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Thread: Help understanding neutral conductor.

  1. #1
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    Help understanding neutral conductor.

    Hello first I would like to say that I am not an electrician. I'm in my first year of college, I am going to be a maintenance technician. I just had my first class in "basic electricity." It was a very basic class just on the history of electricity. There will be several more electrical classes to come but I have a question about something that we sorta just skipped over. I always thought that the white (neutral) wire in a 2-wire circuit always carried the unused current back to the transformer and from there back to the powerstation. After seeing a schematic in my book, it looks like the neutral (unbalanced) wire goes back to the transformer on the pole, taps in the middle of the secondary side and then down to the ground. As you can see I'm clueless on how this works. Does the unused current from the neutral just get asborbed back into the transformer through inductance or does it travel down into the ground? It was stated in class that electricity will not flow unless it can get back to the place it started. I understand how AC alternates back and forth, could the unused current flow back that way. Sorry for the long post, I'm sure this question would probably be answered in one of my upcoming classes but its bugging me. Thanks Dennis

  2. #2
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    I would call that a very astute question. The answer will become more clear as you study further. But here are a few tidbits that will help.

    First, there is no “unused current.” Current flows in a complete path, and all the current that starts down that path will make its way back to the beginning of the path. The thing that is “left behind,” the thing that is consumed by the load, is energy, not current.

    Secondly, in the arrangement you described, the “complete path” starts at one end of the transformer, goes along the “hot” (technically, the “ungrounded”) wire to the load, goes through the load, goes along the “neutral” (technically, the “grounded”) wire back to the center of the transformer, goes through the transformer, and then starts heading back towards the load again. During one “half cycle,” the time that the current is “positive,” current will travel in this circular path about a bazillion times. During the other half cycle, when the current is negative, the current travels in the opposite direction along the same path, and completes the circuit another bazillion times.

    Next, the connection of the center point of the transformer to planet Earth has nothing to do with the path for current flow. There is some disagreement as to its purpose and value. But since your question was about current flow, I’ll leave that topic for another time.

    Finally, welcome to the forum, and best of fortune with your career.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  3. #3
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    Quote Originally Posted by Dennisc
    I always thought that the white (neutral) wire in a 2-wire circuit always carried the unused current back to the transformer and from there back to the powerstation.
    There is no unused current. Unless you have an insulation problem, all the current flows back to the source.
    After seeing a schematic in my book, it looks like the neutral (unbalanced) wire goes back to the transformer on the pole, taps in the middle of the secondary side and then down to the ground.
    The current does flow back to the transformer thru the customers electrical system. The conductor you see going to the ground is the utilities primary system ground. They install these on all transformer poles and on other poles
    along the utility system route.
    Does the unused current from the neutral just get absorbed back into the transformer through inductance or does it travel down into the ground? It was stated in class that electricity will not flow unless it can get back to the place it started. Thanks Dennis
    As I said there is no unused current. It all returns to the source thru the customers wiring and the utilities wiring. You need more instruction before we
    can go into a more detailed discussion.

  4. #4
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    Dennis, here's a really basic lesson, and we'll start with direct current, because the fact that we use alternating current is not relevant to the basics:

    Picture a battery, like you'd find two of in a flashlight. You have a terminal at each end, with a difference in potential of 1.5 volts between them. Right now, no current flows, because the air is not conductive enough for electrons to flow through it.

    Now, we take a light bulb, rated to work at 1.5 volts, and connect it to the battery with a couple of pieces of wire. Now current flows from one battery terminal, through one piece of wire, through the bulb filament, through the other wire, and back to the battery.

    Let's now add another piece of wire to one terminal of the battery, and connect the other end of this wire to a rod driven into the earth. No current will flow through this second wire, because there is no connection from the earth to the other battery terminal.

    Now, we have two conductors; one of them happens to be grounded. We can call one conductor 'grounded' and the other 'ungrounded'. The only difference is that a voltmeter with one lead grounded will read zero volts on one wire and 1.5 volts on the other.

    We can say that one wire, the one not grounded, is 'hot', and we could (improperly) call the grounded one the 'neutral'. Improperly because it doesn't qualify under the definition of 'neutral', so we'll call it the 'grounded' condcutor instead. With me so far?


    Okay, now let's take two batteries, stacked like they would be in a flashlight, with the tip of one contacting the base of the other. They are said to be 'in series'. Let's take a piece of metal and insert it between the batteries; we'll call this the 'center tap'.

    If we test with a voltmeter from the top of the upper battery to the bottom of the lower battery, we would measure 3 volts. If we test from the center tap to either end, we would measure 1.5 volts. Is this starting to sound familiar? Stay with me.

    Now, let's take a piece of wire, and connect it to the center tap, and connect the other end of it to our ground rod. Again, no current will flow, because there is no conductive pathway from any other part of our circuit to the earth; only one point is grounded.

    Let's take two 1.5-volt bulbs, and connect one from the top of the upper battery to the center tap, and the other from the bottom of the lower battery to the center tap. Each bulb receives 1.5 volts from its battery, and each can be turned on and off independently.


    Okay, now we have a conductor, connected to the metallic center tap, that is shared by both of the bulbs. This conductor does indeed qualify as being called a 'neutral'. It also happens to be grounded, but that has no bearing or effect on our circuit.

