I am installing a 120v 3/4hp well pump. I will be putting a spst switch at the well head and have a 30amp breaker at the power box. I am also going to be using a Pumpmaster WPS float switch. The run from the breaker box to the well head is 150ft and it is the same distance from the well head to the water tank with the float switch. I plan on using 10/2 wire from the breaker box to the spst switch at the well head.

The on/off switch at the well head is so I don't have to run back and forth to the breaker box if there is a problem with the pump, or if I need to shock the well. The on/off switch will be wired in series with the float switch.

The float switch comes with 15' of 16 gauge wire.

For that long a run, 150 feet, do I need a to run 10/2 for the float switch or can I just run 16ga? The startup amps for the pump is 28 amps but runs at 9amps. The people at Pumpmaster tell me the float switch will work fine with my pump.

Thanks Dirk

  • 1
    Is there any relay switching in the float switch or pump? I don't see how you could splice in a switch with 16 gauge wire into a 10/2 feed with a 30 Amp breaker.
    – JACK
    Commented May 30, 2021 at 14:53
  • What pump are you using? Seems like some motor starter would likely be needed, the starter should have a control circuit that would allow you to run smaller wire. Commented May 30, 2021 at 15:01
  • Does the well pump motor require a separate starting wire to go down the hole? Commented May 30, 2021 at 15:09
  • Is this 120v 1/2 horse switch the switch you are using? sjerhombus.com/wp-content/uploads/2018/12/9500195C-PM-WPS.pdf Commented May 30, 2021 at 15:18
  • Even if all the stars lined up using a correctly thermally protected motor that allowed just a 30A breaker to provide thermal protection and a 3/4 horse switch then at 120v 9A 150 ft #10 will give you 2.5% voltage loss, then 150 ft of #14 would give you another 6% loss for a total of 8.5%. That's just running current. I wouldn't even think of going there. Commented May 30, 2021 at 15:49

1 Answer 1


Disregard the #16 wire that is included as part of the float switch. That is an appliance, and wires that are part of an appliance are decided by UL (Underwriter's Laboratories) not NEC. NEC concerns wiring in walls, overhead and underground. Its minimum size is #14, but I would not use #14.

Your wires are going to be quite long. If you want the float switch to switch actual well motor current and not simply a relay, then you are talking about a long circuit comprising 300' of cable PLUS the length on down the well of ??? depth.

Such a long circuit will have considerable "Voltage Drop", meaning voltage losses from the resistance of the wire.

You don't want to arrive at the wellhead with significant voltage drop already, only to take more going down the pipe.

And as we bump into larger and larger sizes of copper, cost just gets prohibitive. So I would sidestep into aluminum wire (which has always worked fine at these large sizes)... because it's a cheap way to get lots of ampacity. #6 Al is about the price of #12 Cu, and will keep voltage drop well in line since it's the equivalent of #8 Cu. You'll need MAC Block Connectors (basically miniature Polaris) to splice the wire, but still cheaper than copper by a stretch.

Another option is to eliminate the long run to the tank switch, by converting that to relay operation. Obtain an Aube "relay + transformer" and install it at the well head. It has two low-voltage terminals on it; you short them to make the relay turn on. You wire it so the tank fill switch shorts those two terminals, making a call for water. At this point, current flow is milliamps, so you can run it with the smallest wire allowed (#14 for line voltage, or #18 for low voltage). Make sure to use outdoor rated cable.

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