Electron flow in 240V circuits

electro7

Senior Member
Location
CA, US
Hi,

I was wondering if I could get some help understanding 2 and 3 pole circuits. For whatever reason I have had trouble understanding the theory behind the electron flow with them.

I understand that electrons are always trying to go back to the source. With 120V single pole circuits they flow through the hot, through the load, and back on the grounded conductor.

I understand that in a 2 or 3 pole circuit with a neutral, the unbalanced load is carried back on the neutral.

Here are my questions:
On a 2 pole or 3 pole circuit that say does not have a neutral or has a balanced load, how do the electrons get back to the source? Do I understand right that they flow through the load 180 degrees, or 90 degrees in 3 pole circuits, out of phase from each other and return to the source through the other hot conductor? And with AC current the electrons are moving in both directions, correct? Back and forth, back and forth, back and forth. Then I am trying to understand how a utility meter measures the power used in a building when the electrons are flowing both ways. Do the meters only read electrons flowing one way?

Thanks for your help. I feel like I have a bit of a gap in understanding this concept.
 

mgookin

Senior Member
Location
Fort Myers, FL
Looking at transformer diagrams is very helpful in understanding how the power flows in those configurations.
Look some up on Google for the various types and you'll see how the power is moving.
 

Besoeker

Senior Member
Location
UK
Hi,

I was wondering if I could get some help understanding 2 and 3 pole circuits. For whatever reason I have had trouble understanding the theory behind the electron flow with them.

I understand that electrons are always trying to go back to the source. With 120V single pole circuits they flow through the hot, through the load, and back on the grounded conductor.

I understand that in a 2 or 3 pole circuit with a neutral, the unbalanced load is carried back on the neutral.

Here are my questions:
On a 2 pole or 3 pole circuit that say does not have a neutral or has a balanced load, how do the electrons get back to the source? Do I understand right that they flow through the load 180 degrees, or 90 degrees in 3 pole circuits, out of phase from each other and return to the source through the other hot conductor? And with AC current the electrons are moving in both directions, correct? Back and forth, back and forth, back and forth. Then I am trying to understand how a utility meter measures the power used in a building when the electrons are flowing both ways. Do the meters only read electrons flowing one way?

Thanks for your help. I feel like I have a bit of a gap in understanding this concept.
If it's AC, the electrons just wiggle back and forth a little bit.
 

Ingenieur

Senior Member
Location
Earth
Think of a piston
oscillates in both directions
but no net displacement per cycle (Hertz)
but does work
AC does it on both 'strokes'

current in Amperes = Coulomb/Sec
or charge/electrons passing a point per unit time
it is still current flow if it is the same charge/electron passing back and forth across the same point
 

kwired

Electron manager
Location
NE Nebraska
A two wire circuit regardless of voltage has the current flow from one conductor through the load and back via the other conductor - most people can understand this and it doesn't even matter if you are talking AC or DC current.

A single phase multiwire circuit (two hots and the neutral) is a little more complex but still relatively simple. On typicall 120/240 application you have 240 volts single phase with a tap in the center to derive 120 volts. You mentioned you understood balancing of these systems so I won't dwell on them too much other then to say it is important to have the neutral present to stabilize the voltage on intended 120 volt circuits. Lose the connection to the midpoint of the transformer and you end up with loads in series at 240 volts and varying voltages on 120 volt loads because they are now in series with other loads instead of connected to a solid neutral.

The three phase is what you are likely having a harder time with I imagine.

For a simple example lets take three 240 volt 10 amp heater elements and connect them in a couple different ways. Each element when connected to 240 volts is 2400 watts.

Connect them all in parallel across a single phase 240 volt supply and you have 30 amps draw - 7200 watts, but still individually each element still sees 10 amps and 2400 watts.

