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Transformer Secondary Short-Circuit Current

Estimate transformer full-load current and secondary short-circuit current from kVA and impedance percentage.

Updated September 2026

How it works

Estimate transformer full-load current and secondary short-circuit current from kVA and impedance percentage.

Formula:
Ifl,3φ = S/(√3·V); Ifl,1φ = S/V; Ik ≈ Ifl·100/Z%.

Before you use the result: This is a transformer-limited approximation; source, cable and motor impedances can materially change fault current.

Practical use

Use verified material data and project geometry. Treat results as planning estimates and compare them with the energy/thermal requirements that apply to the project.

Keep units consistent with the source information.
Use project-specific densities, pack sizes and coverage rates where provided.
Round purchasing quantities to supplier packaging and delivery rules.

How to use this calculator: Transformer Secondary Short-Circuit Current

Estimate transformer full-load current and secondary short-circuit current from kVA and impedance percentage.

Quick workflow

Enter the project values from the latest drawing, specification or verified site measurement.
Check units, material properties, allowances and any code- or supplier-specific parameters before calculating.
Review the result together with the formula, assumptions and the related checks shown on this page.

Worked example

Example inputs: System: Three phase; Transformer rating [kVA]: 630; Secondary voltage [V]: 400; Transformer impedance Z [%]: 6.

Using the sample values built into the calculator, the calculation gives approximately: Transformer full-load current: 909.3 A; Approx. secondary fault current: 15 155 A.

What the result means

Calculation method: Ifl,3φ = S/(√3·V); Ifl,1φ = S/V; Ik ≈ Ifl·100/Z%.

Before using the result: This is a transformer-limited approximation; source, cable and motor impedances can materially change fault current.

Common questions

What does this calculator do?

Estimate transformer full-load current and secondary short-circuit current from kVA and impedance percentage.

Which inputs should I verify?

Key inputs to verify include: System, Transformer rating [kVA], Secondary voltage [V], Transformer impedance Z [%].

Can I use this result for final design or ordering?

Use the result as a transparent planning or checking aid. Final design, code compliance, supplier selection and project-specific decisions still need to be verified where applicable. This is a transformer-limited approximation; source, cable and motor impedances can materially change fault current.

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