240 V return path
- Thread starter heymitduh
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Dnkldorf
Senior Member
I suspect your use of "return" path is confusing you.
AC doesn't use a return path, current flows in both directions.
In 120V systems, current flows in one direction on the ungrounded conductor (hot) through the load and then through the grounded conductor (nuetral),
Then it swicthes direction. At 120 times a second.
On 240V it does the same thing but flows through both ungrounded conductors.
Rememmber the complete path?
AC doesn't use a return path, current flows in both directions.
In 120V systems, current flows in one direction on the ungrounded conductor (hot) through the load and then through the grounded conductor (nuetral),
Then it swicthes direction. At 120 times a second.
On 240V it does the same thing but flows through both ungrounded conductors.
Rememmber the complete path?
Dnkldorf
Senior Member
In case you forgot some stuff, look here for a refresher:
http://www.allaboutcircuits.com/vol_2/chpt_1/1.html
http://www.allaboutcircuits.com/vol_2/chpt_1/1.html
The reason that I said is because I've experienced it. I just went down in my basement and tested the amperage on my electric water heater. Variation of close to .5 amp between the two "hots" (ungrounded conductors) And I recall the same situation when I hooked up my neighbor's welder. 50 A two pole that varied about 2 amps between legs.
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
Kirchoff's law, shirley?Dnkldorf said:
If your meter isn't lying, and you really do have a straight 240V two-hot-no-neutral load, then there is some current finding an alternative path, suggesting a busted heater. Undo both hots from the heater and do some resistance measurements to ground?heymitduh said:
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
What kind of meter are you using? Some types of meters give that much variation, and more, just measuring the current flowing along a wooden bench. They are picking up stray magnetic fields, and are not accurately reporting only the current flowing in the conductors within their test clamps.
robbietan
Senior Member
- Location
- Antipolo City
in my neck of the woods, we use 2 hot wires for a 220V system. the electric meters used to have only one CT as the theory goes the same current should be flowing through both legs.
another CT was added to the meter only as it was discovered someone is taking advantage of the single CT by installing a ground and getting 110V off the conductor without the CT.
another CT was added to the meter only as it was discovered someone is taking advantage of the single CT by installing a ground and getting 110V off the conductor without the CT.
ptonsparky
Tom
- Occupation
- EC - retired
Most likely the .5 amps is just the accuracy of your meter. Measure the net current by clamping around both wires at the same time. Should be zero. I am not familiar with that meter so I wonder about the placement of the conductors affecting the accuracy.
As stated, heating elements do fail.
As stated, heating elements do fail.
heymitduh post #1 said:
heymitduh post #4 said:
heymitduh post #6 said:
heymitduh post #9 said:
I would like to suggest a different possibility than stray magnetic fields.
Since your load is 240v and you're taking hot to ground readings, I believe that your readings have nothing to do with your 240v water heater itself. . For the sake of conversation, I'll refer to the 2 hots as black and red.
I believe what you're reading on the black is the voltage drop across 3 different things that are in series, the water heater black and green wires, the black and green wires of a nearby 120v circuit [thru the panel], and the nearby 120v load itself.
I believe that the reading on the red is the voltage drop across 3 different things that are in series but it’s not the same 3 things as the black phase. . The reading on the red is a series voltage drop reading across, the water heater black and green wires, the black and green wires of a nearby 120v circuit that’s a different circuit than the one read on the previous reading for the black phase, and that nearby 120v load itself.
Everything being equal, the voltage to ground would be the same on each leg but the 120v circuits that “snuck” into your readings are probably very different from each other with different wire lengths.
I would be curious what kind of reading you would get if you turn every single breaker in the building off except the water heater 2 pole. . That would isolate out the other building loads. . If you take that reading post the result.
David
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My apologies for my delayed reply. Lots goin' on. Anyhow, David, I did what you advised. First, I verified my previous readings with all brkrs. on. I still read .5-.6 A difference. Then I shut down all brkrs except water heater, measured amperage and they were virtually the same. Maybe .1 A difference. And when I clamped around both red and black I read 0 A. Now I just went back down and measured amperage around both red and black at the same time with all brkrs. on and read .1 A so there is a difference. I'm going to reread your post and try to grasp it better. It's pretty deep!
JohnConnolly
Senior Member
- Location
- Phoenix AZ
heymitduh said:
Wouldn't the "return path" be thru the other phase?
Oops...other "leg"?
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russ__20000
Member
clamp around both...should read 0
Limey Pete
Member
- Location
- Tampa Florida
dbuckley said:
Kirchoffs law reminded me of a problem I was posed years ago:
Imagine a drawn cube and each of the 12 straight lines forming that cube were a 1 ohm resistor the points of convergence were connections. What is resistance between any two diagonally opposite sides?
You can intuit that value but cannot prove it withiut Kirchoffs Law.
Answers please!.
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- Location
- Massachusetts
Limey Pete said:
Something like this?
Credit to NJwirenut.
quogueelectric
Senior Member
- Location
- new york
center taped xformer
center taped xformer
center taped xformer
The reason this is confusing to many people is that the most common xformer that supplies residential power is most likely a single phase 120/240 secondary xformer where the central point of the 240 volt winding is grounded . This makes the voltages from ground never more than 120 however if you go A phase to B phase you will get 240. it is kind of like putting 2 batterys in series you would get +1.5 from the center one direction and -1.5 in the other direction but if you measured across both of them you would get 3 volts. Does this help you to understand it??heymitduh said:
quogueelectric
Senior Member
- Location
- new york
I have more time now
I have more time now
I have more time now
The confusing part is that the former is 1 winding yet they call it A phase and B phase this is because we measure from ground of a center tap so like 2 batterys in series we get a pos voltage to one side and a negative voltage to the other side this is caused by reversing the polarity of the test leads when we measure to ground it is really only one winding yet we say it has 2 different phases. This is why when the 240 volt load is balanced there is no current on the neutral it is the same winding anyway this is all a zero sum game we play.quogueelectric said:
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