Impedance (%Z) — Transformer Glossary

Impedance is one of the most important transformer parameters. It determines: (1) the magnitude of short-circuit current — lower impedance means higher fault currents; (2) voltage regulation — higher impedance causes greater voltage drop under load; (3) parallel operation — transformers must have matching impedance values (within 10%) to share load proportionally. Typical values: 4–6% for distribution transformers, 8–12% for power transformers. The impedance is measured during routine testing by applying increasing voltage to the primary with the secondary short-circuited until rated current flows.