How Is Transformer Impedance Calculated and Why Does It Matter?

Published 27 Aug 2026Updated 27 Aug 20267 min readIntermediateDesign
Quick answer — Transformer impedance (more precisely the per-unit impedance voltage) is the voltage that has to be applied to one winding of a transformer to circulate rated current through the other winding when the other is short-circuited, expressed as a percentage of rated voltage. It is measured in a short-circuit test, largely set by the leakage reactance of the windings, and it governs short-circuit current, voltage regulation and parallel operation.
Contents
How is transformer impedance calculated and why does it matter?

The short-circuit test

With one winding shorted, apply a fraction of rated voltage to the other winding until rated current flows. The applied voltage as a percentage of rated voltage is the % impedance (also called the impedance voltage, often written Z%).

Why impedance matters

It sets the maximum short-circuit current (Isc = Irated × 100 / Z%), limits the fault duty the network and the transformer must withstand, influences voltage regulation (drop across the winding at load), and governs how transformers can be paralleled (they should have similar %Z so load shares correctly).

Typical values

Distribution transformers are commonly 4–6 % impedance; larger power transformers 8–18 % or more. Designers raise impedance, at some cost in losses and size, to reduce fault current and limit inrush/transient effects.

Equation — Z% = (Vsc / Vrated) × 100, and Isc = Irated × 100 / Z%
Worked example

A 10 MVA, 11/0.4 kV transformer with Z% = 6 % has a short-circuit current of 100/6 ≈ 16.7 × rated current, i.e. 167 % of rated … confirm with your design tool before relying on it.

References & standards

Related

Calculator3D modelBook seriesAcademy

Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp

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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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