We have two homes sharing a single well. The other homeowner and I want to change our water system so that we each have our own Pressure Tank and we can each run the well pump with power from our own home. Currently, my electricity and pressure tank, is supplying water to both homes. Our homes are 500' apart, and each home is about 300' from the pressure tank.

I'm looking for an Automatic transfer switch that will send power to the well pump from the Pressure switch of either of our Pressure Tanks. Do any of you know of an Automatic Transfer Switch that will work for this purpose. Our pump circuit is 20 Amp, 240 Volt. It draws only 1600 watts when on. We are using 10/3 wire with ground because of long wire runs. 300' runs from either his or my circuit Panels to the pressure tanks (Installed in a shed on our mutual property line). It is another 250' from our shared Pressure tank shed to the shared well.

For the last two years the well pump has been metered. We know that the pump typically only uses about $5/month worth of electricity. We plan to also meter the water mainline to each of our homes. Since the cost for electricity to run the well is currently low, our main concern is giving my neighbor independent control over his water source.

  • Do you want both of you to power it? Or just one of you? (Both of you gets a lot more complicated.) Nov 11, 2018 at 3:01
  • What happens if both pressure tanks want the pump at the same time? Is there a water valve that will change too?
    – Tyson
    Nov 11, 2018 at 3:01
  • @tyson that seems unnecessary, if a pressure tank is at full pressure, it simply won't want any water. Check valves might be called for to keep the two house systems from simply equalizing via the piping... Like in the old days when two canal companies would connect and one would end up hoovering water supply from the other until they installed a stop lock... Nov 11, 2018 at 3:03
  • @Harper. Yes. We want to each be able to power the pump. When I'm gone for months, my electric will be off. He must be able to run the well from his own Pressure Tank. Nov 11, 2018 at 3:08
  • @Harper what if both have 40min/60max pressure switches. A has used water down to 42 which doesn’t turn the pump on, now B uses a lot of water and he causes pump to come on? Without a valve it fills both tanks. He wants to use the “transfer switch” to properly split the electric bill. This is the same scenario as the duplex laundry room question the other night, but with a new twist.
    – Tyson
    Nov 11, 2018 at 3:10

1 Answer 1


A DPDT air conditioning relay should do the trick

House 2 is connected by default. When House 1 calls for water, it "takes over" and powers the well itself.

Air conditioning relays are rated at 30A and are rated to start motors. It's a 2-pole relay, and it switches both hots, but not ground. This setup must not use neutral: if it does, get a 3-pole DPDT relay, which will be rarer and more expensive. A relay normally connects its two Common terminals to its two NC terminals (Normally Closed, or connected when not energized). When energized, it disconnects NC and connects Common to the NO terminals (Normally Open, or connected when energized).

Each house has wires going to the well pump which are past the presure switch, i.e. They are energized only when that system is calling for water. Those are the wires we will be working with today.

The two wires from the pump are connected to the relay's Common terminals.

The pump wires from house 1 are connected to the relay's coil. That means when house 1 calls for water, the relay will snap over. The pump wires for house 1 also go to the NO (connected when energized) contacts on the relay.

The "common" wires on the relay go to the pump. That will connect house 1’s power to the pump.

The relay's NC (connected when not energized) contacts are connected to the pump wires from house 2. That means house 2 powers the pump whenever house 1 does not.

Note that NC and NO terminals never connect to each other. That is VERY important! For instance there is a scheme with simpler relays that is dead wrong and will short the two houses together. At best, it will trip the breakers. At worst, this will make both houses' meters spin rather faster than you want them to.


Normally in mains electrical, nobody cares about phasing because loads don't care. However in this case we may be switching "live" between two power supplies if both houses call for water. If we switch phases when we switch power supplies, it will cause the motor to jerk rather hard, and that could wear out the motor. The one at the bottom of the well.

We presume that the two houses are powered from reasonably close points on the power grid, that will be close to "in phase". This may not be possible: if they are on different transformers they may be irreparably out of phase, in which case you may need something more sophisticated.

The way you phase them is by running two different color wires (e.g. red and black) starting at the pressure switches. When able, have both houses call for water, and measure voltage between the two blacks. There should be near 0 volts between them. If there is near 240V between them, they are out of phase - swap red and black on one of the pressure switches and re-check. Later, if anybody has work done to their service panel, check phasing again.

From this point up to the relay, be careful to keep red and black matched.

  • Thank you! Is it also possible to have one set of wires (10/3) from the shed containing both Pressure Tanks to the well? Each of us would have our own wires from our home's circuit panel to the pressure switch on our respective pressure tanks. We keep both pressure tanks in a shared shed at our property line. From our two pressure tanks can we each wire into the AC Relay and then from the relay run one 10/3 cable to the well pump? Thank you most sincerely for the excellent cautions on phasing and being certain to not connect the NC and NO terminals of the relay. So appreciated. Nov 11, 2018 at 4:35
  • Yes, I was expecting a single run to the well. Watch your voltage drop. Nov 11, 2018 at 7:48

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