I currently use a DPDT 30 Amp relay to switch between a utility supplied 120 VAC and an inverter supplied 120 VAC. The relay has a 120 VAC coil. I use the inverter 120 VAC power source to energize the relay coil. So whenever the inverter is turned on, it energizes the relay and switches the supply power from utility to the inverter. The problem is, the efficiency of the inverter supplied power is diminished by the amount of current it takes to hold the energizing coil. So, if I could latch that energizing coil to the on position until the inverter is turned off, I could increase the efficiency of this switch. I am concerned about the failure mode though. The failure mode of a solid state solution to this is why I haven't pursued that route. Any thoughts on this would be appreciated. Does anyone have a solution?

  • \$\begingroup\$ What is the holding current of the relay? \$\endgroup\$ Apr 4, 2020 at 15:05
  • \$\begingroup\$ Is utility power limited such that utility power could not hold the relay? \$\endgroup\$ Apr 4, 2020 at 16:06
  • 3
    \$\begingroup\$ If you want an automatic transfer switch you should probably buy one. \$\endgroup\$ Apr 5, 2020 at 4:57

2 Answers 2


I currently use a DPDT 30 Amp relay to switch between a utility supplied 120 VAC and an inverter supplied 120 VAC.


Does anyone have a solution?

It's a double throw relay (DPDT) so energize the coil from the incoming AC (from the mains) and rewire the relay contacts accordingly so that when the relay is inactivated (no AC power), the inverter output is connected to the load.

  • 2
    \$\begingroup\$ Correct, OP is doing it upside down and backwards. Utility power should be treated as the abundant resource. \$\endgroup\$ Apr 4, 2020 at 16:08

Here's a solution, with a standard DPDT relay energised by the utility supply but de-energised by the inverter through a low coil consumption relay.

enter image description here

  • \$\begingroup\$ The OP's mains is always on. This will never switch to inverter. \$\endgroup\$
    – Transistor
    Apr 5, 2020 at 12:11
  • \$\begingroup\$ Hi Transistor, Many thanks for drawing attention to the Op's prime requirement of the inverter being in command. That changeover to inverter should happen only when the utility is down, is a typical mindset. This could be a case of changeover to solar power when available. My post requires editing. \$\endgroup\$
    – vu2nan
    Apr 6, 2020 at 6:44
  • \$\begingroup\$ It's been done! \$\endgroup\$
    – vu2nan
    Apr 6, 2020 at 7:07
  • \$\begingroup\$ Maybe, I should try to clarify this operation. To Michael Harvey, the holding current to the coil is .5 Amp. To Harper - Reinstate Monica, it was never my intention to use the utility power to operate the relay, but to minimize the use of utility power altogether. If my batteries would hold up, my inverter would operate my loads all of the time. \$\endgroup\$
    – sdingman
    Apr 19, 2020 at 22:04
  • \$\begingroup\$ It works beautifully. I'm only trying to minimize coil holding current or eliminate it altogether. To vu2nan, This is what I’ve been looking for, but can’t seem to find. I've been looking for a Latching relay, but a Low Holding current coil would do the trick. \$\endgroup\$
    – sdingman
    Apr 19, 2020 at 22:30

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