I want to build a circuit to switch a 1 1/2hp pump ON and OFF. ON to fill a large tank. OFF when the tank is full. I want to use a mechanical start (push the button) with a switch to cut the power. I see this as a mechanical latching relay application. I prefer to have a 24vac coil to match other devices in the system.

I want latching so that the coil is not powered during the 3 hour fill time but only gets momentary power when the full float completes the coil circuit. After that, the coil and the pump lose power until a finger pushes button.

The catalogs I have read suggest the mechanical portion of the relay is an accessory. How do I spec this relay? I know I need to have contacts for two hot legs to the pump and contacts to control the power to the step down transformer for the coil. I am a hobbyist. Thank you for any clues.

  • 2
    \$\begingroup\$ So... a float switch? \$\endgroup\$
    – Passerby
    Jul 29, 2016 at 20:34
  • \$\begingroup\$ Are you concerned about the relay power consumption during the 3 hour period? \$\endgroup\$
    – Andy aka
    Jul 29, 2016 at 20:37
  • \$\begingroup\$ I can see the requirement for such a system but the circumstances would be pretty special: Manual input required to start the tank filling; pump runs until tank is full - even if a power failure occurs during the fill period. A latching contactor would drop out and not restart after a power failure. \$\endgroup\$ Jul 29, 2016 at 20:46

4 Answers 4



simulate this circuit – Schematic created using CircuitLab

Figure 1. Control circuit.

Your question is a little confused but I think this meets your requirements.

  • XFMR1 is the mains to 24 V AC transformer.
  • RLY1 provides power through its NC (normally closed) contact to the rest of the circuit. When the float switch contact closes RLY1 is energised and power to RLY2 is cut.
  • RLY2 runs the pump when it is energised.
  • Pressing START will cause RLY2 to be energised and run the pump. A second NO (normally open) contact bridges out the start button and keeps RLY2 energised until STOP is pressed or the float switch contacts close or the power fails.

The relays are called 24 V AC control relay. Make sure the contacts are rated for voltage and current at least as great as your load.

If this isn't quite what your require then please edit your question to clarify and post a comment below to notify me.

  • 1
    \$\begingroup\$ Nice and clean, especially the line and load fusing. :) +1 \$\endgroup\$
    – EM Fields
    Jul 30, 2016 at 5:49
  • \$\begingroup\$ The downside of this answer is that it does not maintain the "Set" condition of the pump relay should a power failure occur. This is specifically requested in the question. \$\endgroup\$ Aug 1, 2016 at 12:51
  • \$\begingroup\$ Thanks for the comment, @DwayneReid. Can you show me where I missed that? \$\endgroup\$
    – Transistor
    Aug 1, 2016 at 15:17

Panasonic makes a line of latching relays with mechanical actuators (which can also be electrically controlled, of course).


Or, you could get a dual-coil latching relay and drive it with one-shots. A float switch on the tank triggers a one-shot which drives the reset coil; and your pushbutton triggers a one-shot which drives the set coil.

In either case, using latching relays ensures that the state survives power loss.



Since you are switching mains power, Code requires that you must use commercial off the shelf mains control equipment, not random components from Mouser.

The obvious thing to me is to use exactly what you propose: a latching relay. The General Electric RR7 comes to mind. Anyplace you see light switches which are centered and you push them up or down to turn the lights on/off, that's an RR7 or competitor. It is made to control large banks of discharge lighting such as those found in the ceiling of any big-box warehouse store, typically HID lighting with transformer based ballasts with a wicked inductive kick. As such, the RR7 is well rated for inductive loads such as motor loads. It has two 24V coils, one for on, one for off. You smack one momentarily and it throws and stays.

RiB also makes some relays which mount in the knockout of a junction box.

You could also use a common off-the-shelf air conditioning relay, about $12 and fits handily in a 4-11/16 square deep box with extension. DPST, 30A throw and motor rated. This has a 24VAC coil, and you would need to hold that coil on at all times, it won't latch. You could do the latching in your control electronics, which can be low voltage, and therefore electronics-tier components from Mouser are allowed.


There is a possible easy solution to this: use an off-the-shelf Ground Fault Circuit Interrupter (RCD to our European friends).

Reset the GFCI to start the pump running. Have the upper float limit switch operate a relay that both trips the GFCI AND interrupts the power to the pump motor.

You can force a GFCI to trip by adding a load resistor from the (Hot) output terminal to the Earth Ground lead. This resistor needs to cause at least 25mA of imbalance current to flow.

The purpose of the upper limit relay removing power from the pump is in case the GFCI doesn't trip for some reason.

  • \$\begingroup\$ Using a protective device's tripping mechanism for controlling the system sounds like a very poor plan. \$\endgroup\$ Jul 30, 2016 at 2:22
  • \$\begingroup\$ Original Poster is looking for a mechanical-set, electrical reset relay that can handle pump motor current. Can you suggest any other readily-available and inexpensive options? \$\endgroup\$ Aug 1, 2016 at 12:47
  • \$\begingroup\$ Note that this is NOT an electrical-safety application but instead simply using a GFCI as an electric-trip relay. \$\endgroup\$ Aug 1, 2016 at 12:48
  • \$\begingroup\$ Panasonic makes a line of latching relays with a mechanical actuator. Either can be used. So, yes, I can suggest such an off-the-shelf solution. \$\endgroup\$ Feb 14, 2018 at 18:47

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