1. :Why transformer ratings are in kva?

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3. KVA is the same as kilowatts if the power factor is unity. But the losses and heat generated within the transformer are due to the current flowing through the windings, not the power output. So if the power factor is not unity you cannot run it at the same kilowatt power output. Does this answer your question?

4. yeah,i want little more explanation

5. What units would you think should be used? If you are selecting a transformer, you know it will need to supply a certain load, which is in units of watts, kilowatts, or megawatts. But for the reason I stated, they are not rated in watts, kilowatts, or megawatts. What else do you want to know about it?

6. It must used kilo watts only or any watts kile mega but why kva instead of watts?

7. In DC, the classical relation is W = V.A.

Things are different with AC. The relation between W, V and A is W = V.A.cos(phi) where phi is the phase between current and voltage. This cos(phi) is the power factor mentionned by Harold1470.

For instance if you connect an AC source to a pure capacitance (or inductance), an alternative current crosses the circuit, but the phase being 90°, the transfered energy (W) is null.
Have a look a this wiki page : AC power - Wikipedia, the free encyclopedia

8. Because losses in the transformer not depend on the power factor.Only depends on voltage and current.

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