Connect them in delta configuration across a 240 volt three phase supply and each individual element still has 240 volts across it, each individual element still has 10 amps of current flowing in it. But clamp a meter on any of the three supply conductors and it will read 17.32 amps. This is because the current in supply line A is not only sending current to each of the two elements it is connected to, at the other end of those two elements we also have a connection to supply lines B and C. It gets more complex to explain how/why it divides like it does but it divides at the square root of three factor which is 1.732 plus a never ending series of numbers past the decimal point, but we usually round it off at the hundredth or thousandth level)

Bottom line is with single phase all current flowing on A is the same current flowing on B, with three phase current flowing on A is split by a factor of 1.732 to B and C, net current is a result of some current is going out on A and at same time some is coming back on B and C. They are not at 180 degrees but rather are at 120 degrees so they don't completely cancel one another out. It gets more complex if not balanced but trying to help you with the simplest application here and see how you understand that first. Please reply with any questions or let us know if you understand what I have said so far.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If you look at a three phase AC current diagram for a balanced circuit it's pretty obvious. Current above the line is flowing one way and current below the line is flowing the other. If you look at an instant where the "A" phase is at its maximum above the line, the other two phases are both exactly half as far below the line; in that instant the current away from the source on A is being returned to the source split in half between B and C. Pick any point in time and you'll see that the sum of the currents above the line is exactly equal to the sum of the currents below the line.
 

Arguyle

Member
Location
michigan
A two wire circuit regardless of voltage has the current flow from one conductor through the load and back via the other conductor - most people can understand this and it doesn't even matter if you are talking AC or DC current.
Im the guy that is having trouble understanding, lol. My journeyman and I were discussing this the other day and I have been thrown all out of whack. For fear of hijacking this talk, please let me know how to continue this with Kwired for more about his explanation.

Thank you
 

kwired

Electron manager
Location
NE Nebraska
Im the guy that is having trouble understanding, lol. My journeyman and I were discussing this the other day and I have been thrown all out of whack. For fear of hijacking this talk, please let me know how to continue this with Kwired for more about his explanation.

Thank you
Do you understand for a simple circuit you must have a source, a load, and conductors that make a complete "circuit" from source to load, through the load, back to the source, through the source and then the path starts over again? If not you maybe do need to start a new thread, as the question presented by OP though somewhat similar topic is at a little higher level of complexity.
 

GoldDigger

Moderator
Staff member
...
...
And with AC current the electrons are moving in both directions, correct? Back and forth, back and forth, back and forth. Then I am trying to understand how a utility meter measures the power used in a building when the electrons are flowing both ways. Do the meters only read electrons flowing one way?

Thanks for your help. I feel like I have a bit of a gap in understanding this concept.
If the meter only measured current, you would indeed have a problem with AC. But Power is Voltage times Current.
And at the same time the the current changes direction (sign) the voltage changes direction too, at least for a resistive load. And a negative number times a negative number is a positive number. So the power meter is pushed in the same direction during each half cycle.
I will not go into how an analog meter takes the product of voltage and current, since it is pretty complex physics. But it does.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Think of it even with line-neutral loads, that the electrons are seeking a path to the other phases. With single phase, the current leaving L1 is seeking L2, and the only path there is through the windings of the transformer between N and L2.

With 3-phase, half the electrons leaving A are seeking B, half are seeking C. The neutral is just another path to get there. That is why if you have a 10A load from A-N, and a 10A load from B-N, then 10A will travel on the neutral; that 10A is seeking C, and the shortest path there is to N, then on to C through the transformer windings.
 

mivey

Senior Member
Think of it even with line-neutral loads, that the electrons are seeking a path to the other phases. With single phase, the current leaving L1 is seeking L2, and the only path there is through the windings of the transformer between N and L2.

