Copper losses are directly proportional to the square of the load on the transformer. Wcu=x2Wcufl Here, x is the percentage of a full load of the transformer. Wcufl is the copper losses at the full load. Given that, x = 25% = 0.25 =
1
4
Copper losses at full load =Pcu Copper loss at 25% of full load =(0.25)2×Pcu=