How to Wire Two Floats Together With a DPDT Relay

Written by pauline gill
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How to Wire Two Floats Together With a DPDT Relay
Electrical wiring should be routed carefully. (electrical panel switch image by jimcox40 from

Differential-gap controls allow basement sump levels to be regulated within a level band instead of at a single level point. This practice greatly lengthens the life of a sump pump by eliminating "on-off" cycling of a single bouncing float switch. The circuit also improves the quality of life for the owners of these sumps, providing them freedom from the annoyance of a noisy water pump cycling on and off all night. The circuit can be implemented by wiring two floats together with a DPDT switch.

Skill level:

Things you need

  • Two single-pole float switches
  • One DPDT (double-pole, double-throw) switch rated for 15-amps, 240-volts
  • Existing sump pump
  • Wire nuts
  • Electrician's tools

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  1. 1

    Mount one float switch at the desired high level point, and the other at a lower level point. Connect 120-volt power hot (black) to one lead of the top float switch and the COM (common) terminals of both switches on the DPDT relay. Use insulated wire nuts for all two-and-three-wire lead connections. Strip 5/8-inches of insulation off the wire, twist the stranded wire end and insert the wire under the terminal tab. Tighten the tab screw for all terminal connections.

    How to Wire Two Floats Together With a DPDT Relay
    Switches and relays with routed wiring (the relay image by Victor M. from
  2. 2

    Connect 120-volt power common (white) to one of the lower float switch leads and to the pump motor common (white) lead.

    How to Wire Two Floats Together With a DPDT Relay
    Gap controls for tank level control (Water treatment plant image by Andrei Merkulov from
  3. 3

    Interconnect the remainder of the logic circuit by connecting a wire between the remaining lower float switch lead and one of the relay coil leads. Also run another wire between the remaining upper float switch lead to the other relay coil lead and the normally open (NO) terminal of one of the relay switches.

  4. 4

    Complete the pump's connections. Connect the pump's 120-volt hot power lead (black) to the remaining NO terminal on the relay switch. Connect the pump's ground lead (green) to the ground lead in the electrical box (green).

  5. 5

    Check the entire circuit by visually tracing the logic steps. The intent is to turn on the sump pump when the water level reaches the high level float, and to run the pump to empty the sump until it drains the water to the point of lowering the bottom float, where the pump shuts off. When the water rises above the lower float, it switches power common (white) to one side of the relay coil. However, the coil does not energise to turn on both relay switches until the water continues to rise enough to lift the upper float switch. When the upper float switches on, it energises the other coil lead and the relay coil is energised which actuates both of the relay switches. One relay switch turns on the sump pump. The other relay switch bypasses the top float switch, latching the pump on until it reaches the lower level. When the level goes below the lower float switch, it switches off, and the relay coil is again de-energised, shutting down the pump and unlatching the relay until the next rising-level cycle.

Tips and warnings

  • For an existing 230-volt pump, use a DPDT relay with a 230-volt relay coil.
  • All loose lead connections should be installed inside watertight exterior-grade electrical boxes.
  • This project should not be attempted by anyone who is not completely familiar with domestic electrical wiring practice and all of its terminology.
  • Install a separate breaker for this project, and make sure it is switched off before starting to wire the system.
  • Use safety eye wear on all projects.
  • Use insulated electrician's tools to minimise electrocution risks.

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