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Hot tub wiring 120 vs 240 volts?

KARLSTER's picture

I will be wiring up my hot tub in the next day or two and it has the option of being wired into a 120 volt or a 240 volt circuit.  I am curious what the benefits are of using the 240 volt configuration. 


At first glance it appears that the primary benefit in using the 240 volt circuit will energize a higher wattage coil in the heater allowing it to heat quicker. 


I am not all that well versed in electrical theory and am curious if it is advantageous to have a fairly heavy consumer of electricity drawing its current through both legs of the incoming line as opposed to only one?  i.e. does the meter measure as accurately with the current running in only one leg of the incoming  line as opposed to current flowing through both.


Also has anyone tried an "ozonator" to reduce the need for bromine in the water?


Thanks in advance, Karl

(post #109753, reply #1 of 37)

Karl,

The type of spa that I have, Hotspring brand, had the option of operating with a 120 volt or 240 volt power supply, both requiring a 50A breaker. So I asked the same question. As it was explained to me by the manufacturer, the 120 option will operate the small circulator pump and the heater OR the larger jet pump but not both. The 240 volt option came with a sub panel and powers the heater with 240 volts and the pumps with 120 volt. In my case, the 120 volt option would have required that I provide a GFCI breaker at the power supply. The 240 volt option provided the GFCI breakers in the supbanel. The voltage will not affect the heater output or capacity. If yours is like mine, with extended use with the jets on in the 120 volt configuration, it will not maintain the water temperature and you will have to shut down the jets every so often to let the heater work. I would expect that not spas all are wired the same so your might be different. I would recommend that you use a 240 volt circuit.

The ozonator and other technologies, like silver ion cartridges, only serve to supplement oxidizers like chlorine or bromine and will provide constant protection if you are lax in maintaining water chemistry. My experience, following the manufacturers recommendations, is that no reduction in chlorine or bromine usage is realized, regardless of the supplemental treatments.

(post #109753, reply #2 of 37)

Tim, Thank you for the very thorough and informative answer!


Karl

(post #109753, reply #3 of 37)

Karl, I'm not very well versed in electricity as well. Several years ago I worked for a guy that has a fairly nice shop and nearly every piece of equipment such as table saw, jointer, planer, dust collector etc. were configured to run on 220. He told me that they actually have more power and use less electricity.

Not sure if this is true maybe somebody with more experience can confirm or refute this.


Scott R.

Scott R.

(post #109753, reply #4 of 37)

"Several years ago I worked for a guy that has a fairly nice shop and nearly every piece of equipment such as table saw, jointer, planer, dust collector etc. were configured to run on 220. He told me that they actually have more power and use less electricity. "


This is a common myth.  120/240 volt motors use the same electrical power once they're up an running and produce the same power.  This assumes the branch circuit is wired properly.


However, alot of people end up trying to run 120V motors off of inadequate existing branches, while 240V are run off of new circuits.  Also, 240V wire can be lighter guage, so is more likely to be adequate.  So the common experience is that motors work better on 240V.


In the case of the hot tub, or any heating appliance, it's always a good idea to use 240V.  Heating circuits use so much power you need to wire in a dedicated circuit, which might as well be 240V.

(post #109753, reply #10 of 37)

Power? Might not be the right word as Horse power is = volts times amps. We may be talking about torque.(?) The power is the same at 240 volts as the amps drawn will be 1/2 of the amps drawn at 120 volts.


The saw does get up to speed faster, I can personally attest to that. It might be a matter of slip of the rotor. (1750 rpm vs the theoretical 1800 RMP) due to the fact that higher voltage makes a smaller percentage of volts applied to magnetization of the core.


I have to run (BSA campout this weekend) but I will ponder this over the weekend. I look forward to hearing someone else finish / correct this theorizing.


Here's a coupla facts to feed the fire:


Foot pounds per second x 1.356 = Watts  (or Volts X Amps)


Horsepower x 746 = Watts


Someone please tell me what is missing.


TRVTH
Jack of all trades and master of none - you got a problem with that?

(post #109753, reply #11 of 37)

Most likely it is less line drop, since 220 circuits are usually much heavier duty than generic 110V circuits. Many times the 220 circuit will be capable of 30 or more amps which is 3 times the power of the average 110 volt circuit, but even if it is a #12 20 amp circuit it is still going to have half the line loss of a #12 110v circuit used for the saw.


