Look for Transformer Test report. It will have Iron losses which will remain almost constant through out the loading cycle. Then look for efficiency on the name plate. This corresponds to full load. Now calculate the full load losses. Example if 1000KVA transformer efficiency is 98% then the losses are 2% i.e. 20KW. If the Iron Losses is indicated as 2KW, then the copper losses are 18KW. Now the copper losses are proportional to square of current. Therefore at 50% load copperlosses will be 0.5 sqare times the full load copper loss. i.e. 0.25X18=4.5KW. Therefore at 50% load you will have 6.5KW losses. If you dont know what is the Iron losses of the transformer, then see the noload power consumption of the transformer, it will be very close to iron losses.