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Compare 120Volts to 240Volts - Is one More Energy Efficient?
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- Can we save money by wiring devices to run at 240Volts instead of 120Volts?
- Should we wire our well pump to run at 240V instead of 120V?
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.
Photographs and sketches illustrate voltage, electrical resistance, and other electrical wiring concepts. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
Does Wiring a Motor at 240V Save Electricity over Wiring it to Run at 120V?
As we discuss at VOLTAGE at the SEC the total current (Amps) that will flow through a wire (electrical conductor) is doubled if the pressure (Volts) is doubled. Twice the power or energy can be delivered on a # 12 wire by doubling the voltage and holding the current to 20 amps.
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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 Volts = Watts / Amps. (Volts equals Watts divided by Amps). |
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.
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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?
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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
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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. |
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DEFINITIONS of ELECTRICAL TERMS
Definition of Amps
Definition of Electrical Circuits, shorts
Definition of Volts
Definition of Watts
How a Building Gets 240V and 120V
How many Watts in a Circuit
Definition of AC Alternating Current
Definition of DC Direct Current
Definition of Electrical Ground Terms
Definition of Electrical Resistance
Efficiency of 120V vs 240V Equipment
- Original Authors: Al Carson, Daniel Friedman, Robert Klewitz.
Alan Carson is an ASHI Member, national home inspection educator, author and building failures researcher in Toronto, Ontario.
Daniel Friedman, an original author of this article and the editor and producer of InspectAPedia where this article now appears is an ASHI Member, first ASHI Technical Committee chairman, editor and publisher of the ASHI Technical Journal, licensed home inspector, educator, and building failures researcher in Poughkeepsie, NY. Robert Klewitz is a licensed professional engineer, a professional home inspector, an ASHI Member, and has served on the ASHI Technical Committee as well as in other ASHI activities. His practice is in Issaquah, WA. - Douglas Hansen, Robert Stead. Mark Cramer - technical review.
- Photographs: Daniel Friedman.
- Arlene Puentes, an ASHI home inspector in Kingston, NY, suggested text on the importance of safe electrical grounding. Ms. Puentes has served as Hudson Valley ASHI (American Society of Home Inspectors) chapter president, and is active in professional education in the home inspection field. She can be contacted at ap@octoberhome.com
- N. Srinivasan, MSEE, is a senior member of IEEE with 30 years experience in the electrical industry. Mr. Srinivasan is in Vienna VA.
- Louis P. Babin generously contributed technical editing about the effects of doubling ampacity in an electrical circuit (September 2007)
- Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution
AMPS VOLTS DETERMINATION
DEFINE AMPS VOLTS WATTS
VISUALLY DETERMINE AMPS & VOLTS
VOLTAGE at the SEC
VOLTAGE by EQUIPMENT
LIMITING FACTOR sets AMPS
SE CABLE SIZES vs AMPS
UNDERGROUND SERVICE LATERALS
ALUMINUM SECs & WIRING
MAIN DISCONNECT AMPACITY
PANEL AMPACITY
ELECTRIC METERS & METER BASES
AMPACITY - the LIMITING FACTOR
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- * Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
- Aluminum Wiring Information Website Aluminum Electrical Wiring Hazards and Repairs: in-depth authoritative info, photos, documents including selection of proper vs. ineffective repair methods. E.g.: Ideal 65 "Twister" purple connector fails in field and lab testing with aluminum wire.
- Ampacity of an Electrical Service: How to Estimate the electrical service size or ampacity entering a building
- Circuit Breaker, a bad one fails to trip failure at aluminum bus-to-circuit breaker connection - field report and photographs
- Electrical Panels, How to Inspect in Buildings, safety for electrical inspectors, electrical panel, fusing, wiring defects, defective products. Inspection Class Presentation
- Federal Pacific Electric (FPE) Stab-Lok Circuit Breaker Panel Hazards Website - Latent fire hazards, in-depth authoritative research, documents, advice on Stab-Lok electric panel and circuit breaker failures and what to do when this equipment is found in buildings.
- Lightning Strike Risk Assessment, Protection Systems & Services
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- "Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
- "Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
- "How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
- Rust and Corrosion in Electrical Panels, A Study and Report on Frequency and Cause for Electrical and Home Inspectors at Residential Electric Panels
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