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.

Now I want to bypass the NTCs with a contactor/relay after 100ms or so, in order to reduce the dissipation in the NTCs. The schematic below for reference. enter image description here

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?


2 Answers 2


So I consulted my transformer supplier and together we came to the following circuit: enter image description here

The contactor will make contact after 100ms after power up. After that, full load will be switched on by the control panel. Only issue would be when the contactor fails open while running 70 amps, at which the current will suddenly flow through the NTC's. In this case the NTC's are rated for 80 amps.


The contactor will carry the full load current when pulled in, so should be rated for at least 70A, maybe 110A as that is what your circuit protection is rated for.

You would probably also want a second contactor on the output side of the transformer to prevent load being applied until after the soft start has completed if doing the resistor thing, as otherwise you will likely blow out the resistors, the NTCs have an advantage here.

If you do the resistor thing, make sure the resistors are rated for sufficient pulse power handling (And voltage), you may find specialist carbon composition parts helpful here.

  • \$\begingroup\$ Thanks for your comment. I decided to go with the NTCs together with the bypass, as posted in my anwser \$\endgroup\$
    – Ezraneut
    Commented May 29, 2018 at 12:26
  • \$\begingroup\$ Pretty sure K4 coil is wired incorrectly, the top connection should I think go to the other side of the K4 contact? \$\endgroup\$
    – Dan Mills
    Commented May 29, 2018 at 13:19

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