I am constructing an ATS (Automatic Transfer Switch) for a diesel generator with a max current of 300 A. Now I already have an AC3 contactors with Ith = 100 A and I don't want to waste the money on new ones for the ATS. I know the AC3 contactors are used for squirrel cage induction motors with starting current equals to 5 to 7 times the rated current of the motor (5 to 7 Ith). Can I use these contactors (Ith = 100 A) to build the ATS for the 300 A generator?

  • 1
    \$\begingroup\$ What's an AC3 contactor? In any case probably not, motor start relays are meant to handle very high current for a very short time, and giving them too high of a current continuously will make something melt. \$\endgroup\$
    – Hearth
    Nov 20, 2021 at 15:41
  • \$\begingroup\$ AC3 electrical standards for contactors \$\endgroup\$
    – user137684
    Nov 20, 2021 at 15:48
  • \$\begingroup\$ @Hearth en.wikipedia.org/wiki/Utilization_categories \$\endgroup\$
    – Jeroen3
    Nov 20, 2021 at 20:11

3 Answers 3


Can I use these contactors (Ith = 100 A) to build the ATS for the 300 A generator?

Brand new ones can probably do 300A. But only if you operate them at zero current. Check if the pole losses at 300A are lowed than the pole impedances. Contactors can carry a lot more current than "rated" if you drop the requirement that they will actually be able to operate, and not weld to death on first opening and closing.

So, grid fails -> contactor opens -> contact closes -> generator cranks.

Problem being returning to grid requires pulling the main fuses.

Don't fool around.
The last thing you want is an failure to open destroying your generator and electrical fire.


You need contactors specified for 300 A current and continuos operation of many hours and even several days.

A motor starting current may be 5 to 7 times the rated current but it should last only less than a second or only few seconds. 5 times the current means 25 times of heat production possible only a short time. The contactor would be overheated and destroyed by a current much higher than continuos rated current lasting for many hours. A fire may be caused.


A reliable system would warrant the use of a pair of motor duty contactors with mechanical interlock, as in a 400 V AC 330A 160 kW reversing motor starter. Electrical interlocks in the control circuit would be provided by auxiliary NC contacts.


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