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I want to be able to power my well pump from a battery when the power goes out, but I am learning that well pumps are heavy duty, running at 240V high voltage, even though mine is only 3/4 hp, which is about 560W, which is much less than a microwave oven, which sits on my counter and runs off 120V house current.

What else is going on here besides wattage/horsepower than determines why a well pump needs the higher voltage?

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  • Keep in mind that you'll need quite a beefy hunk of lead and vitriol to run that well pump for any extended period of time...is the goal of the battery backup just to get things running briefly during a power outage so that the pressure tank can be refilled, or are you looking for something more permanently installed/sophisticated than that? – ThreePhaseEel Jul 30 '20 at 23:42
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    Pumps draws the same power either way, for those that are available as 120V or 240V - but as explained in jwh20's answer, less power is wasted in wire resistance running it at 240V. My pump has more than 400 feet of wire and it's not particularly deep or far from the house compared to some places. You will generally need a generator to run a well pump, and if the generator does not provide 240V it's probably not going to run a well pump. Unless you have vast budget, batteries/inverter will give you almost nothing on a standard well pump. My "3/4 hp" pump uses about 1500W typically, BTW. – Ecnerwal Jul 31 '20 at 1:42
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    i.e. nominal rated mechanical power is not equal to the electrical power required to produce that. I happen to have the mindset that I have a power meter on my pump, but you very probably have a rating sticker somewhere in/on your pump controls specifying the maximum safe amperage of the motor. Multiply that by 240 and you'll get a lot closer to your actual need to feed it. Misleading power tool marketing (3 horspower! Plugs into a 15 amp 120V outlet! Must be 125% efficient, or, just possibly marketing BS) may impact your expectations of what a real 3/4 horse motor needs. – Ecnerwal Jul 31 '20 at 1:52
  • using a header tank or a larger pressure vessel may be a better option than a battery backup. batteries are expensive and relatively high maintenance. – Jasen Jul 31 '20 at 2:56
  • Unless you're planning a battery the size of a Tesla WallPack, (which I believe has a 240 inverter built in), you're probably going to be better off with a generator than a battery. Good ones aren't cheap (see, e.g. the Generac line), but they are designed to run indefinitely. And that can be a good thing when a hurricane takes out power for a week. – Carl Witthoft Jul 31 '20 at 17:11
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Need is not the right word here. Preferred is probably a better choice. 240V is preferred because submersible pumps are often well away from the house and deep down in the hole and you must run a wire there capable of supplying its power needs without too much voltage drop in the line.

While you could do this with a large AWG wire at 120V, you can use a smaller and less expensive wire at 240V. So this is commonly done.

I've not shopped for one but I suspect you may be able to locate a 120V submersible pump if you needed one. By the way, keep in mind that a 500W pump may need a 1500W inverter to start it up. Check the specs for your specific pump.

Apart from that, why not get a 240V inverter for your battery backup instead of a 120V one?

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    I don't actually have the battery backup at all, yet. I'm trying to learn what's possible first. My well pump is wired directly into the house current. Is it possible to splice in a switch so that it could be plugged directly into a portable backup device, when the power goes out, or would I have to have the battery wired into specific chosen circuits in a sub panel? – Joshua Frank Jul 30 '20 at 20:06
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Volts is the "pressure" or force behind electricity. Note that 240V power has 20 times the force of a 12 volt battery.

Amps is the flow/volume of electricity.

Power (the thing you ultimately want) is pressure x flow, or volts x amps.

For instance the Oroville Dam has 600 feet of head (water pressure). Flowing 1000 cubic feet per second of water, it can make a lot of power. To make that same power, a Mississippi River dam (with only 20 feet of head) has to drop 30,000 CFS.

So your 560 watt pump can happen either at

  • 2.33 amps at 240V
  • 4.7 amps at 120V
  • 47 amps at 12V

Why 240V well pumps

It's all about voltage drop due to distance. Voltage drop is figured by Ohm's Law: Vdrop = I (current) x R (resistance of wires).

Remember - it's not just the wire length from the house to the wellhead. It's also the wire length down the well shaft. The pump is at the bottom.

To keep the math easy, let's say your well pump is run with 10 AWG wire and it's 500.5 feet out and down. Round trip, the resistance of that 1001 feet of wire is 1.000 ohms.

Now I'm going to show you how to compute 240V, and I want you to follow up and compute the others.

  • Current at 240V is 2.33 amps. Resistance, as said, is 1.000 ohms. Plug that into Ohm's Law: Vdrop = I * R Vdrop = 2.33 * 1.000 ...... Vdrop is 2.33 volts.
  • That means 237.67 volts actually makes it to the pump.
  • 2.33 volts divided by 240V gives us 1% voltage drop. That is perfectly reasonable.
  • 1% x 560W = 5.6 watts - acceptable loss.
  • Now compute these for 120V and for 12V.

Ideally we aim to keep under 3% voltage drop, but a little flex is OK.

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  • And well pumps will likely be rated to deal with higher than normal voltage drop anyway – ratchet freak Jul 31 '20 at 12:49
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I'm a Canadian Electrician - so CES is Canadian Electrical Code, but I'm sure physics in the US will be close. Table 45 states a 3/4 HP motor draws 13.8A @ 115V or 6.9A @ 230V Since "nominal" voltages are 120V and 240V - the motor is already assuming a 4% voltage drop.

From some informal on-line searches, people are suggesting the 3/4 HP pump will draw 6A - 8A. I supposes it depends how much load the water is putting on the pump (head & flow).

I'll use 10A @ 115V for this example (1,150W electric).

One characteristics of most motors is they continue to produce the same output even when provided with low voltage (unlike a resistive load/heater or incandescent lightbulb which will produce less output/light when voltage is less).

So, if voltage drop in your wire causes the voltage at the pump to drop to 103.5V (10% less than 115A rating - which is 13.75% voltage drop from the 120V supply) the amperage will increase by about the same percentage to maintain same power. So about 11A which is over-loading the pump windings by 10% - not great.

From CEC table D3 assuming the pump was fed by a #12 AWG wire a 400' run would cause the 13.75% voltage drop when the pump is drawing 11A.

So using the same 400' of wire, but now wired 240V (so the pump wants 230V @ 6.9A). Again using table D3 suggests 4% voltage drop. (note the actual voltage drop is about 9.6V, which would be 8% of 120V, but since we've upped the voltage, it is only 4% of 240V)

That 4% voltage drop - as mentioned earlier brings the 240V supply down to the motor rating of 230V at the bottom of the well, so you're both saving almost 10% in line losses (that saving 100W of power) you are now also running your pump in-spec, so less likely to over-heat or have early failure.

I hope that all makes sense!?! AND I hope I don't have any errors or typos in my calculations.

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