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| Author Information | Topic: 120v or 240 v |
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posted January 04, 2003 at 09:03 PM
Im thinking about installing a 600watt light. What Id like to know is, which voltage rating should I purchase? I heard that if I buy the 240 volt type over the 120 volt type that it would be cheaper to operate per the electrical calculation P/I/E (watts law). Is this true? My freinds however (phone guys) say that a 600 watt light will draw the same current whether it is rated at 120 v or 240 v, due to the fact that the electric meter on the side of the house is a watt meter and does not care what the voltage is. I personally disagree. Per Watts law P/I/E the higher the voltage, the lower the current draw which in turn saves energy and money. Am I right and my co-workers wrong? Thank's Otto IP: 64.12.96.73 |
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posted January 04, 2003 at 09:26 PM
Pat, You pay for watts. The current will be double on the 120 volt circuit, but the watts will be the same. Watts = volts times current. The only svaings would be that since the current is less there will be less I squrared R (Current squared times resistance) losses in the wires that feed the circuit on the higher voltage. This would be a very very small amount of energy. Don IP: 205.188.208.73 |
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posted January 04, 2003 at 10:24 PM
My question is, does a 600 watt light fixture operate cheaper on 120 v or on a 240 v power source? According to watts law the current usage is considerably less when wired at 240 v, in fact the operational cost savings would be quiet substantial. I disagree that per your comment the energy saved would be very very small. I already know that per watts law 600 watts is 600 watts regardless of voltage. My only comment is that a 600watt light operating at 240 v will use A LOT LESS electricity than a 120 v light at 600 watts. 600w div by 120v = 5a or 600w div by 240v = 2.5a . Does any one else agree with this? IP: 64.12.96.73 |
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posted January 04, 2003 at 10:33 PM
Pat, The meter that measures your electrical energy consumption is a kilowatt-hour meter. It multiplies watts X hours X 1000. Ed IP: 142.154.16.15 |
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posted January 04, 2003 at 10:41 PM
All I want to know is will a 600 watt light wired at 240v versus 120v save me substantial money after one year? Also if I had an electric motor that was compatible wired for 120v or 240v, would I be better off ($$$) wiring the motor at 240v after one year ? IP: 64.12.96.73 |
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posted January 04, 2003 at 10:54 PM
Pat, for starters it Ohm's law, and not Watt's law. As Don and Ed stated you are paying for Watt Hours, which is the product of the voltage and current. The only savings you will gain on a 240-volt lamp is I squared R losses encountered on the wire, and it will be minimal…… Your analogy is like saying 10 dimes is worth more than a 1 dollar bill. IP: 67.234.91.78 |
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posted January 04, 2003 at 11:03 PM
Slightly, perhaps a buck or two over a year. As has already been stated, the light, motor or other dual-voltage load would convert (consume) the same amount of energy at either voltage. The small saving would result from less energy being converted into heat in the supply wiring, due to the lower current draw at the higher voltage. Ed [This message has been edited by edmac@attcanada.ca (edited January 04, 2003).] IP: 142.154.16.15 |
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posted January 04, 2003 at 11:19 PM
Ed, I did some rough calculations on the assumptions that the light is left on 24 hours, 7 days a week, with .4 ohms conductor resistance, and .05 cents per kilowatt hour. On a 120 volt system powered consummed would be 610 watts. 240 volt would be 603 watts. The savings on a 240 volt is $3.05 per year. Or the light would cost $264.11 at 240 volts per year, or 267.16 at 120 volts per year. Not much! IP: 67.234.91.78 |
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posted January 05, 2003 at 12:23 AM
Dereck, Wouldn't the voltage drop caused by the I^2R losses actually reduce the voltage at the lamp and therefore reduce the wattage? I'm assuming that with the small voltage drop, that the hot resistance of the lamp does not change enough to make any difference. If so the 600 watt lamp would have a 24 ohm resistance at 120 volts and 96 ohms at 240 volts. If we add the 0.4 ohms for the wire to each of these we get 24.4 and 96.4. Using P = E^2/R we get 590 watts for the 120 volt circuit and 597.5 watts for the 240 volt circuit. Or am I doing this all wrong? Don IP: 205.188.208.73 |
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posted January 05, 2003 at 06:02 AM
PAT. NO NO NO NO NO NO NO NO IP: 209.240.198.62 |
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posted January 05, 2003 at 07:17 AM
I feel like such a fool. Sorry if I said anything out of line. Now I know why the chicks dig you guys ! IP: 64.12.96.73 |
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posted January 05, 2003 at 09:32 AM
Pat, your not a fool, you just didn't understand the physics. Don, no your not wrong, your numbers are accurate, , my numbers were theroretically correct, but the difference is still 7 watts, OK 7.5 watts. LOL DC IP: 67.234.27.198 |
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posted January 05, 2003 at 09:40 AM
The misconception voiced by Pat is undoudoubtabley the most common one Elctricians hear on a daily basis. I generally just explain that the utility Co. is charging for Watts only and sometimes point out that higher voltages can allow for lower installation costs by utilizing more pratical conductor/equipment sizes. And also that at 480V 3Ph. "Squirle Cage" motors can be employed that provide very high efficiency. ........ This is when the coustomer wants to know if they can have 3ph put in their garage.............. That's when I tell 'em about my new Perpetual Motion machine........... IP: 66.118.195.217 |
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posted January 05, 2003 at 09:51 AM
Following this line of logic..... at 960 VOLTS electricity is free. At 1,920 Volts the utility Co. pays the customer......... At 3,840 Volts....Electricians will rule the world !! IP: 66.118.195.217 |
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posted January 05, 2003 at 11:46 AM
Dereck, But in my calculations, the 240 volt circuit costs more to run than the 120 volt circuit. In your calculations the 240 volt cost less to operate than the 120. Don IP: 152.163.189.136 |
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posted January 05, 2003 at 11:08 PM
Don Yes. Your calculation shows that you pay more for 240V supply, but because less energy is wasted, you get far more light for your money. But, another way, incandescent light bulb for higher voltage (lower current) has thinner filament and can not be powered for so high temperature as thicker - lower voltage, higher current filament. Thus for the same energy used, light from higher voltage bulb would be dimmer. LOW VOLTAGE BULBS ARE MORE EFFECTIVE, but waste more energy on supply wires. I agree with the others that the difference is not signifficant, some times on plus, some time on minus. For real and very accurate engineering calculation we should use not only pure physic laws, but also other technological factors. IP: 206.215.76.78 |
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posted January 05, 2003 at 11:10 PM
Using your values: 120 volts: 24 ohms + .4 ohms = 24.4 ohms 240 volts:
This calc assumes that the lamp resistance versus applied voltage is constant, which in reality it is probably not (especially if it is not incandescent). IP: 69.3.93.118 |
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posted January 05, 2003 at 11:25 PM
you guys sound like potential customers for my Perpetual Motion Machine......... IP: 66.118.195.245 |
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posted January 06, 2003 at 08:11 AM
We did a job this sommer in a horse barn and stable. The lighting was all 240 volt incandescent lights. The customer wanted everything rewired to 120 volts what ever his saving was in electricity never even came close to the cost of replacing the bulbs, the cost of the 240 volts bulbs was ' Killing him '. Also they were always special order, I dont see any benefit in installing 240 volt bulbs. WOC IP: 152.163.189.136 |
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posted January 06, 2003 at 09:15 AM
Sorry guys. Don your right. I rounded my current up to 2.5 and 5 amps which introduced an error. [This message has been edited by dereck.campbell@mindspring.com (edited January 06, 2003).] IP: 165.122.123.80 |
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