In the context of transformer protection, if a transformer has 3% impedance, which percentage describes its impedance?

• A. 2%
• B. 4%
• C. 3%
• D. 7%

Answer: (C)

Percent impedance is the value used to describe a transformer’s ability to carry current during a short circuit, expressed as a percentage of rated voltage. It tells you how much voltage is needed to push the transformer to its rated current when the secondary is shorted. If a transformer is specified as having 3% impedance, that percentage itself describes its impedance. In other words, the impedance is 3% of the rated voltage.

For context, a lower impedance (smaller percent) means a higher short-circuit current, while a higher impedance means a lower short-circuit current. So 3% implies a certain fault-current level—roughly 1 divided by 0.03 (about 33 times rated current) in per-unit terms. The other numbers would correspond to different fault-current levels, but the given specification defines the actual impedance as 3%.