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I have a pump that is 350 ft from my house. Here are the specs for the pump:

Rated power - P2: 1 HP
Main frequency: 60 Hz
Rated voltage: 1 x 115 V
Maximum current consumption: 8 A
Power plug: US plug 115V

The breaker is a 20 amp GFCI breaker (I can't seem to find good GFCI breakers that are Square D larger than 20 amp). I know from calculators that I need larger than 10/2 wire, but I ran the 350 ft with some 10/2 ufb wire anyway, to see if it would work ok. I did this because I already had the 10/2 wire. It works intermittently, but I've stopped using it because I don't want to damage anything. I'm getting mixed signals based on which electrical calculator I use saying I either need #6 or #4. Wire's insanely expensive, but I need this pump working.

I know there are other alternatives like running 240v, but I can't seem to find a step down to 120v that will be happy in an outdoor environment. So, ignoring the fact that I could run 240, with a step down, how can I accomplish this?

Is it ok to keep the 10/2 on both ends (since the wire is about as small gauge as I can get and still get it inside the boxes I need easily) and splice 300+ ft of #4 or #6? I'm not sure how the splicing would effect the run.

And assuming splicing is ok for this..... since it's hard to find 500ft 6/2 wire in ufb, and I need 300ish feet, would it be ok to buy a few 125ft lengths and splice them?

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    Side question - how old is the current pump? How much service life is left? You may be better to run the cheaper wiring for 240V and uprate the pump. Comment because not really an answer.
    –  Criggie
    May 5 at 23:46
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    @Criggie Excellent question. The pump is basically brand new. I've had it for 3-4 months. It's an excellent pump and I really don't want to replace it. Also, for some reason where I'm at getting pumps has been difficult recently, and 240V pumps nearly impossible. SO, definitely staying with the 110.
    –  trueCamelType
    May 5 at 23:50
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    Your core question of "how to buy wire runs longer than 250'" is to stop limiting your shopping experience to only Home Depot/Lowes/Menards and talk to other suppliers, e.g. electrical supply houses.
    –  Harper - Reinstate Monica
    May 6 at 19:01
  • " I can't seem to find a step down to 120v that will be happy in an outdoor environment" - that is insane - these are commodity items for British builders, where 240v is standard for domestic, and 120v is common for tools.
    –  MikeB
    May 7 at 14:01
  • @MikeB I'm sure the fault is mine that I can't find them. Maybe I'm looking for the wrong thing. I've never had to use anything like this, but every one I'm finding seems to be for lab use or requires a pig tail in and out. I haven't seen one that is hard wired. But I'll be taking a trip to a new commercial electric distributor today.
    –  trueCamelType
    May 7 at 14:10

3 Answers three

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Your run is too long for your GFCI

Your GFCI is tripping, not necessarily because your pump is faulty or your GFCI is broken, but because you are well outside the 250' maximum cable length limit in your GFCI's instructions .

As a result, you'll want to get a small encapsulated (NEMA 3R rated) 240V/120V stepdown (1500VA is fine) and put it out by the pump location, then use a "spa panel" sized breaker panel to house the GFCI there, with a 2 pole 15A breaker back at the house to protect the transformer and run.

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    Which would also mean OP can use 12 AWG wire without a problem, based on the ~ 4A load @ 240V = 8A @ 120V.
    –  manassehkatz-Moving 2 Codidact
    May 6 at 2:56
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    To expand on the reason of the maximum cable length limit: there is capacitive leakage from the cable to it's ground wire and to the surrounding soil. The GFCI cannot distinguish the capacitive leakage from actual faults, which leads to it tripping.
    –  jpa
    May 6 at 10:13
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    To say nothing of the wire going down the well itself. If it's a 200' well you have a 550' length.
    –  Harper - Reinstate Monica
    May 6 at 17:57
  • @Harper-ReinstateMonica There is no well, and the 350' is probably closer to 370' because 350 is straight line path. This is all very interesting, and thank you for all of the responses.
    –  trueCamelType
    May 7 at 5:49
  • @jpa I didn't know about capacitive leak (or several other things) thanks for that. I'll look more into it, but that definitely sounds like my problem.
    –  trueCamelType
    May 7 at 5:50
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Wire's insanely expensive

The trick with bumps for voltage drop

This is the death of countless power projects: "Either I didn't check, or I know the Load Calc will force me into several wire size "bumps" from my thin copper wire I normally use. And since I'm bumping from copper, I'll bump to copper , and not really think about any other wire types . Nope! Cannot afford that copper, I'll use the copper I'm willing to pay for even though I know it'll fail".

