Transformers are rated for power output in kVA or MVA based on continuous operation under specified temperature rise and cooling conditions. Nameplates show rated voltages, currents and frequency, from which apparent power is computed. For example, a three phase transformer with rated line voltage V and current I has rating S = ?3 × V × I. The rating assumes particular cooling class and ambient temperature. Some transformers also have overload or dual ratings for different cooling stages, such as ONAN and ONAF, with higher kVA when forced cooling is active.