Alternatively, the percentage of a transformer can be described as the percentage of the nominal voltage in the primary that is required to circulate the rated current in the secondary.

The impedance of a transformer can be measured by means of a short-circuit test.

The secondary of the transformer whose percentage impedance is to be measured is shorted. The voltage on the primary is gradually increased from zero till the secondary current reaches the transformer's rated value.

The percentage impedance of the transformer is calculated as

Z%= (Impedance Voltage/Rated Voltage)*100

Thus a transformer with a primary rating of 110V which requires a voltage of 10V to circulate the rated current in the short-circuited secondary would have an impedance of 9%.

The percentage impedance of a transformer a crucial parameter when operating transformers in parallel. It also determines the fault level of a system during faults.