Thread: Less expensive?: 120 or 240

I teach this wonderful stuff all over the country and am always coming up with questions in class to confound the unwary. The latest is: which is less expensive to run? 120 or 240. 240 is certainly more efficient if a motor, for instance, is wired that way, but is less expensive? The question came up when I was showing folks basic Ohm's Law to figure out wattage. Since we are charged by kwh, someone pointed out that 240 is twice as expensive. I feel like I'm having a brain fart here and missing something.

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Originally Posted by Terry Fitzgerald

I teach this wonderful stuff all over the country and am always coming up with questions in class to confound the unwary. The latest is: which is less expensive to run? 120 or 240. 240 is certainly more efficient if a motor, for instance, is wired that way, but is less expensive? The question came up when I was showing folks basic Ohm's Law to figure out wattage. Since we are charged by kwh, someone pointed out that 240 is twice as expensive. I feel like I'm having a brain fart here and missing something.

P= IxE. So if the motor etc. uses 100w there is less current (I) with a 240v supply. Still uses 100w of power. The savings comes with smaller conductor size requirements. Motors that are 120/240 have two sets of coils that are configured either parallel or series with jumpers to use 120 or 240v. 240v motors can be made much smaller than same hp 120v motors.

I don't know if this helps but its the best I have right now.

I think that where my brain is screwing up is that if you have a 30 amp circuit using #10 awg you have the potential to use 240 volts and 30 amps. What really matters is not the potential but what the motor actually uses. A 240 motor might only pull, say, 100 kwh where a similar 120 motor might use 200 kwh. Does that sound right?

240 volts is more efficient if long circuit runs are involved and/or if the motor is fairly large.

It is seldom cost effective to wire fractional horsepower motors for 240 volts unless they are installed in an environment where the 240 volt supply is easy to access, like industrial and some commercial installations. To this end, you often find 120 volt motors in a residential dryer that has a 240 volt supply because the same motor can be used in a gas dryer that only has a 120 volt supply. The issues of cost effectiveness in higher motor voltages really begin to show up around a one or one and a half horsepower motor unless the motor experiences freqent stops and starts and then smaller motors may benefit.

Basically, watts = volts times AMPs. In a dual voltage motor the coils are connected in series for high voltage and in parallel for low voltage. The result is that by connecting the coils in series you can use twice the voltage at half the AMPs of the parallel connection. So, the net result is that half the AMPs at twice the voltage is the same number of watts as the configuration that uses twice the AMPs at half the voltage. In either configuration the wattage will be the same.

Consequently, since you use the same number of watts either way the KWH for either configuration is the same and consequently the cost of operation will be the same.

I would offer that if you are going to teach this "stuff" all over the country you take some basic courses so you know whereof you speak. This is one of the most basic electrical principles. It's called OHMs law.

Thank you so much for the "helpful" suggestion, Bill. As I said, I was teaching Ohm's Law regarding watts when this question concerning 120/240 came up. An electrician in the class said that since 240 appliances made more efficient use of current that was why Europe and most of the world uses it instead of 120. There are three other electricians in the class and everyone thought it was less expensive to use the 240. Off the top of my head and having not really thought about it before, I wasn't positive one way or the other so I told the class that I would think about it and get back to them.

I'll have to be careful to not ask any more questions here since I wouldn't want to offend anyone's sensibilities.

Terry,

I doubt that Bill meant any offense. A thick skin is needed to post and receive posts on this board. If you have been reading for a while, it should be obvious.

The reason that Europe (and much of the rest of the world, actually) uses higher voltage than we do is simply that the wire size can be reduced. You already know this, given a specific wattage, higher voltage means lower amperage, which means smaller wire diameter needed.

This is a huge savings. Wire savings in every home, every appliance, everything that uses electrical power. We use really high voltage for the transmission lines from the generators and then step-down with transformers near the end-use. Same amount of power, just smaller wires to get it from there to here.

Urban legend or truth? I heard that one of the reasons that Europe ended up using 240 volt was that Edison had the patent for the 120 volt, 60 HZ AC systems, and was also patenting it in Europe. He was pretty cut throat, and may have pushed enough. Anyone else know more about that?

Thanks Gunnar, you are probably right. Appreciate your reply.

Originally Posted by Jim Robinson

Urban legend or truth? I heard that one of the reasons that Europe ended up using 240 volt was that Edison had the patent for the 120 volt, 60 HZ AC systems, and was also patenting it in Europe. He was pretty cut throat, and may have pushed enough. Anyone else know more about that?

Couldn't say for sure. My understanding was that Edison lobbied for DC and Westinghouse lobbied for AC and won. But, I could be mistaken.

Originally Posted by Gunnar Alquist

Couldn't say for sure. My understanding was that Edison lobbied for DC and Westinghouse lobbied for AC and won. But, I could be mistaken.