Edited 11/15/2002 5:36:15 PM ET by MARKH128

(post #109753, reply #12 of 37)

"Foot pounds per second x 1.356 = Watts (or Volts X Amps)

Horsepower x 746 = Watts

Someone please tell me what is missing."

Lots

First Watts is NOT EQUAL to volts x amps.

Power (in Watts) = V*A*pf (power factor).

While in electrical equipment is common to measure power in watts there are other units, horsepower and btu/min are too other common measurement systems.

The 1 hp = 746 is just the conversion factor between unit systmes.

What what kind of power is this. It is the power into the motor.

There are losses in the motor So that mechanical power delivered by the motor is input power * efficiency.

For the 1 - 3 hp, single phase motors that are common in woodworking equipment the combine power factor and eff is in the range of .55 to .75 when running at rated power.

But you basic premise is correct. A 120/240 motor has two sets of windings. They are in parallel for 120 or series for 240. But exactly the same amount of current and the same voltage across each winding. If there is equivalent wiring there won't be any difference.

The starting difference is that starting takes a large surge current so any small drops in the wiring are more noticable. With 1/2 current on 240 and twice the supply voltage. The affect is much less.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #109753, reply #5 of 37)

compare electricity to say gasoline, let's say a gas of 100 octane gets you 50 miles to a gallon, but a gas of 50 octane only gets you 25 miles to a gallon because it has less energy(I know this isn't true but it's a good analogy). 


well a voltage of 240 volts has more energy than 120 volts and so less of it is required to do the same amount of work, the amount of energy is measured is amps or also referred to current.  so you should always when possible use 240 volts because it requires lower volumes of current, half to be exact.  the motors run cooler, and less current is running through the wires which causes less heat, and in extreme cases could prevent a fire.  to put it simply would you rather have 20 amps runing throught one leg (120 volts) or would it be beter to split that into 2 legs with 10 amps in each(240 volts)?


as far as being cheaper I don't believe that is really true, the electric company measures 240 current volume, if you turn on a 120 volt light the electric meter adjusts to the fact you are only using half of it's supply.


as far as the ozonator i have no experience with them other than a co-worker has one on his spa and really likes it, says he uses very little chemicals to treat his spa.


good luck, be carefull


 

          Strong words indicate a weak argument     

(post #109753, reply #6 of 37)

Sorry, but your analogy is wrong. A motor running on 120 volts is using say 746 watts. The same motor running on 240 volts is using half the amps but still 746 watts.  There is no practical difference. Assuming the wiring is adequate for the load, there will be no difference at all.

(post #109753, reply #7 of 37)

"Sorry, but your analogy is wrong. A motor running on 120 volts is using say 746 watts. The same motor running on 240 volts is using half the amps but still 746 watts.  There is no practical difference. Assuming the wiring is adequate for the load, there will be no difference at all."


Yup.


When wired for 120V operation, the two windings in the motor are in parallel.  When wired for 240V operation, they're in series.  Either way, each winding gets 120V across it, and draws the same number of amps.  It makes no difference to the motor what voltage the branch circuit is.  Again, this assumes the wiring is adequate for the load.

(post #109753, reply #8 of 37)

if it makes no difference than why do they even make a 240 volt motor, if it is all the same from what you are saying there is no need to ever have a 240v motor, or what about the 75 hp 480volt motors running some of the machines i program? why not just plug them into the wall? 


the reason is because its current that causes heat and by upping the voltage that a motor runs on you reduce the gauge of the conductors required to carry and use the load.  and I never stated that the wattage was different!  it does make a difference, as far as current only is concerned, the higher the voltage used the less current thus the less potential for overheating the circuit.  if you take this motor and run it under constant load it will run cooler at 240 volts as opposed to 120 volts, the reason is because at any given time the windings in the motor are carrying half the current, the other half of the current is in the other set of windings.  if you run at 120 you tie those winding together thus doubling the current going through them.  do what you want, both are acceptable, but 240 is safer and better!

          Strong words indicate a weak argument     

(post #109753, reply #9 of 37)

Get a basic textbook on motors. Your theory is seriously flawed.