With 3-phase, half the electrons leaving A are seeking B, half are seeking C. The neutral is just another path to get there. That is why if you have a 10A load from A-N, and a 10A load from B-N, then 10A will travel on the neutral; that 10A is seeking C, and the shortest path there is to N, then on to C through the transformer windings.
That is just so wrong. That is as bad as saying current wants to get to Earth. Sorry I don't have time to elaborate at the moment but you should re-think that. Suffice it to say you could take bolt-cutters to "C" and it would not matter.
 

electro7

Senior Member
Location
CA, US
So the electrons wiggle back and forth a little bit. I understand. So then would the amount of current be how fast they are moving back and forth (Coloumb/sec)?

Then if a load requires 30 amperes and there is #14 AWG wire used the heat produced by the electrons moving back and forth is to much for that wire and would cause the insulation to melt. Am I thinking about this correctly? And the load may not operate because the resistance may be to much to allow that much current in that small of wire.

Now for a DC circuit, since it is direct current do the electrons make the full loop, moving from the source, through the wire, through the load, back through the second wire to the source? I am thinking of DC because we install solar PV systems and sometimes solar PV battery back up systems.

Thanks for your help!
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Now for a DC circuit, since it is direct current do the electrons make the full loop, moving from the source, through the wire, through the load, back through the second wire to the source?
Yes, but it takes an individual electron a very long time to make the loop.
 

Ingenieur

Senior Member
Location
Earth
Think of the 'speed' as the frequency which in our case is fixed

a #10 can carry 1 A or 30 A
the 'speed' is the same
what varies is the current density or gradient
current/area

more electrons are excited (driven to move, either migrate the whole ckt or vibrate/oscillate) the higher the voltage
assume a 1 ohm load
1 v = 1 A
increase the driving potential or electromotive force to 30 v you get 30 A
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
That is just so wrong. That is as bad as saying current wants to get to Earth. Sorry I don't have time to elaborate at the moment but you should re-think that. Suffice it to say you could take bolt-cutters to "C" and it would not matter.
Is it so wrong that it does a disservice to an electrician working in the field? I don't pretend to understand the intimate intricacies of exactly why electricity behaves the way it does, but I've always considered this to be an acceptable shortcut to visualizing what's going on. I'm sorry you find it detrimental to the OP. Pray, what simple illustration do you have to offer?
 

electro7

Senior Member
Location
CA, US
So just so I have it solidified in my understanding, in a DC circuit the electrons flow through the hole loop (circuit) and in an AC circuit they just wiggle back and forth a bit. Is that correct?

In an AC circuit the amount of electrons that wiggle back and forth is the amount of current in that circuit rather than the speed at which they move, correct?

With the hole wire size question, I guess it comes down to how many atoms are in the copper wire with valence electrons that can be excited out of orbit. So can a #10 AWG wire carry 200 amps? The answer to that questions would probably be "Yes" but the wire and insulation would be burnt to pieces. Maybe a #22 AWG could not carry 200 amps because there are not enough valence electrons to excite out of orbit. Am I thinking along the right lines here?
 

kwired

Electron manager
Location
NE Nebraska
So the electrons wiggle back and forth a little bit. I understand. So then would the amount of current be how fast they are moving back and forth (Coloumb/sec)?

Then if a load requires 30 amperes and there is #14 AWG wire used the heat produced by the electrons moving back and forth is to much for that wire and would cause the insulation to melt. Am I thinking about this correctly? And the load may not operate because the resistance may be to much to allow that much current in that small of wire.

Now for a DC circuit, since it is direct current do the electrons make the full loop, moving from the source, through the wire, through the load, back through the second wire to the source? I am thinking of DC because we install solar PV systems and sometimes solar PV battery back up systems.

Thanks for your help!
Following statement I copied from a Wikipedia article.

"One coulomb is the magnitude (absolute value) of electrical charge in 6.24150934(14)×1018 protons or electrons"

What that means is one coulomb is a measurement of charge. Current is a measurement of movement of a particular amount of charge. Whether that movement actually means subatomic particles travel through the current path, IDK, and am not sure if anyone actually knows, but what is being measured is the amount of charge that is moved through the circuit path regardless of how it actually moves.

