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I am trying to figure out the acceptable distance to pull 4ea. 8 gauge wires through 1 inch sch. 40 PVC conduit. This conduit is carrying power from my house to my well pump. The trench has a few small bends to avoid trees but in general is pretty straight, meaning no 90, 45, 22.5 bends, the exception being a 90 at each end where it goes into and comes out of the ground.

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    You need a ground, even on 3-phase "wye". The ground can be smaller than the conductors (and bare as well) assuming it's more than a 30A circuit. (A 30A circuit requires #10 wire and #10 ground, and if you enlarge hots, you must enlarge ground in proportion). Jul 17, 2023 at 20:52
  • What is the voltage/current/power rating of the pump?
    – pathfinder
    Jul 18, 2023 at 0:49

2 Answers 2

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OK, so apparently the well pump documentation, presumably a UL listed pump, says #8 is OK for 1625'. That's a bit surprising but not hugely, since well pumps often have a long vertical cable run down the well, and so they're accustomed to high voltage drop situations. OK then.

Use aluminum for the long run; #6 aluminum is the equivalent to #8 copper. Use a disconnect as a cheap way to do three splices from aluminum to copper; use #8 copper for the final few feet to the pump equipment.

Do NOT guess - actually calculate voltage drop

(yes, I've already run them).

A common mistake when doing long-distance circuit is what I call the "bamp" - rather than actually run a voltage drop calculation, they just "bump" or "bamp" 1 or 2 wire sizes arbitrarily as if that oughta do'r.

And I gather your "bamp" is all about the price of copper. You don't want to see what the voltage drop calc would say, because you ain't paying for that much copper!

Well, no, you're not :)

Because outside of small branch circuit wiring in an unfortunate experiment in the 1970s where rules were violated, aluminum wire is actually wonderful stuff. If you're afraid of aluminum, actually research what went wrong and why. Entirely foreseeable. Not a surprise in hindsight.

So even if #8 is a valid choice (5 amp pump???), use #6 aluminum instead.

If your pump is more like 12 amps like I'd expect, use #2 aluminum, that's a popular commodity size.

You're especially going to want aluminum the day the pump quits, and you go down there and find somebody stole it for the copper value. Aluminum is far less worth stealing.

To make a legal splice from aluminum to copper, use any disconnect able to take the wire size.

Alternative: Battery (and charge the battery)

An alternative is take advantage of battery system prices being in free-fall thanks to wrecked EVs, and stick a battery system out there. Now you don't need to sustain the impulse load of the pump starting and running, you only need to supply low (average) load over those wires. So they can be smaller (so #6 aluminum; I still wouldn't use copper).

Another alternative is do the battery but with solar. Why solar? Because solar is cheap, and solar+battery is probably going to be cheaper than the #8 copper you were planning to buy, nevermind the conduit.

And depending on your terrain, there's another battery you can use: height. Every 2 feet of rise gives 1 PSI in pressure, so you can select the pressure you want in your house and stick a storage tank that much higher. Then your ordinary draws are from the storage tank and your pump can simply run as needed to replenish the storage tank. And you don't lose pressurization if you don't lose power. So you could have something like solar feeding a small battery that runs everytime the battery gets full (and runs til the battery is at 30%). Unless told to stop due to the storage tank being full.

Or step up to 480V

And I recommend 480V here, even though wires are listed to 600V, because 480V kit is easier to find used.

Here, you have a pad or pole mount transformer with the conduit exiting directly out the bottom of the transformer, and going directly into the bottom of another transformer at the well pump. No one should ever have a reason to open up these transformers! You connect the transformers back to back on the 480V primary side, then backfeed the house one with 240V, and take 120/240V off the transformer at the well head. (yes, you get 120V at the well head in this scenario).

No neutral is included. Arguably, ground is not required either since the 480V is entirely isolated. It's only 480V between the transformers, so the wire should not visit any other equipment (because consumer tier equipment is not rated for 480V). Now, your #6 aluminum can support 20-25 amps of well motor at reasonable voltage drop, or a whole RV stand at less reasonable drop (around 6-7%). If you want RV stand + well, then #2 aluminum.

