information, manufacturer data and the applicable project requirements.

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