My well pump is fed by a 40a 240v circuit. I noticed that sometimes when the power cycles, or I manually cycle the breaker, that the circuit is only getting 120v. If I flip the breaker a few more times this resolves itself. Do I need a new breaker, or is this likely something else?enter image description here

  • Stranger things have happened than a worn out breaker. How often is it used as a disconnect switch? – isherwood Jan 14 at 17:36
  • Not that often, but it is pretty old I think, and it is outside (in a Nema box of course). – Robin Davis Jan 14 at 17:38
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    That's a HOM240 breaker. It's $10 at any home store. Do not buy any other brand, they will seem to fit but will then burn up the bus. – Harper - Reinstate Monica Jan 14 at 21:27

Almost certainly a bad breaker. Square D Homeline is the cheaper model than the QO breakers, but you'll have to replace it with another Homeline breaker since the QO breakers won't fit in a homeline panel.

BTW, I noticed that you are running 10ga wire. That isn't rated for 40 amps. And to be running 40 amps to a pump, it must be a heck of a big pump. What's it being used for? Commercial irrigation?

Also, it may be a loose connection someplace, like the wires themselves or the feed to the panel.

  • Thanks for pointing out the undersized wire! This is just a pump that serves 2 residences each with a fairly extensive irrigation system. I think it was installed maybe 10 years ago, long before I bought it. I don't know what the actual amperage of the pump is, or why they decided a 40amp breaker was appropriate -- maybe the 40amp breaker is necessary to accommodate start-up? Maybe I can just use a clamp ammeter to measure this? – Robin Davis Jan 14 at 19:13
  • Ok I just checked and it's a 5HP pump. So I think the 40 amp breaker is necessary. Also, aren't some 10 gauge wires actually rated for 40 amps? Though I'm not sure what type of wire that is exactly. – Robin Davis Jan 14 at 19:51
  • Regarding your question about wire size, as usual: "It depends". 10ga in cable is rated at 30 amps. individual wires in a race way or free air can go up to 40 amps, but it has to be rated for a high temp environment. There are others here who know more about this than me, hopefully they will chime in. In any case, 10ga seems pretty skimpy to run a 5 hp pump on. What type of pump? How far from the breaker to the pump? Voltage drop could an issue. Pumps aren't like table saws that rarely draws it's full amp rating, pumps will draw at a normal but high level continuously when the are running. – George Anderson Jan 14 at 21:33
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    #10 should be fine for 5hp, since that only draws 28A under normal conditions (source: elliottelectric.com/StaticPages/ElectricalReferences/…). The 40A breaker is likely needed to supply the startup current, but that is allowed by code and shouldn't cause any issues. OP, I wouldn't bother replacing the wiring unless you determine that it's what's causing your problems, which is unlikely. – Nate S. Jan 14 at 22:05
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    To be clear, everything works fine. I only noticed this flakey breaker because I was installing some other equipment down there and had my multimeter out, and decided to spend a little more effort troubleshooting why the pump controller was in a fault state rather than just cycling the breaker like I had in the past. There is a float switch in the tank, and then a radio signals the pump controller to turn on & off. They're about 1/2 mile apart. – Robin Davis Jan 14 at 23:32

Also check the wire connections on the breaker.

That is THHN wire which has the size written right on it. It's 10 AWG which needs a little thinking about:

240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(l) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.
(G) Overcurrent Protection for Specific Conductor Applications. Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(0).

Motor and motor-control circuit conductors: 430, Parts II-VII

And in 430.32 they allow overcurrent device to be 140% of motor rating. And in Table 430.52 they allow 250% of Full Load Rating. Not actually seeing the motor nameplate, most likely the 40A breaker is legit.

  • OP mentioned in the comments on another answer that the pump is 5 HP, so only 28A at 240V fully loaded, so both the wire and the breaker are the right size. – Nate S. Jan 14 at 22:08
  • Hey Harp. Not quite following your answer: At first you say the 10ga is a problem, but close out your answer by saying it's probably legit. Learning more about the question thru the comments, we certainly don't have to worry about ambient temps. Still, with the distances involved, I'd have run 8ga rather than 10. But now we're getting opinion based, so I'll shut up now! LOL – George Anderson Jan 14 at 22:35
  • For all I know the long run is 8ga. This wire is just what's going to the pump controller, about 1' away. – Robin Davis Jan 14 at 22:44
  • Well (no pun intended!) you need to get your hands around this and learn as much as you can about it. Pull the cover off the pump controller with the power off and inspect it. AGAIN WITH THE POWER OFF. Tighten any connections, inspect for damage (arcing, melting, discoloration, loose wires, etc.) of the wires, etc. You didn't say how the pump is turned on and off, so I can't help on that. Given, your system, you should probably form a nice friendly relationship with a good "pump guy". – George Anderson Jan 14 at 22:54
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    @GeorgeAnderson Good point, fixed. Given the short run I'd have done it with #8 just to quell any alarm-raising busybodies such as myslelf :) – Harper - Reinstate Monica Jan 15 at 0:29

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