(post #109753, reply #17 of 37)

 


go here and read it for yourself, maybe you should review that basic text book?


http://www.solarseller.com/alternative_energy_efficiency.htm


"There is an advantage to using the 240 v.a.c. wiring points in this motor. If it is wired for 240 v.a.c. it will draw 2.5 amps and still consume 600 watts (240 x 2.5 = 600.) This motor, running at the higher voltage, can use smaller wire for a given distance and the motor will also run cooler (and last longer) since it is using half the amperage. Amperage in a conductor (be it wire or the windings in a motor) = HEAT.  "


i'm sure (like al gore) you will refuse to concede so i wll stop here and not waste any more of my time with you.  have a good weekend.

          Strong words indicate a weak argument     

(post #109753, reply #18 of 37)

"This motor, running at the higher voltage, can use smaller wire for a given distance and the motor will also run cooler (and last longer) since it is using half the amperage. Amperage in a conductor (be it wire or the windings in a motor) = HEAT."

He is right about the size of the SUPPLY wiring.

He is COMPLETELY WRONG about what happens in the motor. He does not understand how the motor is constructed he will understand that exactly the same amount of current flows through each winding. Regardless of whether they are setup for 120 or 240 the currents in the coils are not affected. There are two sets of coils that are either connected in parallel (120) or series (240).

The motor will not run cooler operating on 240 vs 120.

Bill (BEE, MsEE)

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #109753, reply #32 of 37)

> There are two sets of coils that are either connected in parallel (120) or series (240).


Someone else on this forum once said that while the run windings are arranged either series for 240 or parallel for 120, that the same starting winding is used in both cases.  From a look at the wiring diagrams for the Baldor on my Unisaw, that looks like it could be correct.  In that case, the motor would start a lot better on 240.


 


-- J.S.


 

 

 

-- J.S.

 

(post #109753, reply #33 of 37)

I was under the impression that the start winding was across one of the main windings. So when it was connected for 240 it the main windings acted as a autotransformer so that the start winding was still on 120.

But I wonder about the capacitor start/capacitor run motors. Those would almost have to have two sets of "start" windings.

But I have never torn into the windings to see what is going on internally.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #109753, reply #34 of 37)

The start winding on every motor I've ever seen that had a start winding (ie split phase, cap. start/induct. run, cap. start/cap. run) was wired in parallel with one run winding.  Wired for 115V operation, the start winding sees 115V, since all three windings are in parallel.  Wired for 230V, there's a shift from 0V at the mid-point (during starting only), since it is now a series-parallel configuration with one run winding in series with a run/start parallel winding pair.  The start/run pair will drop somewhat less than 115V due to the lower impedence.  I've measured this momentary voltage difference to ground, and I seem to remember it being in the 20V-30V range at it's peak, dropping to a volt or two after start.  In one experiment on a 1/2 hp motor starting a fairly high inertia load, the motor howled a bit during starting at 230V, but went nearly silent when I connected a neutral to the common point, thus locking each winding into 115V.  It didn't start any better, it just got quieter.  I thought that was interesting, though probably meaningless in any practical sense.


I've attached a connection diagram and a schematic (sort of) for a 3 hp Baldor [Unisaw] motor [(Unisaw because it has the special attachments for that saw)], but as a connection diagram it is not unique, nor is the motor itself otherwise unique.  The start portion of the circuit is in green to make it easier to visualize it not being connected after start.  A capacitor run motor has a run capacitor in parallel with the start capacitor, but wired around the start switch so that the "B" winding and run cap. remain in the circuit after starting.  These are used mostly on larger single-phase motors, with the run cap. creating a second phase, allowing more power output and higher efficiency from a smaller frame.  Sort of a 2-phase motor.


I've also tossed in a sketch of two 60W light bulbs in series and parallel, as an analogy for how these small motors are connected.  Same voltage drop, same current, same power, just different line voltage and current.  The series case looks just like a load center and two branch circuits, if you connect the neutral.


