Assuming you are talking about a 3 phase system, the kva per phase is the total divided by three. So for your 1 kva transformer, the rating would be 333.3 voltamps per phase. To get the current, divide the voltamps per phase by the voltage per phase. Voltage per phase for your example is 415 divided by sqrt 3, or 239.6. So your current per phase is 333.3 divided by 239.6, or 1.39 amps. I usually divide kva by phase to phase kv then divide that by sqrt 3.
It has been very helpful to me to remember some of the numbers for voltages that I work with so that I don't have to calculate it each time. So I just remember that 1000 kva at 12.47 kv is about 46 amps, so at 25 kv it's about 23. Then you can rapidly convert voltamps to an approximate number without having to calculate it.
