To find a transformer’s power rating, you normally read its nameplate, which lists rated primary and secondary voltages, currents and kVA or MVA. If kVA is not given, you can calculate apparent power as S = V × I for single phase or S = ?3 × V × I for three phase, using rated line voltage and current. Always use RMS values. The rating also depends on cooling class and ambient temperature. Some transformers have multiple ratings associated with different cooling modes like ONAN and ONAF.