Why transformer rating in KVA:
Actually, every one says it is due to core losses and copper loss of the transformer, but actually, that is not the correct answer. Let’s see the exact answer for Why transformer rating in KVA
Simply we can say that the machine which is the ability to deliver real and reactive power which machine can be rated in KVA. Actually, the Iron losses and copper losses are common for all electrical machines such it may be either motor or alternator or transformer. But in Transformer that is not the reason for calculating losses.
The real reason is that all AC sources such as transformer or alternators are rated in VA (or KVA or MVA depending upon size requirement) is because the load power factor of current is decided by the load connected with. And since the load power factor is not known at that time of transformer is being manufactured, they are rated considering resistive load (PF = 1).
If the load is inductive + resistive (for example, induction motor), some part of power from the transformer will go into supplying the reactive power (VAR) and remaining for supplying the active power (W). But the power supplied from the transformer (apparent power, VA) will still be the vector sum of VAR and W. Transformer will deliver both real and reactive power at a time that’s why transformers are rating in KVA.