Why do well pumps need 240V when the wattage is not that high?

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?

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.

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.