The core of the whole "faster starts, more power" perception of 230V operation is in how the speed/torque/efficiency/power factor curves are effected by deviations in voltage from namepate value.  The torque curve of a typical motor drops as the square of the voltage reduction from nameplate.  In other words, with a 10% drop in voltage (to 103V from 115V) at the motor terminals, locked-rotor torque, pull-up torque, and breakdown torque are reduced to 81% of full-voltage values.  At 20% voltage drop (92V, down from 115V), these values drop to 64%.  With startup current for induction motors ranging from 2 to 5 times full load current, voltage drop in branch circuit conductors can be substantial.  Doubling the voltage (and halving the current) has a huge effect on performance, all other things being equal.  Using a 1.5 hp motor as an example, with 50A locked-rotor current at 115V and 50' x 2 12 ga conductors (.162 ohm), that's an 8V drop, or about 7%, which is a drop in locked-rotor torque of about 13.5%.  Going to 230V, 25A and the same 12 ga wire, 4V dropped on 230V is about 1.7% for a locked-rotor torque reduction of only about 3.4%.  Same wire, much different performance.  Performance loss is even greater, obviously, with light extension cords and/or long wire runs.  Torque reductions are less pronounced as the motor speeds up and the current draw decreases/impedance increases (the start winding messes this up; three-phase is a smooth curve), but the perception is that the motor starts easier and "has more power".  But, of course, if a motor was wired to an infinitely stiff voltage source, 115V or 230V at the lugs, there will be no difference in performance.   My understanding of the NEMA MG-1 motor nameplate design voltage requirements of 115, 230,460V etc. as compared to the nominal supply voltages of 120,240,480V, etc. is that it is an effort to build some branch circuit voltage-drop-under-load compensation into the motor design to further reduce this effect.  An underloaded motor draws less current anyway, so a slight over-voltage is allowable and inconsequential, just as a light motor load under reduced voltage is OK.  As long as nameplate current is not exceeded. 


4LORN1 wrote "Both theory and observed effects touch gloves and retreat to their respective corners to emerge and fight another day."   I think they do agree.  Now, if someone would like to take a stab at how similar transformers and induction motors are, and how rotor slip and back EMF controls impedance and thus current draw, I'm going out for coffee and to rest my brain.


edit: References to Unisaw deleted; that particular motor is 230V only.  Diagrams are for similar dual voltage Baldor motors.  Sorry for the error. 



Be seeing you...


Edited 11/19/2002 2:16:58 PM ET by TDKPE

Be seeing you...

(post #109753, reply #35 of 37)

Thanks, TDKPE.  I've saved your message and drawings, they're the kind of thing that comes in handy sometimes.


 


-- J.S.


 

 

 

-- J.S.

 

(post #109753, reply #36 of 37)

Thanks for all of the details.

I see that the motor that you posted the diagram for is reversilbe by swaping the connections for the start windings.

I checked the 1.5 hp motor on my Jet CS table saw and it only has two sets of wires for 120/240 and it is none reversible. The start winding is permanately wired internally across one of the main windings.

I am seeing more cap start/cap run motors being used to reduce current loads on 120.

Delta CS is CS/CR and speced at 1.5 hp, 12 amps.

And Jet's new Supper saw is rated at 1.75 as is the somewhat similar DeWalt.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #109753, reply #37 of 37)

Wow.


Clearly and concisely put. Line resistance would have been far down the list on my guesses for the behavior of motors at 220 vs 110. Likewise, I printed your post.


As for the transformers and their behavior, I cannot go into your level of definition but gross understanding is that the motors and transformers both work on a magnetic principle. That is the first 10ms of their electrical turning on is occupied "filling" the magnetic fields in the silicone steel cores.


Transformers and motors require time delay fusing to allow for this inrush.


As a description  of what I suggest: In an AC motor with a variable speed control (I'm describing industrial models here) There is a voltage boost adjustment to provide for the magnetizing current at startup and low speed operation. Normally a motor wants to run at a linear relationship to voltage & frequency. A 480 volt motor running at 1/2 speed would like to see 240 volts to match the 30 Hz output from the VFD. As you get down to the bottom of the frequencies the low voltage being out put in this linear relationship is not enough to keep the motor running. The voltage boost is added in so that the voltage put into the motor will not drop (for a 480 volt motor) below 60 volts.


That sort of puts "clothes" on my magnetic explanation.


I owe you a coffee for sharing your post.

Jack of all trades and master of none - you got a problem with that?

(post #109753, reply #20 of 37)

Sorry if I was offensive.

(post #109753, reply #21 of 37)

One small point that may make Bill Hartmann's explanation a bit more understandable:


I think the key concept that you are missing in Mr.Hartmann's explanation is the functional difference between series and parallel circuits.


When the motor is rigged to work on 120v the two windings are placed in parallel and each winding is directly contacting both the 120v hot and neutral. Each of the coils "see" , get, 120v. In this case each winding carries half of the total full load current, amperage. A good thing because a motor on 120v pulls twice the amperage, current, than the same one on 240v.


