Why Transformer rating is given in kVA?

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When power transfer from primary winding to secondary winding of transformer there are two losses in transformer one is called core losses this is due to the alternating flux in core and second is copper losses this is due to current flowing in the winding when the transformer is loaded.

The I2R or copper loss is depends on current (I). And core loss is depends on voltage (V). These two losses are not depends on power factor (cos Φ) of the load. So, the temperature rise and rating of transformer is decided by the two losses one copper losses which depends on current (I) and second is core losses which depends on voltage (V). So, losses of transformer depends on V and I that's why the rating of transformer is given as the product of voltage (V) and current (I). This product of voltage and current is called as VA rating.
The VA rating of both side of transformer is same. And this rating is generally expressed in kilo volt ampere rating.

kVA = V1I1/1000 = V2I2/1000
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Transformer Rating is in kVA: The life of insulation of transformer depends upon temperature. Temperature rise results from losses of transformer. The copper loss of transformer depends on the current and the iron loss depends on the voltage. Hence total transformer losses depend on volt-amperes and not on phase angle between voltage and current. The losses are independent of load power factor. To prevent transformer from damage due to temperature rise, it is highly essential to limit the losses. The limiting values are referred as rating. To limit the losses, the operating voltage & current must be maintained within limits. Hence transformer rating is in kVA.
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Output power of transformer is given by P= VICosØ, for different types of load i.e (resistive, capacitive, inductive) cosØ changes so, for same voltage and current output power will different, so transformer is designed to operate at particular voltage and current levels and it not designed to deliver particular output power that is why rating of transformer is in KVA.

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As copper loss of a transformer depends on current and iron loss on voltage, Hence total transformer loss depends on volt-ampere and not on phase angle between voltage and current ie. It is independent of load power factor. That is why rating of transformer is in KVA.