    We could also add a 3-volt bulb, connecting it from the top of the upper battery to the bottom of the lower battery. It can operate independently from the other two bulbs. In fact, we could add a plethora of both 1.5-volt and 3-volt bulbs to our batteries.

    If we happen to add matching wattages of 1.5-volt bulbs to the two 1.5-volt halves of our 3-volt system, no current will flow through the wire to the center tap. However, if we have an imbalanced set of loads, the difference current will flow on the neutral.

    For example, let's say we have 2 amps flowing through the upper wire, and three amps flowing on the lower wire. That's a difference of one amp, and that will flow on the neutral conductor to the center tap. However, nothing will flow into the earth.


    Now, let's see if we can induce some current to flow into the earth. We already have one wire, the neutral, grounded via the ground rod. The only way we can get current to flow through the rod's wire is to ground one of the other battery terminals.

    In other words, we would have to connect two different points of the circuit, with a voltage difference (aka potential) between them, to two different points of the earth. We call this a 'ground fault', meaning an accidental grounding of a conductor.

    The current from the accidental grounding will only attempt to flow toward the intentionally-grounded point of the circuit, where the neutral is grounded, and no farther. The current will flow through any availabe pathway between potential differences.


    Okay, now let's convert this 3-volt battery supply into a utility transformer that delivers 240 volts between the two ungrounded conductor, and 120 volts between either ungrounded conductor and the neutral, which also happens to be grounded.

    There is one main difference: unlike the direct current (DC) from one or more batteries, the utility delivers alternating current (AC), which simply means that the polarity swaps back and forth. The main reason for this is that transformers require AC.

    As I said earlier, the differences between AC and DC are not relevant to this discussion, except that the transformer's secondary winding receives the energy to produce the 240 volts from the electricity delivered to the primary.

    Any current from a ground fault on the secondary side will only attempt to flow toward a point with a potential difference, which would be the grounded neutral conductor, and no farther. The transformer isolates the secondary from the rest of the world.

    The only time current in the secondary system might attempt to flow farher upstream, towards the sub-station supplying the primary system, is in the case of a primary-to-secondary fault within or outside the transformer enclosure.

    So, that one conductor of any system is grounded has no effect on the normal flow of current. In a manner of speaking, it makes an accidental contact with a hot wire more dangerous, but it also limits the secondary voltage in case of a primary-to-secondary fault.


    I hope this helps in your understanding, and doesn't add to your confusion.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  5. #5
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    Quote Originally Posted by charlie b
    ...During one “half cycle,” the time that the current is “positive,” current will travel in this circular path about a bazillion times. During the other half cycle, when the current is negative, the current travels in the opposite direction along the same path, and completes the circuit another bazillion times.
    That's a popular misconception, Charlie. Actually, current is much, much, much slower.

    A recent article I read put it in these terms: In a copper wire having a cross section of 0.5 mm² carrying a current of 5 A, the drift velocity¹ of the electrons is of the order of a millimetre per second.

    While current may be slow, its effect is not. As a matter of fact, it is [nearly] instantaneous because electron drift occurs at all cross sections along the circuit path [nearly] simultaneously.

    ¹Drift Velocity is the average velocity that an electron attains due to an electric field.

  6. #6
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    Thanks alot guys. You all were very helpful. I've been trying to understand all of this useful information. I'm sure I'll pick it up with my future classes in electricity. I was the usual do-it-yourselfer that thought there was nothing to electricity, but after each class I attend I see the dumber and dumber I am. I had no clue there was so much to learn about electricity. Thanks for the help. Dennis

  7. #7
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    Quote Originally Posted by Smart $
    That's a popular misconception, Charlie. Actually, current is much, much, much slower.

    I am very well aware of the notion of "drift velocity." I am also very well aware that this is a "fact" that has no real value in our business. It came into play in my graduate course called "electron devices" (and no I did not misspell that word, it was not "electronic"). But it plays no role in our day to day work. So I prefer to disregard it utterly, and hope that nobody notices. I prefer to think of electrical current in terms of single electrons traveling through the wire at nearly the speed of light, and making the circle from source to load to source many many times per second. It is a lie, but it is useful lie. :smile:

    We don't need no stinkin' drift velocity! :mad:
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  8. #8
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    Quote Originally Posted by charlie b
    I prefer to think of electrical current in terms of single electrons traveling through the wire at nearly the speed of light, and making the circle from source to load to source many many times per second.
    After knowing the fact(s), what you choose to think is your prerogative...

    It is a lie, but it is useful lie. :smile:
    ... but knowingly propagating a lie as the truth is a different matter.

  9. #9
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    Quote Originally Posted by Smart $
    ... but knowingly propagating a lie as the truth is a different matter.

    I am not propagating a lie, as though it were truth. I am acknowleging the lie, and declaring it to be irrelevant.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  10. #10
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    Quote Originally Posted by charlie b
    I am not propagating a lie, as though it were truth.
    I was referring to your first post in this thread...

    I am acknowleging the lie, and declaring it to be irrelevant.
    ...not your second and third.

    Any elaboration beyond that is outside the scope of this forum.

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