I think many accept the fact that current flows through a conductor in a similar fashion to how a "Newtons cradle" works, energy is introduced into a conductor at a certain point, by several possible methods, but in all cases is conveyed through the conductor by bumping sub atomic particles into adjacent particles similar to the way a Newtons cradle works.
 

JFletcher

Senior Member
Location
Williamsburg, VA
So just so I have it solidified in my understanding, in a DC circuit the electrons flow through the hole loop (circuit) and in an AC circuit they just wiggle back and forth a bit. Is that correct?

In an AC circuit the amount of electrons that wiggle back and forth is the amount of current in that circuit rather than the speed at which they move, correct?

With the hole wire size question, I guess it comes down to how many atoms are in the copper wire with valence electrons that can be excited out of orbit. So can a #10 AWG wire carry 200 amps? The answer to that questions would probably be "Yes" but the wire and insulation would be burnt to pieces. Maybe a #22 AWG could not carry 200 amps because there are not enough valence electrons to excite out of orbit. Am I thinking along the right lines here?
An uninsulated, free air #10 conductor could carry 200A easily, indefinitely.. in the Arctic Circle. A #22 would probably melt near instantly, even in liquid nitrogen. It boils down to resistance, current, and time. Battery jumper cables are very small for the loads they carry, yet putting 1000A across #2 wire for 3 seconds doesnt result in a pool of molten copper... try it for 2 minutes, maybe a different story.
 

jaggedben

Senior Member
Hi,

I was wondering if I could get some help understanding 2 and 3 pole circuits. For whatever reason I have had trouble understanding the theory behind the electron flow with them.

I understand that electrons are always trying to go back to the source. With 120V single pole circuits they flow through the hot, through the load, and back on the grounded conductor.

I understand that in a 2 or 3 pole circuit with a neutral, the unbalanced load is carried back on the neutral.

Here are my questions:
On a 2 pole or 3 pole circuit that say does not have a neutral or has a balanced load, how do the electrons get back to the source? Do I understand right that they flow through the load 180 degrees, or 90 degrees in 3 pole circuits, out of phase from each other and return to the source through the other hot conductor? And with AC current the electrons are moving in both directions, correct? Back and forth, back and forth, back and forth. ...

Thanks for your help. I feel like I have a bit of a gap in understanding this concept.
For a basic understanding of 240/120V split phase, I think that an analogy with DC and batteries can help.

Imagine two AA batteries with the negative of one connected to the positive of the other, and wires connected at both ends and in the middle. Like this:
*[+1.5V-]*[+1.5V-]*

The asterisks (*) represent wires you can connect loads to.

You can connect a 1.5V load to either outside wire and the middle wire, but the middle wire is positive for one set and negative for the other. You can connect a 3V load only to the outside wires.

If you have two identical (i.e. 'balanced') 1.5V loads connected to each side at the same time, that is the same as a single 3V load, and the middle wire is not necessary. If you understand Ohm's law you can run the formulas for each load separately or both loads together and see that the current is the same in all situations. When both loads are connected, no current flows on the middle wire; you can think of it as canceling out, or just you've turned the loads into a 3V load. But if you want to control the two loads separately then you need the middle wire.

Now to convert the analogy to AC, imagine a little magic elf flipping the batteries around 120times a second. :thumbsup::lol: The important thing to understand is that if your loads are not polarity sensitive, then the mathematical relationship is the same (i.e. mirror image) with the batteries flipped around. This is true even with a real AC sine wave that varies the voltage.

A split-phase transformer is just two windings stuck together exactly the same as the batteries, except that being AC the outside terminals are not designated negative and positive. But imagine them as having one polarity half the time and the other polarity the other half the time, courtesy of the elf. ;) This will help you understand which direction current flows during either half of the AC cycle.

This analogy is not so immediately useful with 3-phase, but if it can help you fully wrap your head around split phase then you're on your way to understanding 3 phase too.
 
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