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 DIY Chatroom Home Improvement Forum Does 240v use less energy than 120v?

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03-10-2010, 03:12 PM   #16
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Quote:
 Originally Posted by mopowers I'm not sure of the wire size. But it would be whatever size needed for a 50amp circuit. I do know that the wire is aluminum. It looks really big to my untrained eye.
How old is the circuit? Could it be tinned copper rather than aluminum?

03-10-2010, 03:21 PM   #17
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Quote:
 Originally Posted by mopowers I'm not sure of the wire size. But it would be whatever size needed for a 50amp circuit. I do know that the wire is aluminum. It looks really big to my untrained eye.
Assuming #4 Al:

VD for 17 amps @240 volts is 0.5 volts or 0.4%

VD for 34 amps @120 volts is 1 volt or 0.8%

Both of these are withinn acceptable limits.

 The Following User Says Thank You to brric For This Useful Post: mopowers (03-10-2010)

03-10-2010, 03:24 PM   #18
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Quote:
 Originally Posted by brric How old is the circuit? Could it be tinned copper rather than aluminum?
I guess it could be. I have no idea. How would I tell? The house was built last July in CA if that matters.

03-10-2010, 03:26 PM   #19
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Quote:
 Originally Posted by brric Assuming #4 Al: VD for 17 amps @240 volts is 0.5 volts or 0.4% VD for 34 amps @120 volts is 1 volt or 0.8% Both of these are withinn acceptable limits.
Thanks man. So the difference would only be 0.4% I'm guessing I would never see that in my wallet.

03-10-2010, 03:29 PM   #20
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Quote:
 Originally Posted by mopowers Thanks man. So the difference would only be 0.4% I'm guessing I would never see that in my wallet.

The power used is the same. There is no savings differential!!!!!!!!!!

03-10-2010, 03:44 PM   #21
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Quote:
 Originally Posted by brric The power used is the same. There is no savings differential!!!!!!!!!!
You're right. Thanks!

 03-10-2010, 05:46 PM #22 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,832 Rewards Points: 5,246 There is a small difference in total power consumption due to the greater losses in a circuit running at 120 volts versus 240 volts. The power consumption of the compressor does not change, however some power is consumed in the wiring up to the compressor. The power dissipated by a wire can be computed from the equation P = I * I * R where I is the amperage and R is the resistance of the wire in ohms. The current I for a 120 volt circuit is twice that for an equivalent power 240 volt circuit. The resistance of the wire is essentially independent of the voltage, but does depend on the size of the wire. Therefore the power loss in a 120 volt circuit is 4 times the power loss of an equal length 240 volt circuit if the wire is sized for the amperage draw at 120 volts. If the wire at 240 volts is sized for 240 volts, you will probably use a smaller wire than the 120 volt case, and the resistance is higher, but the square factor of the amperage typically overcomes the somewhat higher resistance, and you still dissipate more power. Remember, you pay for the power loss in the wiring up to the device. Example: A compressor designed for 17 amps at 240 volts would require #12 wire, at 34 amps at 120 volts you would use #8 wire. #12 Cu wire has a resistance of 0.159 ohms/100 ft, while #8 wire has a resistance of .063 ohms/100 ft. Assuming a 100 ft run, the losses in the wire are as follows: 120 volts: 34 x 34 x .063 = 73 watts 240 volts: 17 * 17 * .159 = 46 watts In this case, the 240 volt circuit saves 27 watts of power.
03-10-2010, 05:58 PM   #23
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Quote:
 Originally Posted by Daniel Holzman There is a small difference in total power consumption due to the greater losses in a circuit running at 120 volts versus 240 volts. The power consumption of the compressor does not change, however some power is consumed in the wiring up to the compressor. The power dissipated by a wire can be computed from the equation P = I * I * R where I is the amperage and R is the resistance of the wire in ohms. The current I for a 120 volt circuit is twice that for an equivalent power 240 volt circuit. The resistance of the wire is essentially independent of the voltage, but does depend on the size of the wire. Therefore the power loss in a 120 volt circuit is 4 times the power loss of an equal length 240 volt circuit if the wire is sized for the amperage draw at 120 volts. If the wire at 240 volts is sized for 240 volts, you will probably use a smaller wire than the 120 volt case, and the resistance is higher, but the square factor of the amperage typically overcomes the somewhat higher resistance, and you still dissipate more power. Remember, you pay for the power loss in the wiring up to the device. Example: A compressor designed for 17 amps at 240 volts would require #12 wire, at 34 amps at 120 volts you would use #8 wire. #12 Cu wire has a resistance of 0.159 ohms/100 ft, while #8 wire has a resistance of .063 ohms/100 ft. Assuming a 100 ft run, the losses in the wire are as follows: 120 volts: 34 x 34 x .063 = 73 watts 240 volts: 17 * 17 * .159 = 46 watts In this case, the 240 volt circuit saves 27 watts of power.
You are assuming different sizes of wire which is not what has been discussed here.

03-10-2010, 07:08 PM   #24
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Quote:
 Originally Posted by brric You are assuming different sizes of wire which is not what has been discussed here.
It is a valid assumption, since the original question postulated using the same compressor wired at different voltages. Even with the larger sized wire for the 120 Volt connection, you still save a few watts wiring it for 240 Volts.

If one were to use the larger conductor wired for 240 Volts, one could argue that more watts could be saved. But the difference would not be worth it.

The rule of thumb on this or any other motor load is always use the highest available voltage when installing such circuits.
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