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This article describes the difference between wiring an electrical motor, such as a well pump, to operate at 120 volts compared with 240 Volts and answers the common question of whether or not wiring an electrical device to operate at higher 240 Voltage level will save on the electrical bill by using less energy. The short answer is no, and details are provided below. Sketch courtesy of Carson Dunlop Associates. Photographs and sketches illustrate voltage, electrical resistance, and other electrical wiring concepts.
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As we discuss at both AMPS VOLTS DETERMINATION and at VOLTAGE at the SEC the total current (Amps) that will flow through a wire (electrical conductor) is cut in half if the pressure (Volts) is doubled.
Twice the electrical power or energy can be delivered on a # 12 wire by doubling the voltage and holding the current unchanged, for example at 20 amps. Doubling both voltage and also doubling the amperage would deliver four times the power or energy. Thanks to reader Bob Hawley, for careful reading & editing of this point.
But doubling the power or energy does not mean that we cut our electrical bill in half. Why not? (Our photo at left shows a 240V pump control switch.)
As we introduced at Definition of Volts, Volt, formally, is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power.
This definition is not very helpful to consumers. Using a water-in-pipes analogy, volts is analogous to water "pressure" in the electrical system.
Having higher "pressure" in a pipe (or electrical conductor) means that conductor is capable of delivering more energy to the user. Later in this article we further explain electrical potential.
Mathematically the following simplified formula relates volts, watts, and amps in an electrical circuit
Electrical Energy Usage Measured in Watts
The watts consumed (and therefore the size of your electric bill) for running a water pump or other electric motors will be almost exactly the same regardless of whether you are running the pump wired at 120 Volts or 240Volts.
Using an imperfect "water pressure" analogy, sending water through a pipe to move a water wheel, if we double the pressure (volts) at which we are supplying water energy to push the wheel, the number of gallons per minute (amps) we need to do the same work is cut in half.
So if we keep our pipe and water wheel and all else the same, but send water through the pipe to push the wheel at 240 psi, we would need half as much water quantity (measured in gallons per minute or "amps") to turn the wheel at the same rate as if we were pushing on the wheel at 120 psi.
[Click to enlarge any image]
Electrical motor voltages are similar in this regard. If we have an electric motor that is designed to run at either 120V or 240V (not all of them are) then the label on the motor will tell us that at 120V (pressure or potential) the pump motor will draw about twice as much amperage (current) as at 240V.
Our label (photo at left) shows that this 1/2 horsepower jet pump electric motor can run at either 115V at 10.8 Amps of current draw, or at 230V at 5.4 Amps of current draw.
With this example, if our electric pump motor draws 10.8 Amps when running on a 120V circuit, you see from the motor label that it draws 5.4 Amps when in use on a 240CV circuit.
10.8 Amps = ?Watts / 120 or doing the algebra, 120 x 10.8 = 1296 Watts when our pump motor is running at full load and wired on a 120 Volt electrical circuit.
05.4 Amps = ? Watts / 240 or doing the algebra: 240 x 5.4 = 1296 Watts when our pump motor is running at full load wired on a (nominal) 240 Volt electrical circuit.
The electric meter at a building measures electricity usage in kilowatt hours (KWH). If you run a 1000W electric heater for one hour, you've just used 1 KWH of electricity. So the pump is costing essentially exactly the same to run at either voltage level.
So what advantages do we get from running an electric motor at 240V rather than 120V?
Most electric motors will be a little easier to start turning at higher voltage. And for the same amperage draw, at 240V a smaller diameter circuit wire may be permitted - thus the circuit is a little less costly.
Finally, on a long circuit, the voltage drop due to resistance of the wire itself will be less at 240V than at 120V. However, separately, some experts point out that higher voltage circuits are potentially more dangerous.
In sum, the electric bill is about the same, but at 240V the pump has a little easier time starting, it may thus have a little longer life, and the wiring might be less costly.
The improvements in efficiency of use of electrical power is slightly better at higher voltages (less energy loss) but running a water pump will not be noticeably more or less costly between the two options.
See DEFINITIONS of ELECTRICAL TERMS for definitions of Amps, Volts, Watts, Ohms, etc. Our photo at left shows the data tag inside of a pump pressure control switch and shows that the switch might be wired for either 120V or 240V if other site conditions and equipment specifications permit.
Don't Rush to Re-Wire your Well Pump from 120V to Change it to 240V
The pump pressure control switch in our photo at left has contacts that permit it to be wired to operate at either 120V or 240V. But we don't know if the pump motor itself can run at either voltage. First make sure that the pump motor and control are labeled to indicate that either voltage level is permitted with the equipment you have installed.
Second we don't know that the pump motor, even if it can run at 240V, will not also require a heavier-duty pump control switch than this relay.
Second, there will be some additional costs when changing the voltage level at which an electric motor is going to be operated.
Even if the electrical wires can be left un-changed (when moving UP from 120V to 240V) the overcurrent protection (circuit breaker or fuses) will need to be changed to properly and safely protect the electrical circuit. In either case, if we exceed the current rating of an electrical wire, it will get hot, risking a fire. That's why we use fuse devices (or modern circuit breakers), to limit the current flow on electrical conductors to a safe level to avoid overheating and fires.
Some but possibly not all motor controls such as well pump control switches can be re-wired to operate at either voltage. But if not you'll need to replace the pump control switch.
The wall switch to turn off the well pump will need to be changed and rewired if you change the voltage serving the pump.
So when might we make these changes? Certainly when a new well pump is being installed or when other electrical work is being done on a building that would be the time to ask your licensed electrician about pump wiring and circuit voltage.
Frequently Asked Questions (FAQs) comparing 120-volt to 240-volt electrical circuits
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