I am designing a soft start circuit for a 70KVA, 70A, 480/230 delta/star transformer, since the fuses kept blowing without.
I've been using 3 NTC thermistors rated for 80A in series with the lines limit the inrush current, but obviously get very hot at nominal load current.
Since I am not very experienced with high power systems, I have a couple of uncertainties:
What should the current/KVA rating of the contactor/relay be? It will not be switching on the full load, only sourcing when already energized. It can however be happening that someone switches the main disconnect switch on full load. Does this create a arc in the relay, or is this limited by the NTC's which are still in series?
Do the inrush current limiters need to be NTC's or can I use regular (i.e. 5 ohm) power resistors?
Is it important to have the relay rated for US 480V, or will a EU 400V version do?