Why transformers are rated in KVA instead of KW?

  • The copper loss (I2R) in the transformer depends on the current through the winding.
  • The iron loss or core loss of the transformer core depends on the voltage as frequency of operation is constant.
  • None of these losses depends on the power factor(cos Φ) of the load.
  • So losses decide the temperature rise and hence the rating of the transformer.
  • As the losses depends on V and I only, the rating of the transformer is specified as the product
  • of these two parameters (V x I).
  • Thus the transformer rating is in KVA and not in KW.