Both transformers need to have the kVA for the load to be served.

On bending...

It's not about the number of 90 degree bends. It's about the total bending in degrees. The bending between a pulling point cannot exceed 360 (and if you're a DIYer, better to aim for 90 and accept 180 at the worst).

So if you have two 90s, two 60s and three 30s, that's too much and you need a pulling point in there.

For the NEC, radius does not enter into it. However a broad radius is easier to pull as a practical matter. A DIYer is going to want to keep bending far below NEC maxima, because operating near that maxima means needing the electrician's full suite of pulling tools.

Although conduit size is a large mitigating factor, so three #8 and a bare #10 ground would honestly fit in a 1/2" conduit and probably won't be so bad in 1" conduit.

Remember data is not allowed in power conduit, unless it's non-conductive fiber.

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  • Thank you. The total distance of the run is 1,100 ft. I will turn up 90s into a J-box at certain intervals if necessary, but hoping I can pull the whole distance in one shot.
    – Ian
    Jul 17, 2023 at 21:07
  • @Ian 1100 feet? HOLY SMOKES! You're really going to want to do a Voltage Drop Calculation on that. Lots of people just do a "bamp" which I call taking the normal wire size and bumping a size or two on general principle, and that trick never works. It either wildly oversizes, or wildly undersizes the wire. Jul 17, 2023 at 21:11
  • 1100 ft. I think I would tell the local high school football team they can watch the cheerleaders in T-shirts wash cars if they pull the wire.
    – crip659
    Jul 17, 2023 at 21:32
  • I have not done a voltage drop calculation myself, I don't even know how to. I am getting the #8 wire size from my well pump controller product data. In the manual, it allows for a total distance of 1,625' from the controller to the well pump.
    – Ian
    Jul 17, 2023 at 21:48
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    @Ian wow, I'd want to see the amp draw off that, but the product documentation on a UL-listed product is in fact reviewed by UL. Perhaps well pumps are designed to tolerate big drop. Nonetheless, disregard #8 and substitute #6Al. They have the same ampacity. The last few feet from disconnect to well controller need to be #8 copper because of the instructions. Jul 17, 2023 at 22:19
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I can't tell you if 8ga wires are appropriate for your distance/current/voltage drop as you haven't given enough information. Regarding conduit fill, depending on the cable type you are using, you're likely to be at 17-21% full. That means your conduit is plenty large enough for the cable. If it's a particularly long pull, you may need to use rope instead of standard pull string. If it's very, very long you will want to use a pull scale or tension meter to make sure you don't exceed the pulling force on the cable, or use multiple pull strings so you can pull it from the middle once it's part way through. Make sure to use plenty of lube, and you may want to leave a string in the conduit alongside your cable.

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  • Thank you. The total distance of the run is 1,100 ft. I will turn up 90s into a J-box at certain intervals if necessary, but hoping I can pull the whole distance in one shot.
    – Ian
    Jul 17, 2023 at 21:07
  • That's a long pull. At 1100 feet you're going to have to do some math about cable tension - it's a significant amount. You also should do the voltage drop calculation to see just how much power you'll be throwing away in the wire. There's no requirement for a pull box at this length, at least none in code that I'm aware of, but 200 feet, 300 feet, and 500 feet between boxes are all common in various scenarios. I'd probably put a midpoint handhole in, myself.
    – KMJ
    Jul 17, 2023 at 21:55
  • I have not done a voltage drop calculation myself, I don't even know how to. I am getting the #8 wire size from my well pump controller product data. In the manual, it allows for a total distance of 1,625' from the controller to the well pump.
    – Ian
    Jul 17, 2023 at 22:00
  • @harper got you covered on voltage drop. I'm still thinking a pullbox is a good idea. Also you might be able to save a bunch of money on wire if you put the control box at the wellhead instead of back at the house - because you'll only need your two power wires and a ground.
    – KMJ
    Jul 17, 2023 at 22:29
  • I should have noted that the well pump is 230 volt, 3-Phase with a start-up draw of 12 amps.
    – Ian
    Jul 19, 2023 at 16:02

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