What are you trying to do with the results of that computation?

No, it is not 10 amps.

At the exact moment when one leg is at 5 amps the other leg is at a lower value (assuming that the 5 amp reading was at the peak level).

Has to do with being out of phase and the type of system WYE or DELTA hookup at the generation station.

It is a bit confusing for the novice but really not that hard to understand why. You could check out a basic electricy book or do some googling on the internet to learn about multiphase systems.

ronnatalie, I am trying to find an electricity hog. My KwH usage has increased in the last couple of months with no reason. A few years ago had the same problem and my pool pump was failing and that was the culprit. So, I am trying to determine if I have another device that is failing.

budlite, so is 5 Amps the correct amount? or are you saying it is difficult to determine unless I can get a reading on both wires at the exact same time?

Let me ask a different questions also, depending on the answer to my previous question. What is the best way to determine amp usage on an appliance that uses a double pole breaker?

You are doing the right thing, just clip the ampmeter on one leg and read it, it should be exactly for other leg. For what you are doing that should be enough information. If your current draw is higher that whatever the specs say for your device then you should investigate why. Usually the nameplate ratings of whatever (refers, motors etc)(the running amps listed along with the voltage on that plate behind refers and on motors) are higher than the actual usage you will notice.

The amperage draw on each leg could be different, especially if the breaker protects a multi-wire branch circuit. Clamp around one conductor at a time. If the only load is truly 240 volts, both readings should be the same. If the loads could be 120 volts, the readings may differ from each other.

Thanks all!

Well you can look at it one of two ways then. You have either 5 Amps at 240 V, or 10 at 120. In either case the VA is going to be the same. As bus_driver points out, if some of the current flows back via the neutral, then you've got a 120V load connected there. Still adding the two single legs (and multiplying by 120) will give you the answer.

bud_lite, WYE or DELTA has no bearing on single phase power like this. Kirchoff's law says the current going in has to equal the current going out. There's no "extra phase" for it to be split across here. What he does has to worry about is that the phase of the AMPERAGE and the VOLTAGE are not necessarily aligned. Hence WATTS != VOLTS times AMPS.

ronnatalie

Residental current is our of phase between the two hot conductors and I do believe it does why and delta do have a bering on it, I would have to go back and check my theroy books but I think residential systems are from a delta wired transformers which are center tapped for the neutral. The two conducters you get at your home are out of phase (even though we still call residental home systems single phase, they are really kinda 2 phase!). So you are wrong about what you say about no "extra phase" for it to split.

Like I said the two hots are out of phase and one is at max while the other is I think 120 degrees behind.

The two hots are 180 degrees out of phase with each other in a residence. But for the purpose of measuring current, it doesn't matter, as long as you clamp one wire at a time, ronnatalie is correct.

Residential power is 120/240 V from a single centertapped winiding on a pole transformer. The hot legs are 180 degrees apart compared to the center tap.
This is NOT 2 phase power. 2 phase power is 90 degrees apart and requires 4 wires to deliver (no common).
Inductionmotrs operating on 120/240 V power have zero startgin torque. To overcome this problem a start (and sometimes a run) capacitor is used to create a phase shift (always less than 90 degrees, but adequate for starting). The run capacitor allows the motor to deliver more power and drive heavier loads without pulling to much power sionce the second phase is always present.
3 phase induction motors have starting torque, do not require slip rings or start switches, and are about as rugged as an electric motor can be made. Starting very large 3 phase motors can take some extra hardware, but it is to prevent drooping the supply lines and ensure a smooth startup.
3 phase requires either 3 wires (delta) or 4 wires (Y) to deliver and has 3 legs 120 degrees from each other.
The 4 wires required for 2 phase produced its quick death since a 3 phase delta can use one less wire and deliver greater power over the same size conductors.
The penalty to delta systems is the possibility of circulating currents in the delta from phase imbalance. Ths can lead to trasnformer saturation, overheating, and damage.
A Y eliminates this problem by providing a neutral point to return imbalance currents and having no loop current can circulate in.

Reposted a portion of what was removed (thanks moderator)

Residential power is derived from a center-tapped stepdown transformer
(7.2KV to 240V usually) fed from a single phase (one voltage waveform).
There is no Wye or Delta transformer involved. The center tap provides
for a split-phase power system, providing the 120/240 VAC power we are
used to dealing with. This is basically two in-phase voltage sources
connected in series. Think of it as two batteries connected in series (+
of one connected to the - of the other): Between the + and - of one you
have 120V, between the + and - of both you have 240V.

