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I am replacing an old submersible pump with a new one. Old one has 3 wires, new one has 3 wires plus ground. I want to reuse the 3 wires currently there and just add a ground, which I will attach to the well casing (the house electrical also uses the well casing at a ground, in addition to rods). But I can't find submersible-type green-coated wire locally.

Questions:

  1. Why is this extra ground necessary, when the pump is submerged in water that is already in a grounded metal casing? Can I just cap the wire?
  2. Or can I just use THHN 10 wire?

wiring diagram

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  • 3 wires plus ground usually can mean a 240v device that uses neutral, but odd for a submersible pump. What does the wiring diagram/instructions say for the new pump?
    – crip659
    Commented Nov 10, 2023 at 12:51
  • @crip659 Diagram says to wire red, black, yellow to 3-wire 240v coming from control box, and to ground the green 4th wire. It just feels silly to bring a ground cable from a 200' deep hole to the surface to then attach it to the casing
    – Cheery
    Commented Nov 10, 2023 at 12:56
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    Is that a solid yellow or green with yellow stripe? Yellow is not a neutral or ground colour. Are you sure this is not three phase wiring? Can you post a picture of the wiring diagram? Let the experts take a look. This is for US wiring?
    – crip659
    Commented Nov 10, 2023 at 13:30
  • @crip659 Just added a diagram. I'm sure it's single phase, and I tested pump and it's working, so the rest of the wires are good.
    – Cheery
    Commented Nov 10, 2023 at 13:40
  • 3-wire pump (4 with ground) is a pump with the starting circuit on the surface. This made more sense when electrical components were less reliable than they are in the 21st century, but if it's what you have it's what you have. Or sometimes it's 3-phase delta, but we know that's not the case here from prior questions
    – Ecnerwal
    Commented Nov 10, 2023 at 13:57

2 Answers 2

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Explicitly grounding the pump provides a degree of increased protection and is bog standard in current codes.

These days, there's unlikely to even BE such a thing as "12-ga interior-rated green-color cable" - First off, it's a wire, not a cable (a cable is multiple wires bundled together) and secondly if you go buy a roll of green wire the odds of it not being rated THWN (among other ratings such as MTW and THHN and possibly THWN-2) are approximately zero from standard hardware store stock. You could get more options from an electrical supply, but getting one not rated for water is still unlikely.

The W in THWN means the insulation is rated to spend its entire life submerged in water. That's required for any outside conduit (which is where most "rolls of wire" end up going) and is thus the normal, default rating of "insulated wire on a roll."

IIRC that makes it a higher temperature class of insulation than what I got from the pump supplier (might have been RHW, or THW - I forget - they figure it's going in a nice cool well and it probably costs them a hair less) but in any case, it's rated for water submersion, and the higher temperature rating is not a problem.

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    I went to an electrical supplier. They said THWN is rated for wet, not submersed in water. The current pump wires do say "submersible" in them. They sent me to the plumber's supply, where I bought the pump. Those guys said electrical supplier. So I guess the only solution is to buy a new 3+1 wire submersible full length, as nobody seems to have single submersible wire.
    – Cheery
    Commented Nov 10, 2023 at 15:23
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Such a thing exists, it's just something that's not trivial to source or use

While you're right that garden-variety submersible pump cable is all multiconductor, there is such a thing as single conductor pump lead wire in the form of Paige's P7305-SP. It's only made by a single vendor that I know of (Paige Pumpwire), though, and it's of an uncommon multi-rated type (submersible RHH/RHW-2/DLO) atop being fine stranded, so you will need specially rated equipment at the terminations as well.

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