Open any transformer nameplate and you will find its capacity written in kilovolt-amperes (kVA), not kilowatts (kW). This is not arbitrary; there is a precise engineering reason behind it, and the question comes up in almost every interview.
The Core Reason: The Manufacturer Doesn't Know Your Load's Power Factor
Real power kW depends on the load's power factor (kW = kVA × PF), and the power factor is determined by the consumer, not the manufacturer. The manufacturer doesn't know whether the transformer will feed air conditioners, heaters, or motors, so the transformer is rated by the apparent power kVA, which does not depend on the power factor.
The Second Reason: Transformer Losses Depend on V and I, Not PF
- Iron losses in the core depend on the voltage (magnetization).
- Copper losses in the windings depend on the current (I²R).
- Neither loss is related to the load's power factor; the thermal limit of the transformer is determined by the values of V and I, i.e. the apparent power S = V × I.
A 1000 kVA transformer feeding a load with a power factor of 1.0 delivers 1000 kW. The same transformer with a load at a power factor of 0.7 delivers only 700 kW, even though the transformer carries the same current and heats up to the same degree in both cases. That's why the fair rating is in kVA.
Sample answer: For two reasons: first, the manufacturer doesn't know the power factor of the loads that will be connected to the transformer, and real power kW depends on it. Second, the transformer's losses and thermal limit depend only on the voltage (iron losses) and the current (copper losses), i.e. on the apparent power S = V × I, regardless of the power factor.
Answering "because kVA is larger than kW" is a shallow answer that costs you points in an interview. The required answer must connect to the load's unknown power factor and to the nature of the transformer's losses.
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