When the motor is hooked up to run on 240v, the windings are hooked up in series, one attached to the end of the other, the two windings share the 240v. Each winding only 'sees' 120v. In fact if you were to measure the voltage at the point that the between the two windings to ground it would read very close to 120v. Each winding carries the full current drawn by the motor.


In series circuits voltages divide, are shared, but current, amperage does not. In parallel circuits current are shared, divided, but the voltage does not. In either case, the motor being rigged for 120 or 240v, the windings, each one being considered separately, see both the same voltage and current. The difference being that the motor will draw more current, amps, on less voltage, 120v, but more voltage and fewer amps at 240v.


The only real cost savings would be that if the house wiring were marginal for the amperage the wiring would heat up, in effect becoming a long but weak space heater. This would both raise the usage of electricity, as plugging in a space heater would, and lower the performance of the motor, or ozonator as the voltage would drop. Switching the setup to 240v would half the current draw, cause the wiring to run cooler, preserve the voltage at the load and could save a bit of money. But the real savings are because you are correcting for weak wiring not saving electricity at the motor.


Both theory and observed effects touch gloves and retreat to their respective corners to emerge and fight another day.

(post #109753, reply #22 of 37)

Well Said !


Wouldn't a chalkboard, some batteries, some lights, and a fluke make this so much easier to explain?


The "efficiency theory" of 240 is related to the inefficiency the same sized conductors due to higher current loads when running 120. 


I don't know if it was mentioned anywhere, but didn't someone say that if the tub was hooked up 240 that they were able to take advantage of running 2 things simultaneously?  The reason for that is that with the lower current draw of the motor at 240, there is "room" left on the conductor for the extra amps for whatever that other device was.


 


Steelkilt Lives!

http://jhausch.blogspot.com 
Adventures in Home BuildingAn online journal covering the preparation and construction of our new home.

(post #109753, reply #23 of 37)

Hi guys  - 


 


I'm new on this forum and have to jump into this discussion late.  Glad that the motor 120/240v questions were cleared up.


I run a small electrical contracting business and hook up at least a couple of residential tubs a year.  The issue about 120v vs. 240v is USUALLY this:


The 120v configuration USUALLY requires a single 20a 120v dedicated circuit.  The heater and jets don't run simultaneously. The heater, a linear resistive load, has 120v applied across it and might be on the order of 1500 watts.  It takes a long time to bring the tub up to temp and if you're in a cold-weather climate and are in the tub, the heat loss can exceed the ability of the heater to keep up.


So, enter 240v.  Suddenly the same heater has 240v applied to it.  Because of its' fixed resistance and Ohm's law, the output is quadrupled, say to 6000 watts. The draw is about 25a.   You run a 50a circuit, #6-gauge wire, and you can also run your 1/2hp 120v 2-speed pump on high while the heater works.


Like I said, this isn't how all tubs are set up, but MOST residential self-contained types are like this.


Ozonators are good, they reduce the level of sanitizer considerably.  However, the cheaper ones tend to rust or oxidize, then leak a little current to ground and  trip the GFI breaker.


Yers truly..........LEE 


Edited 11/17/2002 11:27:00 AM ET by Ed

(post #109753, reply #24 of 37)

4LORN1,


One thing I have been curious about is whether or not the utility companies meter can accurately measure both 110 and 220 volt loads.  In other words if you apply a 50 amp load on one 110 volt leg of the line coming out of the meter, will it spin at the same speed/record the same consumption as if you were drawing 25 amps at 220 volts from the two incoming hot wires.


Thanks, Karl

(post #109753, reply #25 of 37)

Actually those meters are actually very accurate, considering the environment and conditions they are forced to work in, and operate on a principle that eliminates inaccuracies due to voltage. I wouldn't worry too much about trying to save money by rejiggering the voltages of appliances or such. Even a best case and assuming a very inaccurate meter the differences are miniscule. Many hours of time could be wasted trying to squeeze out a penny or two of savings.

(post #109753, reply #28 of 37)

Since the subject of residential meter accuracy has already been brought up, I've always wondered what the meter is reading, ie kVA-hrs or kW-hrs.  Or, in other words, is the meter influenced by low power factor reactive loads such as unloaded motors and transformers, light ballasts, etc.  I've always wanted to ask, but it wasn't worth starting a whole thread over.

Be seeing you...

Be seeing you...

