Why is it a single phase rating in kVA?

Answer 1

So to really understand this one, you must be familiar with the power triangle, which can be seen at this link (near the bottom of the webpage): https://www.allaboutcircuits.com/textbook/alternating-current/chpt-11/true-reactive-and-apparent-power/

A good understanding of true, reactive, and apparent power will help, so read the whole page if you're not yet familiar with these concepts!

Particularly, this convention arises because of the power factor of the sinusoidal power. When we rate AC devices that consume power, we actually rate them in kW, because we know the power factor of the device, so we know an exact "true power" which comes from V*A*pf where pf = cos(Θ).

However, when we rate devices that supply power (such as transformers), we do not know the power factor of the load it will supply. So to rate this, we use the "worst case" in which case the pf would be 1. In this worst case, all of the apparent power is true power, and we have no reactive power (so think of the power triangle with a θ = 0°. In other words, we use kVA because because we do not know what the true power will really be.

So the summarized answer is that we use kW to rate loads and kVA to rate single phase supplies because of the power factor.