Normel,

I was with you until you said that our power is "basically two in-phase voltage sources". Residential power is instead two 180 degree out-of-phase sinusoidal voltage sources. The difference between the two, since they're perfectly out-of-phase "averages out" at about 240V.

As Brickeyee points out, the real advantage of multi-phase power is for starting (and running) motors. To a motor, our residential 240V power is no better at producing starting torque than a single 120V line. So, from a motor's perspective, and to keep things from being confused with a 90 degree out-of-phase 2-phase delivery system, we call our residential power single phase.

I know this is just a subtlety, but I didn't want anyone looking at our power system with an oscilliscope to be confused.

In less-densely populated areas, such as my area, the POCO pulls one primary line from one phase of the 3-phase line plus one "neutral" (grounded) as their linemen call it and that two-line primary may run 5 to 7 miles. It is single phase- period. Here it is 7200 volts. Single phase transformers are set near each service. The primary of the transformer has just two connections and the secondary has three, the center (center tap) one being grounded and also connected to the POCO "neutral". The other two are the so-called "hot wires". Measuring the voltage between the two hots shows about 240 volts (245 at my house). Measuring voltage from one hot to the neutral/center tap shows 120 volts ( 122.5 at my house). Any measurement begins at a point of reference. This repeats much of what has been previously posted, but perhaps yet another phrasing will help someone with understanding.

I'm not an engineer, so someone please correct me if this is in error. However, I thought that one of the defining characteristics of multi-phase power was that the different phases were generated by different windings on the generator. Phases are X degrees apart (120, 90, whatever) because they are literally that distance apart on the generator's armature.

Unless I'm wrong, this isn't the case with 240v residential power. So how can it be "real" two phase? Or am I missing something here? Perhaps I'm simply naive?

An engineer with one of the large power companies gave me a personal tour of one of the larger coal-fired power plants. The generator is actually three generators on one shaft, spaced 120 degrees apart. That is the three phases. In this plant the generated voltage is just 18k, but it is stepped up for transmission to about 500K volts. The generators themselves are surprisingly small. All the rest of the plant supports the generators. Much of the plant has duplicate equipment so that a breakdown of one component does not cause a shutdown. Some of the blower motors on the "burners" are 1200 Hp and they have several water pumps of 750 HP. The coal is ground to a powder and burns almost like a gas. It takes a tremendous amount of power from elsewhere just to get the plant started. The generator never completely stops, even when out of service it is turned slowly so that the bearings do not develop any flat spots. The generator is sealed when in operation and the atmosphere inside is hydrogen because it has less drag than does air. It is amazing what is done to make and send us the electricity. I did tour one nuclear station when about 50% finished, but it is off limits now.

DavidR,

Phases are nothing more than comparing multiple electrical waves with respect to time. So assuming two waves, if they both peak at the same time, they're said to be "in" phase, or 0 degrees "out of phase".

When the power company creates power, they do exactly what you suggest, and put the windings 120 degrees apart, and deliver 3 phases of power. That's why if you look at overhead transmission lines, they're always in groups of three.

However, that's not the only way to create a phase difference. As Brickeyee pointed out, single phase motors use capacitors to start (and sometimes run) the motors. What they're doing with the capacitors is creating a second phase. Residential US power comes from a single phase which is delivered to the house via a center-tap transformer. By definition, the two hot legs are 180 degrees out of phase with each other. By convention, though, we don't call this 2-phase, because at one time, power was delivered as 90-degree out of phase, and was called 2-phase. (I'm not sure if this still exists anywhere). One thing that you might find interesting is that the fans on many computers and audio equipment are 3-phase to both prolong their lives, and to make them quieter. All it takes is a little bit of electronics to transform single phase into multiphase power.

All it takes is a little bit of electronics to transform single phase into multiphase power.

I guess that depends on your perspective, and maybe on the quality of the resulting power.

One of the long-term projects for which I'm accumulating bits (as I find them cheaply) is building an electric car. I have the motor and inverter already. The motor, a 34kW 3-phase induction machine, is surprisingly small. However, the inverter is amazingly large and complex. It actually has a dedicated microprocessor just to shape the waveform, or so I'm told.

