The Answer:
Copper losses depends on current which is generally passing through transformer winding while iron losses or core losses or insulation losses depend basically on voltage. Therefore, that's why the transformer rating may be expressed in kVA/not in kw.
In addition, KVA =kilo × Voltage × Current. When we design a transformer we do not know for which type of load it will use the load. It may be Resistive, capacitive or inductive. We only know voltage and current and its design process does not depend upon the power factor.
In case motor the rating in KW=kilo×voltage×current× p.f Regarding motor, we know for which load will be work on motor. (Resistive, capacitive and inductive ) we know power factor of motor like in case resistive load I think it will be 0.8 but in transformer case we do not know power factor because we do not know which type of load it will be work. And, transformer loss depends on voltage (iron loss) and current (copper loss). It's not depend on power factor because in design time we do not know power factor of transformer.
