I want to wire a well pump into my homes 200 amp breaker box using a 20 amp breaker and #12 wire?

my guestion is . there is a voltage slector on the rear of the pump witch setting would i use 115 volt setting or 230 volt

If your pump draws 20 amps at 115 volts, it will draw 10 amps at 230 volts. As wires get longer, there is a voltage drop in the wire due to the wire resistance and the current through the wire. That means a given size wire has a maximum length depending on the current. #12 wire will carry 20 amps for about 50 ft without producing unacceptable voltage drops. By switching to 230Volts (and wiring to a 230V breaker), you can use #12 for a 100 ft. run. Any further and you will need larger wire.
That probably indicates that the pump could be wired either way...The real question is how did you wire it? The 230V setting would have 2 hot wires out of phase by 180°, a neutral, and probably a safety ground. The 115V setting would require only 1 hot, 1 neutral, and 1 safety ground.

If you havn't done it yet, the 230V method is probably more energy (cost) to run, but the wire for the 115V will be cheaper, easier to get, and easier to run.

If the setting is in the incorrect position, it could have fatal effects on the pump.
My husband works for Chambers and Phillips in Canton, Texas.They drill water wells. They would need more info from you regarding your question, the web site is www.chambersandphillips.com. You may also contact them via telephone at 903-567-6091.
Use the 115 volt setting if wiring into your service entry. That is the 200 amp box you describe.
The 230 volt setting would be more efficient as it would require less current. Your 200 Amp Breaker Box should have both 120 and 240 Volts available. A different breaker is required if you want to use 240 Volts as opposed to the breaker used for 120 Volts.
Use the 115 volt setting. I am assuming this is a single pole circuit breaker.
Since there are conflicting answers, I'd like to add that Vritualguy got it right. I don't know why rscanner got a thumbs down, because he is also right, although he didn't have much detail, while it is true it makes little difference to the motor what voltage it is running at, but as Virtualguy said, there will be less loss in the supply lines at the higher voltage, so the overall system will be more efficient at the higher voltage.

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