120 vs 240 1 Phase efficiency

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LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If deciding between the two choices, I'll opt for the higher voltage every time, unless there's a compelling reason not to.
 
Flicker Index

Flicker Index

Senior Member
Location
Pac NW
Occupation
Lights
Curtlre said:
I have been an electrician for 38 years and am retired now. I am totally second guessing myself and need some input. To me watts are watts weather a load is 120 volts at 10 amps or the load is 240 volt 1 ph at 5 amp you are consuming 1200 watts and the efficiency of the equipment is the same. Seems very simple to me but again I am second guessing myself since a neighbor's pool guy him it's more efficient to run his pool pump and heater at 240. Thanks in advance for your help.
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You aren't incorrect in theory, but he isn't incorrect in practical sense either.

Take for example, 1500W space heater. If it's drawing 12A and you're seeing 119v under load, 124v unloaded.
The unit is producing 1428W. If it's still 124v at the transformer primary, then you're losing 60W between the transformer and the heater. All else being equal, the loss is 15W if it was operated at twice the nominal voltage.

For something more complex than a heating element...
T8 32W lamp ballasts with identical load, 120-277v rated ballasts often consume about a watt or two less per ballast on 277 compared to 120 due to increased converter efficiency, but the consumption doesn't drop further, and sometimes goes up with a 347v version. Even PC power supplies run slightly more efficient at 240v vs 120v. These differences are more than negligible when you have many floors of office or tons of servers in a data center.
 
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P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Flicker Index said:
You aren't incorrect in theory, but he isn't incorrect in practical sense either.

Take for example, 1500W space heater. If it's drawing 12A and you're seeing 119v under load, 124v unloaded.
The unit is producing 1428W. If it's still 124v at the transformer primary, then you're losing 60W between the transformer and the heater. All else being equal, the loss is 15W if it was operated at twice the nominal voltage.
Click to expand...

To lose 60 W of heating in the wires @ 12 Amps you would have to have .417 Ohms in the wires going to the load. Less than 200 feet of copper #14.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Flicker Index said:
You aren't incorrect in theory, but he isn't incorrect in practical sense either.

Take for example, 1500W space heater. If it's drawing 12A and you're seeing 119v under load, 124v unloaded.
The unit is producing 1428W. If it's still 124v at the transformer primary, then you're losing 60W between the transformer and the heater. All else being equal, the loss is 15W if it was operated at twice the nominal voltage.

For something more complex than a heating element...
T8 32W lamp ballasts with identical load, 120-277v rated ballasts often consume about a watt or two less per ballast on 277 compared to 120 due to increased converter efficiency, but the consumption doesn't drop further, and sometimes goes up with a 347v version. Even PC power supplies run slightly more efficient at 240v vs 120v. These differences are more than negligible when you have many floors of office or tons of servers in a data center.
Click to expand...
If the bulk of the loss occurs within the space to be heated - is isn't really lost either.
 
G

garbo

Senior Member
Curtlre said:
I have been an electrician for 38 years and am retired now. I am totally second guessing myself and need some input. To me watts are watts weather a load is 120 volts at 10 amps or the load is 240 volt 1 ph at 5 amp you are consuming 1200 watts and the efficiency of the equipment is the same. Seems very simple to me but again I am second guessing myself since a neighbor's pool guy him it's more efficient to run his pool pump and heater at 240. Thanks in advance for your help.
Click to expand...
If wire lentgh is short would be very close. With 120 volt you might have to increase wire size. With 240 volts the 2 pole GFCI circuit breaker could cost at least 3 times the price of a single pole GFCI. What is the heater wattage. You never ever want to run a 240 volt heater on 120 volts. At halve the rated voltage believe that the ampere would be 25% and only 25% of the BTU'S produced. You are correct watts is watts same as kilowatt billing hours. I moved several machine & injection molding shops where they wrnt from 240 to 480 volts and customers swore that they were going to save a lot of money on their electric bill. Told them that the thousands of dollars that they will be spending for 600 volt buss duct switches, panels & 3 pole breakers will make them forget on the few dollars they might save. Also they had to purchase and install 480 to 120/208 volt transformers for general use 120 volt receptacles.
 
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