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A friend wants me to help add a electrical socket in his well house. The house is OLD, and was not wired to code (rural, multiple additions over many decades). The well is about 100yds from his house, and the power to the well is supplied with #12/2 w/ground, protected by a 30AMP 2-pole breaker. (Not what I'd run, for sure. I have no idea as to the motor size/draw, but he's not had any problems with the breaker flipping, over many years. Surprise!)

He wants to have an outlet he can plug a 100W bulb into, to keep the well from freezing. I suggested running his supply line into a small box, and putting a new breaker in the box for the well, and another, 15AMP, for a GFCI outlet. I know this is not code acceptable, but he wants it done, and can't afford to pay to have it done right.

Is there any reason to go up in wire size from the box to the well/outlet? I can't see any point, but....

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    At 300' even with a small pump larger than #10 wire would be needed to be code compliant 30 amp breaker requires #10. This may be an issue with trying to sell the property in the future and may cause pump failures at a higher rate due to voltage drop. – Ed Beal Jan 4 '17 at 22:22
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    Can you get us the nameplate numbers from the motor? We'll need those to determine if the breaker is actually oversized or not... – ThreePhaseEel Jan 4 '17 at 23:13
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    That length of wire is 0.953 ohms or "close enough" to 1 ohm. That makes the math ridiculously easy, since VoltageDrop = Amps x 1. Hence he can determine amps by measuring voltage at the service panel with the pump running, then measuring voltage at the pump with pump running. The difference is voltage drop. That = amps. – Harper - Reinstate Monica Jan 5 '17 at 0:24
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12/2 may require a 20A breaker

Note ThreePhaseEel's comment, where he notes certain motors require a 25A or 30A breaker (NEC 430.52) yet are permitted to use 12AWG wire (430.22 notably 430.22E), if so, this may have been legal after all. I would, at least, upgrade to 10AWG until it leaves the house. I care if it burns down your house. I don't care if it burns down your yard.

If the amp draw is well under 20A, then go ahead and fit a 20A breaker and now you're code legal in any case. And due to a happy coincidence of the numbers, your wiring run is almost exactly 1 ohm of resistance. That means the voltage drop equals the amps. It's a lot easier to measure voltage drop than amps: measure the voltage at the panel, measure the voltage at the pump, subtract. The pump must be running while you're measuring.

Yes, you can run 20A on 12AWG for unlimited length. Your load might not like the voltage drop, though. That's why I suspect the actual amperage is well under 20A.

Don't use a lightbulb for a heater. Use a heater.

Light bulbs burn out and you won't know when they do. Just go to McMaster-Carr, Grainger, or other industrial supply and buy a heater module. For instance they have a strip heater, 240V, 3 inch wide, 120 watt for $19. Any common strip heater will do fine.

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Or even better, they also sell pipe wrap heating, which you can then wrap with insulation. Could take well under 100W!

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There's no way to string 120V out there

The pump has 240V hot-hot-ground 3-wire electrical service out there. Neutral is not ground so there is no way to obtain a neutral to get 120V service. No big deal since you can get 240V heaters.

If you must do this thing with incandescent light bulbs, get 3 bulb bases and wire them in series to 240V. Then put 3 identical bulbs in there. Each bulb will see 80 volts (1/3 of 240v). Since that's 2/3 of its normal voltage, it'll run 4/9 (2/3x2/3) of its normal power. The bulbs will last a great deal longer, but when one fails, they all go dark.

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    A 1.5-2HP, 240VAC, single phase motor requires a branch circuit overcurrent device of 25-30A as per 430.52, whereas 14 or 12 AWG wire could be used as per 430.22...(I really want to see the nameplate on this well pump motor!) – ThreePhaseEel Jan 5 '17 at 0:35
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    I did basically the same thing in my brother's pumphouse. In his case, the run to the pumphouse (600 feet) was sufficiently large gauge wire, but there was no neutral. I installed a 240V baseboard heater -- the smallest available -- and that has kept things from freezing for over 10 years. Before I put in the heater, we had two light bulbs wired in series, and just replaced the bulbs every season whether they were working or not. – Jonathan J Jan 5 '17 at 5:55
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IF your current draw is only 16 amps 240v #8 copper would be needed to run 300' and keep the voltage drop below 3%. Here is a link to an easy to use voltage drop calculator. A 1/2 HP pump would work on #12 wire but there is really not much ampacity left for additional loads. The voltage drop to be code compliant needs to be 3% or less. We really need the size of the motor to give accurate advice. As stated above 12 gauge wire is a code violation with a 30 amp breaker.

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