True...........

Edison was always a fan of DC power and hooked up parts of New York City with low voltage DC until he got into losing battle with George Westinghouse who, along with Nicola Tesla (who really got robbed by Edison when he worked for him, by the way), promoted AC. Edison was also active in Europe in the 1880-90's, but his system was DC.

Watts is watts, a 120 volt load drawing 20 amperes and a 240V one drawing 10A still is 2400 watts....

Alternating Current

Alternating current (AC) is one of two types of electrical flow. The other, Direct current (DC), was the predecessor of alternating current. Following the invention of the electric battery by Alessandro Volta in 1800, the next eighty years saw the invention and development of numerous devices that utilized DC. It was not until 1884 that AC posed a real challenge to DC. Unlike direct current, which flows in one direction, alternating current oscillates forward and backward at a specific frequency, usually 50 or 60 cycles per second (a cycle per second is referred to as Hertz, abbreviated Hz). In other words, the current peaks first in one direction, drops to zero, peaks in the other direction, drops to zero, and then repeats the cycle. AC in North America oscillates at 60 Hz, so it takes only one-sixtieth of a second for a single cycle. In other parts of the world 50 Hz is common, and 400 Hz is common on aircraft, to make possible lighter electrical machines.


Alternating current was the brainchild of Nikola Tesla, a brilliant Croatian electrical engineer initially employed by Continental Edison in Paris. In 1884 Tesla came to the United States to work with the chief proponent of DC, Thomas Alva Edison, and to convince him of the benefits of AC. Alternating current has a number of advantages over DC. Alternators (generators designed for AC operation) did not require the slip-rings and commutators (brushes) upon which their DC cousins depended. AC operates on the process of electrical induction, which was discovered by Michael Faraday in 1831. Inducing the flow of electricity from one coil of wire into another eliminated the arcing that plagued DC generators. As mentioned above, an even greater advantage to AC is that its voltage can be stepped-up to higher levels with a transformer, sent great distances through high tension wires, and stepped-down at its destination. Alternators at power stations produce "three-phase electricity;" they have three coils equally spaced around their primary coil, each of which is induced to produce a 60 Hz alternating current for three circuits. Three-phase electricity can supply as much current through three thin wires as it would normally take two thick wires to carry. The advantage in using a thinner wire is to minimize the electrical resistance a thick wire would produce.

Since the current of an alternating current peaks in opposite directions over one cycle, the average voltage for the cycle is zero. The voltage of the current intensity, or amplitude, as displayed on an oscilloscope, is that of a sine wave. In three phase power, as the voltage in one wire peaks, the voltage in the other two are halfway to peak (one increasing, the other decreasing).

Unfortunately for him, Edison rejected alternating current. First, he thought, the world was geared to DC; there were no AC applications so there was no need to change. Secondly, Edison considered alternating electricity to be "killer current," and would not accept arguments to the contrary. Tesla remained with Edison for only one year before quitting in disgust. The opportunity for AC to prove itself came in 1893. Tesla, with his backer George Westinghouse, underbid Edison for the contract to provide power for the Columbian Exposition in Chicago. This was the first electrical fair in history, and Tesla's polyphase (two-phase) AC system was a spectacular success. The consummate showman, Tesla put on impressive performances; at one point he sent a potential of 200,000 volts of AC through his body and challenged Edison to do the same with DC. (Edison may have had the last laugh; after the state of New York began using AC to electrocute prisoners, Tesla became convinced that Edison had helped establish the system to prove to the public that alternating current was indeed deadly.) Also during the fair, Westinghouse demonstrated a rotary converter that, ironically, changed the polyphase AC into direct current to operate a DC motor and run a railway car. The case in favor of alternating current was further advanced by Charles Steinmetz. He established the law governing hysteresis,the residual magnetism that occurs in generators and motors. Hysteresis causes a loss of power and was little understood at the time. His theoretical studies of AC in 1893 resulted in making what was then a very complex field understandable to the average engineer and electrician.

What a concise little history lesson! Cool!

Originally Posted by A.D. Miller

Alternating Current
Since the current of an alternating current peaks in opposite directions over one cycle, the average voltage for the cycle is zero. The voltage of the current intensity, or amplitude, as displayed on an oscilloscope, is that of a sine wave. In three phase power, as the voltage in one wire peaks, the voltage in the other two are halfway to peak (one increasing, the other decreasing).

Actually, the average value is the root mean square of the peak value of the sine wave, or 0.707 times the peak value for single phase.

A lot of the distribution wiring in Europe had to be replaced after WWII and the reduced quantity and cost of material was a big factor in their 240 volt system.