And the right answer is to sidestep into aluminum. 2-2-2-4 (90A nominal), costs barely more than 10/3 UF (30A) . They also make a 6-6-6-6, which is too small for your 120V well pump, and a 4-4-4-4, but may not be available underground-rated, and doesn't enjoy commodity pricing like 2-2-2-4. Why four wires? Availability, and the ability to put other stuff out there with that ample feeder.

Huge 100k-subscriber off-grid living channels have ignominiously failed at home power projects because they did the above mistake, causing sharp voltage fall-off.

Breakers and GFCI

There's no affordable way to have a 2-2-2-4 circuit be less than 60 amps (70A on one brand) because #2 won't fit on smaller breakers. That means you will need a small subpanel (cheap) at the wellhead to attach to the #2 wires and have an appropriately sized breaker for the well itself.

Here, the breaker can be GFCI, solving the problem ThreePhaseEel points out (assuming your well isn't too deep). Make sure to pick the brand of subpanel that matches the GFCI breaker you already have; they are NOT interchangeable even if they seem to fit.

As far as GFCI protection, the long haul is unprotectable because of its length, and since it's >=60A and a feeder, it doesn't need it anyway.

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    Excellent points, and yes, I can see this being a common problem for people like me who don't 100% understand how electricity works. I'm going to do some more testing, and assuming it's due to the gauge of the wire I'll look into this.
    –  trueCamelType
    May 7 at 6:01
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    @trueCamelType: You said "don't 100% understand how electricity works.". In my opinion, this is a big no-go for doing DIY projects on electricity. What, if for some reason the safety measures fail and somebody comes to death? You are responsible for what you are doing. Go and call a professional electrician and let help you. This is way cheaper than to pay for any damages that may occur either to things or human beings.
    –  Wolfgang Roth
    May 7 at 7:16
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Using the Southwire voltage drop calculator , 350 feet, 120V, 8A:

  • 10 AWG = 4.9 %
  • 8 AWG = 3.22 %
  • 6 AWG = 2.06 %

10 AWG is probably enough, but 8 AWG is better if it is really running at 8A.

For lower current/smaller wire, see if you can find a 240V pump - i.e., designed for 240V. It may even be possible to reconfigure the pump you have, but that depends on the design.

Splices must be in accessible junction boxes. With buried wire that's a problem. The preferable thing - higher up-front parts cost but you don't have to dig as deep - is to use conduit with individual wires instead of direct-bury cable.

10 AWG or even 12 AWG for short parts is fine - the voltage drop is based on resistance and is a cumulative effect. The wires inside the pump are even smaller - but only for a very short distance.

This circuit must be GFCI protected . If it is already GFCI protected with a GFCI breaker and the breaker trips are actually GFCI rather than overcurrent then the problem has nothing to do with voltage drop and everything to do with water getting where it shouldn't - e.g., a problem with the cable sheath and insulation.

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    (man idk how to tag that name lol) Unfortunately I can't convert the pump to 240V. That's one of the first things I looked into. I really don't want to change the pump. This 10 AWS definitely isn't working. On a side note, this will be run above ground. I can't dig where I'm at, so it will need to be ufb. It will probably be in conduit eventually, but for now it's running across the ground (I know it's terrible, but Its in the middle of the woods where no one would run across it).
    –  trueCamelType
    May 5 at 23:58
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    Splices can also be inside the well casing, with appropriate underwater splice kits. 8 AWG copper begins to get into "much larger aluminum wire costing less" for the amperage/resistance/voltage drop)
    –  Ecnerwal
    May 5 at 23:59
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    At 300 feet trhough the wods poles may make more sense fiscally than conduit on the ground for acceptable wire protection.
    –  Ecnerwal
    May 6 at 0:23
  • @Ecnerwal I agree, and that may be the route I go, but first I have to stop this breaker from tripping :) .
    –  trueCamelType
    May 6 at 0:27

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