A single phase transformer has no load of 64 W, as obtained from an open circuit test. When a short circuit test is performed on it with 90% of the rated currents flowing in its both LV and HV winding. The measured loss is 81 W. The transformer has maximum efficiency when operated at:
1
50.0% of the rated current
2
64.0% of the rated current
3
80.0% of the rated current
4
88.8% of the rated current