The 120/240 volt system we use here has the advantage of limiting the line voltage to ground to half of a 240 volt only system, and for those lucky enough to have experienced getting across both voltages at some point most will tell you 120 volts is a bit more forgiving than 240 volts is. 240 volts is easy to come by in both residential and commercial places where needed. It's definitely not needed for most lighting and appliance uses. Most home uses that can make good use of the reduced conductor sizes 240 volts allows are dryers, ranges,water heaterselectric heat, and larger home shop equipment.

None of this is to infer that some savings aren't possible from all all 240 volt supply but a good majority of the stuff we use would take many years to realize even a dollar's worth of savings.

Here's a little more info Mains electricity - Wikipedia, the free encyclopedia

Originally Posted by Bill Kriegh

Actually, the average value is the root mean square of the peak value of the sine wave, or 0.707 times the peak value for single phase.

A lot of the distribution wiring in Europe had to be replaced after WWII and the reduced quantity and cost of material was a big factor in their 240 volt system.

The 120/240 volt system we use here has the advantage of limiting the line voltage to ground to half of a 240 volt only system, and for those lucky enough to have experienced getting across both voltages at some point most will tell you 120 volts is a bit more forgiving than 240 volts is. 240 volts is easy to come by in both residential and commercial places where needed. It's definitely not needed for most lighting and appliance uses. Most home uses that can make good use of the reduced conductor sizes 240 volts allows are dryers, ranges,water heaterselectric heat, and larger home shop equipment.

None of this is to infer that some savings aren't possible from all all 240 volt supply but a good majority of the stuff we use would take many years to realize even a dollar's worth of savings.

Here's a little more info Mains electricity - Wikipedia, the free encyclopedia

BK: I'll leave it to you to argue the finer points (uninteresting minutiae) with the electrical engineer who wrote the article. I posted it to correct all of the wrong answers to the reasons why we use AC vs. DC.

A.D.
Wasn't trying to pick on you.

To some folks it doesn't matter. To others the first question is how AC gets anything accomplished if the average voltage is zero.

If you Wiki root mean square the you can find the math behind calculating the average value of a sine wave

Originally Posted by Bill Kriegh

A.D.
Wasn't trying to pick on you.

To some folks it doesn't matter. To others the first question is how AC gets anything accomplished if the average voltage is zero.

If you Wiki root mean square the you can find the math behind calculating the average value of a sine wave

BK: I managed to master enough math to cut roofs, build stairs, and balance my check book (usually). A sine wave, though it sounds important, has no meaning to me. I have looked up the definition several times in my life - to no avail. I do not even understand the definitions!

Higher voltage is cheaper to work with in regards to heat loss from current. Of course then you must have better insulation, switches that are less resistant to arcing, on and on.

Good posts. I learned a little more today than yesterday.

Watts are watts, however you slice it.

The question is, how many watts are lost due to heat? In theory, fewer amps means fewer watts spen heating the wire - but this factor pales in comparison to the expense of running fatter wire. The numbers work in a similar manner for the single phase vs. three phase argument.

A second variation of the argument comes up with control circuits, where the higher cost of the higher-voltage rated components helps offset the cost of having a transformer, or running a neutral.

The reason 240 volts and 50 Hz is used in Europe is purely political.

When electricity was coming into Europe from the US, they refused to use 60 cycles and instead used 50 cycles as 50 cycles is a division of the metric system they were using - 60 cycles was not. They also selected to use 240 volts instead of 120 volts for similarly political reason, which I forget now.

When the US went over and helped out in WWI and WWII, many areas were using both systems, the 240 volts 50 cycles and 120 volts 60 cycles, mainly left over from the US war effort helping them (we brought our own power generating equipment with us, so wherever the US went, so did 120 volts 60 cycle power, which was used to re-build after the war effort.

Political decisions to make all of Europe consistent created the need to make a decision, and the decision was to go with 50 cycles as it was a derivative of their metric system, and the standard with 50 cycles was 240 volts. So now Europe has 240 volts 50 Hz (50 cycles).

Jerry,

Better watch out. You are on vacation and are going to get in trouble with your wife!

By the way, I believe your location is incorrect. You are not currently in Florida.

Oh, yeah. Since you are on, check out the end of this topic. I think you need to speak up on behalf of Jerry Mc and myself. http://www.inspectionnews.net/home_i...rse-s-ass.html

Originally Posted by Gunnar Alquist

Better watch out. You are on vacation and are going to get in trouble with your wife!

So true.

By the way, I believe your location is incorrect. You are not currently in Florida.

Also so true.

Oh, yeah. Since you are on, check out the end of this topic. I think you need to speak up on behalf of Jerry Mc and myself. http://www.inspectionnews.net/home_i...rse-s-ass.html

Done.

Anyone who does not believe us can meet me at John's Grille when we drive back down to San Francisco to fly out back to Florida.

Europe has harmonized their voltages to 400Y/230V, in between countries still using 380Y/220V & the UK which used 415Y/240V.


In Germany it's common for 400Y/230V 3 phase in a residence.