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I'm running an 8 gauge wire from the breaker box to my pump (which is 200 feet away from the house).

I want to install a switch near the breaker box and want to know if I can use 10 gauge wire from the breaker to the switch and still maintain the proper amperage for the pump (i.e. so it won't overheat).

The hypothetical diagram is:

220V breaker
  |
  | 10 gauge wire (under 10 feet long)
  |
switch
  ||
  ||
  || 8 gauge wire (approximately 200 feet long)
  ||
  ||
 pump

The question is: Will the 10 gauge wire mess up the run or will the pump still work right?

  • Could you post a picture of the nameplate of the pump? How much current does it need? – Pigrew Aug 3 '13 at 11:28
  • The pump's current draw is an important factor, but if the breaker is more than 30A you cannot use 10Ga no matter what. (well, maybe if it had some super duper heat resistant insulation it might be OK) – bcworkz Aug 3 '13 at 17:51
  • Insufficient data. Need to know the required amperage in order to give an accurate answer. – The Evil Greebo Aug 5 '13 at 12:58
2

You are free to use a conductor that is rated greater than the ampacity of the feed, but not less than. If the breaker is 30 amps, you can go above #10, but not below it. Likewise, if the breaker is 40 amps, you can't go smaller than #8.

If you're using #8 just to compensate for the drop over distance, then yes - you can come out of the switch with #10 if #8 somehow makes termination impractical at the pump. It's not uncommon to see #8 pulled to hot tubs and the like even with a 30A breaker simply because the electrician is not sure what kind of motor is actually going to show up, so they go with the larger conductor just in case.

This, of course is assuming copper wire with THHN insulation.

  • 1
    To be clear: "If the breaker is 30 amps you can go above #10" - in wire sizes, above #10 sounds like a higher number but it actually means a lower number. The lower the gauge, the thicker the wire, and the higher the amp rating. – The Evil Greebo Aug 5 '13 at 12:57
  • @TheEvilGreebo I should have made that clear, d'oh! But you've done such a splendid job of it :) Would you mind if I just moved your comment into my answer so we're sure it sticks around, or just edit it in yourself? – Tim Post Aug 5 '13 at 13:46
  • Go right ahead. :) – The Evil Greebo Aug 5 '13 at 13:47
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200 ft is a long distance. You should check the voltage drop of the wire to keep to less than 3% at the rated current. You might find that #6 or even #4 is required for this circuit.

http://www.csgnetwork.com/wiresizecalc.html

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As others have pointed out, the question cannot be answered without knowing the breaker rating.

That said: In my experience most household well pumps are about 1/2 horsepower. On a 220v circuit they will draw about 7A. Running such a pump on a 15A breaker is fine. And a 15A breaker doesn't need anything like 8 or 10 ga wire.

Because of the long electrical runs, many well pumps are designed to tolerate low voltage. But many people will go to 1 wire size heavier to decrease the voltage drop. Recommended minimums are usually included in the pump installation manual.

Some years ago my friendly neighbourhood pocket gopher chewed through my well powerline. The line wasn't installed to code, being only about 6 inches underground, and running for much of it's length under a rock garden, so rather than try to find the break, I installed a new line. That line was 12 ga. for a 1/2 hp pump that in summer runs 24 hours a day. Pump typically draws 5.7A in use. (Limited by the recharge rate of the well bore)

Note in passing: If this is for a serious duty pump, consider putting in a 3 phase pump with with variable frequency drive. This can reduce pump start/stop cycles, decrease starting currents, decrease energy usage somewhat. Easier on both pump and plumbing.

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