Why Transformer Rating is in kVA not in kW

Two Reasons why are transformers rated in KVA?

1. The first reason is loss and,
2. Second reason is load connected to transformer

Losses in transformer:

1. Only Copper losses and Iron losses occurs in transformer. (No Windage & Friction)
2. Copper loss occurs in winding due to flow of current through it during transfer of power from the primary to secondary winding.
3. Iron loss consist of Eddy current loss and hysteresis loss which is function of voltage.
4. Therefore total losses are due to voltage (V) and current (A) and depends on power factor.

Load connected to transformer:

1. The loads that will be connected to the transformer are never known to the manufacturer at the time of transformer design.
2. Hence it cannot be rated in kW. The load kW depends on the power factor of the load.

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