The person I bought the drive system from (I think she's a EE) told me that it's not that hard to make ugly 3-phase power, but making clean 3-phase power, power which will drive a motor at peak efficiency at a wide range of speeds, requires sophisticated hardware and software. (Take it for what it's worth, as I know very little about this stuff - I never got that far in EE.)

I do know that there are such machines as rotary converters, which make 3-phase power from single phase, but I have the impression that industrial electronic motor controllers that do this work similarly to my electric car inverter - they first rectify the incoming AC, then build up 3-phase AC from scratch. Again though this is all a bit hazy for me, so forgive me if I'm remembering incorrectly.

I agree with what you said, I may have been a bit glib with respect to how easy it is to create clean multiphase power. But back to the original post, residential 240V power is mathematically pretty clean power with two phases, but since it isn't especially useful for motors in this configuration, we just call it single phase. The primary reason for multiphase power is to drive motors efficiently.

BTW, your car example becomes more complicated because of the 34kw motor. It's a lot easier/less expensive to produce 2w of clean power for a computer fan.

Is this thread closed? My last post never appeared.

The typical residential distribution transformer has one primary winding which is supplied from one phase by the POCO. The transformer has one secondary winding which is center tapped- meaning a connection in the center of the winding as well as one at each end. This provides three connections on the secondary. The center connection is grounded, but also carries current for the 120 volt loads as necessary. If the end connections are labeled as "A" and "B" and the center is called, for the moment, the neutral, then our possible loads on the transformer secondary would be "A" to "B" (240 volts). "A" to neutral (120 volts) and "B" to neutral (120 volts). If we displayed the sine curve of all of these loads at an instant on an oscilloscope, or on three oscilloscopes, the sine curves would be identical at that moment except for the indicated voltage. Not 180 degrees out of sync nor any other angle.

"Not 180 degrees out of sync nor any other angle."

Well, actually the ends would appear 180 degrees out if they are refereneced to the center tap.
Go back to the battery analogy. You have 2 batteries inseries, say 12 volts each. The + side of one batery is connected to the - side of the other (+-+-). If you measure across both batteries from the unconnected + to the unconencted minus, you will read 24 V (+24 if you put the negative lead of the meter on the negative open battery terminal).
If you put the negative lead of the meter on the center point (AKA 'neutral) and measure to the unconnected positive end you will read +12 V. If you then go to the unconnected negative end you will read -12 V.
This is exactly how a center tap transformer operates. Since AC lines are measured RMS, the concept of negative and positive looses meaning. If you used a 2 channel o'scope with ground on the neutral the 120 V hot ends would appear 180 degrees apart. When one reached ~+168 Vpeak, the other would be at -168 Vpeak.

Brickeyee, we agree perfectly. The only differences are in the voltages, not in "phases".

When describing the instantaneuos voltage waveforms 'phase' is typically used to indcate the the sine waves are not perfectly aligned but have some time offset in the zero crossings (and thus the entire wave). For 3 phase power they appear 120 degrees apart and the sqrt(3) gives the realationship between line-to-line and line-to-neutral RMS voltages.
For a center tapped single phase supply the RMS voltages add directly bacause the peaks are 180 degrees out from each other.
It is a nice way of providing double the voltage for large loads while preserving the safety of a 120 V system. Each line remains at 120 V to neutral (that is also ground/earth referenced). To receive a 240 volt shock you would have to touch both hot wires at the same time.

BTW, your car example becomes more complicated because of the 34kw motor. It's a lot easier/less expensive to produce 2w of clean power for a computer fan.

Less expensive, I'll bet, since the power electronics uses MUCH less silicon. But easier? Maybe, but I suspect they don't even try. It's hard for me to imagine that your basic $5 computer fan (which probably costs about 20 cents in container lots) is running on a very clean waveform. I'd suspect that its circuit is pretty crude and the waveform far from optimum.

But then it doesn't really have to operate with very much efficiency. If that 2 Watt computer fan is (pulling a number out of the air) 40% efficient, in most cases (such as a 250 Watt computer) the power waste is just about negligible - lost in the noise. It's a fan, after all, so any waste heat is trivially easy to get rid of (not that 1.2 watts makes much heat). It's also apt to be powered from the mains, so the energy supply is (we hope) plentiful.

On the other hand, an electric car has a battery that holds (at best) about as much energy as a gallon of gasoline. The motor and inverter had better be as efficient as possible. The claim for this inverter is 96-98%; for the motor, 90%, so the system efficiency is 86-88%. I can't imagine that it attains that efficiency all the time, though.