Step up and step down transformers are always rated but unlike bulbs, their power rating is always given in kVA whereas bulbs are rated in watts. Why is it so? Is their any specific reason to this?
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It is because losses in transformer depends on voltage and current that is V and I and not not power factor.
i.e. KVA= K * voltage * current
Hence rating of transformer is given in KVA and not in watt