Transformer ratings are known to be measured in kVA and not in kW. A transformer's rating or the rating of any electrical machine shows its ability to carry mechanical load without showing any signs of overheating. Rises in temperature, which are known to be major threats to insulation, comes up as a result of internal loss within the machine.
Two known types of losses exist in a transformer;
- Insulation losses or core losses or Iron losses
- Copper losses
Copper losses (I2R) are uneven losses that are mostly dependent on the amount of current that passes through the windings of the transformer while Insulation losses or Iron losses or Core he voltage losses are mostly dependent on the voltage.
This shows that the transfer is designed for Iron losses-rated voltage and copper losses-rated current. It is not possible for us to make a prediction on the power factor during the process of designing the machine, since power factor is dependent on the load which is not stable.
When manufacturers produces UPS, transformer, etc., they are never able to predict the exact type of load that would be used and consequently rate the device based on its maximum output of current that can be safely carried by the conductors (at unified Power Factor) as well as the conductors' insulation rating (Voltage & temperature). That is why the transformer rating can be in kVA, and not in kW.