The equivalent series winding Z = R + jX, in ohms, at rated frequency and reference temperature, across the terminals of one winding of a pair, when the terminals of the other winding are short-circuited and further windings, if existing, are open-circuited.

For a three phase transformer the impedance is expressed as phase impedance (equivalent star connection).

In a transformer having a tapped winding, the short-circuit impedance is referred to a particular tapping. Unless otherwise specified the principal tapping applies.