(post #109753, reply #29 of 37)

OK I said I was done with this discussion but additional information has been brought to my attention so I'll throw a few dried pine needles on this fire.


I work with an extremely talented electrical/computer engineer, he's one of those rare people you can find at a university teaching, but has the real talent to put his knowledge into industry, probably the most intelligent person I have ever known.  well I asked him about residential wiring and how the meter works.  according to him (and I have never been able to prove him wrong on anything in any subject), he stated that the shunts used to measure the electricity measure pure current, it doesn't matter which leg of the 220 you use, so if all of the 110 circuits in your house were being used and all wired to only one leg of the 220 brought to your home your meter would run twice as fast than if you split those circuits in half using the other leg of the 220 and the total load being equal.  I had no idea this was true,  he also stated that it is important that your box be balanced or your ground leg brought to your house will be carrying excessive current that can reek Havok on some of your electronics.  this being true, it is cheaper to run a 220 motor at say 10 amps as opposed to running it with 110 at 20 amps.


by the way, a few posts back there was an argument that residential wiring is single phase 220, that isn't true, if it was single phase the a meter across the two leads into your box would show 0 volts no potential difference, they need to be phased 180 out to read 220.


ok i'm ready.....  hit me

          Strong words indicate a weak argument     

(post #109753, reply #30 of 37)

"by the way, a few posts back there was an argument that residential wiring is single phase 220, that isn't true, if it was single phase the a meter across the two leads into your box would show 0 volts no potential difference, they need to be phased 180 out to read 220."

This is more of a problem in terminology rather than any thing.

Yes it is 2 DIFFERENT phases. As an engineer you can measure and write mathemical descriptions for the two different legs showing that they are 180 degrees out of phase.

But to the electrican and user they are not "poly-phase". In other words they can not be used to operate a poly phase motor.

Poly phase meaning more than one phase. Of which currently 3 phase is the only common one, but there where others.

(BTW, there where true 2 phase systems in the past where the phases where 90 degrees apart. They where used on some servo control systems and maybe some power systems).

I started to descibe how a motor worked, but was getting to envolved and I am not that good (actually terrible) trying to describe something like this with words.

But a induction motor (common AC motor) requires a rotate magnetic field. If you try to use that with the "2 phases" in a 240 volt system you just have the magnetic field flopping back and forth 180 degrees. There is no tendency for the motor to want to turn either CW or CCW.

In poly phase motors the other windings are spaced around the motor. For example with a 3 phase motor they are 120 degrees apart mechanically and well as having electrical signals whose peaks are apart 120 degrees in phase (time). That causes a magnetic field to be formed and the rotor tries to follow that field and turns.

For single phase motor they have to artifically generate a 2nd phase and that is most common done with a capacitor as in a "capacitor start single phase induction motor". Once it is started then the rotationo the rotor acts as a generator and keeps the 2nd phase going.

About the Kw-HR meter you friend is wrong. But I have to do some research to see how to explain it.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #109753, reply #31 of 37)

Your friend is all wet.

First while shuts can be used to form a voltage that can measured to compute the voltage that is not how watt hour meters work. For higher currents they use a current transformer to reduce it to a lower value for the meter, but still no shut.

The have current coil in them. The Dot product between the current coil and the voltage coil are what drive the disk in the meter. For 240/120 systems they have two current coils so that the affects of each leg are added magnetically.

Thus it does not matter if it is 120@20 amps are 240@10 the watt hour meter will register the same.

Look at http://www.usbr.gov/power/data/fist/fist3~10/3~10_3.htm

and

http://www.themeterguy.com/Theory/watthour_meter.htm

http://www.themeterguy.com/Theory/three_wire_single.htm

I believe that someone asked about whether they measured power or volt*amps.

They measure power, that is that portion of the current that is in phase with the voltage.

BTW, commericial services often have surcharge for the peak load (maximum watts) used over a short period of time (demaind factor).

They also charge for the VAR-hours. That is volt amps reactive or the amount of current that is 90 degrees out of phase with the voltage. It does not do any work so they don't charge directly for this. But the power company has to pay to shuffel the reactive current back and forth and power is lost in the line resistance.

The difference between real power and reactive power is called the power factor.

That is why you see lamp ballast that are listed as High Power Factor. Using those will help the business reduce their surcharges.

Now days lots of the commerical service use an electronic meter with an LCD display that cycles between the different measurements.

. William the Geezer, the sequel to Billy the Kid - Shoe