I have a 120V coil contactor relay connected to a pump. The pump is connected on the control terminals of the relay. I would like to switch another circuit when the pump circuit is turned on, but this problem has to do with the control circuit. When I energize the circuit, the relay clicks but the pump does not turn on. On the other hand, if I jump the relay (bypass the relay), when I energize the circuit the pump does turn on. I’ve tried two different relays with the same result. Pump is here https://www.pexuniverse.com/armstrong-astro250ci-circulator-pump Relay is here https://canada.newark.com/carlo-gavazzi/gdp251ss120v/contactor-spst-no-120vac-panel/dp/45Y3714 enter image description here

2 Answers 2


It sounds (and looks) like you've put the relay coil in SERIES with the pump, not in PARALLEL with the pump. Correct that and both should work as expected.

So, hot from whatever turns the pump on goes to pump and one end of relay coil. The other end of the relay coil goes to neutral, as does the other side of the pump.

  • I agree the coil needs 120 to pull in then the contacts close on the hot or both hot and neutral contacts to power the pump.
    – Ed Beal
    Commented Oct 13, 2019 at 18:04
  • From reading the question: There's already an upstream relay powering the pump. The new relay is to power "something else" when the pump is powered. So all that's needed is to put the new relay's control coil in parallel with the pump, NOT to put the pump on the new relay's load contacts.
    – Ecnerwal
    Commented Oct 13, 2019 at 21:59
  • @Ecnerwal, exactly, the pump is on the control circuit. I didn’t realize it had to be wired in parallel, though now that I look at the diagrams again they all indicate this. The “something else” is a boiler, the installer didn’t plumb in a flow check valve. The relay will ensure that the boiler fires only when the pump is running (this doesn’t account for the case where the pump breaks but still draws current but better than not having it). Thanks for the help.
    – Michael C
    Commented Oct 14, 2019 at 1:02

Your relay needs line and neutral to operate. The motor also needs the same thing but it is controlled by the relay. You never interrupt only the neutral line so the relay interrupts the line side.

Here is a back of business card drawing of what I am saying.

Relay and Motor drawing

The power for the relay and motor could be the same or different depending on the size of each. Note that a large motor could have two or three hot line which each need to be interrupted by the relay however that does not seem to be the case here.

For a small system the two line and the two neutral wires would then be jumpered together. This would be the case if the motor is also 120 V and low horsepower. If the motor is 240 V, you would need a two pole relay. If the motor is several horsepower, I would assume you would want your wire separated like my drawing.

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