Electric Utility Neutral Return Wire

Discussion in 'General Electronics Chat' started by twodotmike, Jan 16, 2015.

  1. twodotmike

    Thread Starter New Member

    Jan 16, 2015
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    I have questions based on my understanding of the 3 wire, single phase, home electrical system (USA). My questions are not about the equipment ground (green or bare) wire, although I realize they are bonded together at the utility pole transformer center tap point and the house Mains Panel.

    My understanding (please bear with me):

    Electrical power to the house Mains Panel comes from the secondary of the utility company transformer in the form of 3 wires: 2 phase (hot wires) and 1 neutral return wire (center tapped on the secondary and grounded). Inside the house Mains Panel the 2 separate phase wires are connected to the 2 separate hot buses and the neutral return wire is connected to the Mains Panel neutral bus. At the house Mains Panel this neutral bus is also bonded to the ground bus.

    In the house Mains Panel the branch circuit hot wires are connected to the buses through circuit breakers and the branch circuit return neutral wires are connected to the neutral bus. Within the house Mains Panel branch circuits the current flow alternates from phase to phase for 240v and from phase to phase through the neutral bus for 120v. If the phase to phase current flow is balanced then virtually no current flows back to the transformer on the utility neutral return wire. If there is an unbalanced current flow, then, only this unbalanced current flows back to the transformer.

    My questions:
    1. Does this mean the return current from the house Mains Panel neutral bus to the utility transformer flows only in one direction, from the house Mains Panel to the utility pole transformer, and does not alternate (on the neutral return wire) as the current in the phase wires does?

    2. If it is a one way flow, then, is this unbalanced current returned to ground at the transformer center tap ground point or is it somehow reintroduced into the phase wiring?

    3. If the return current to the utility pole transformer return neutral wire alternates, what keeps it from impeding the current flow of one of the phase wires? If the return current alternates, it seems like it would be "out of phase" with one of the 2 phase wire? If you have made it this far, thank you for your patience.

    Thank you,
    Mike
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Number one: this is a single phase transformer with a centre tapped secondary, there is no 2 phases at work here. (whether the tfmr primary was fed from two phases of a 3 ph tfmr or one phase and a star neutral).
    If the loads are exactly balanced then there is no flow in the neutral conductor, only when an imbalance occurs WRT C.T. is there a normal AC current in the neutral conductor.
    Max.
     
    Last edited: Jan 16, 2015
  3. twodotmike

    Thread Starter New Member

    Jan 16, 2015
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    Thank you, I appreciate your time and your information.

    I understand that this is a single phase system. A poor choice of phrasing on my part, perhaps I should of said "leg to leg" or "leg to leg on same phase"? I have it in my mind how the system works, but I am a little confused on the flow of the unbalanced load on the neutral leg.
    Leg to leg (?) flow on the phase wires makes sense, as one leg is not "opposing" the flow on the other leg. If you will, one leg pushes, the other leg pulls. It seems that if the neutral return current alternated then it would be in opposition to the current flow of one leg.

    Mike
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Since all the currents are AC, then there is no direction to speak about. If L1 is more heavily loaded than L2, then if you clamp an AC ammeter on N, you will read the difference |L1-L2|. If L2 is more heavily loaded than L1, the clamp on ammeter shows |L2-L1|, but since the ammeter knows nothing about the relative phase angle (0 or 180 degrees) of the current in N, then it reads the same absolute magnitude either way. Only if you looked at the voltage in a current shunt with an O'scope would you see the 0 or 180 degree phase angle.
     
  5. twodotmike

    Thread Starter New Member

    Jan 16, 2015
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    MikeML.,

    Thank you very much for your response. I'm glad you took the time to answer my question.
    I did just that this past weekend on my Mains Panel, placed my clamp meter on the utility leads and noticed the different amp readings as I turned on/off items around the house.

    Mike
     
  6. twodotmike

    Thread Starter New Member

    Jan 16, 2015
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    MikeML.,

    Does the "unbalanced" return current, then, once it gets back to the transformer center tap point, flow through the center tap ground wire, through "earth" back to the sub-station? OR...?

    Mike
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There is no current flow to earth ground unless a ground fault exists, your supply transformer centre tap point will be earth ground referenced at the transformer, and re-referenced to earth ground at your service entry point.
    The earth reference is for safety only.
    Any current flow to the centre tap completes the circuit, as far as the tfmr is concerned.
    Max.
     
  8. twodotmike

    Thread Starter New Member

    Jan 16, 2015
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    MaxHeadRoom,

    Thank you. That more or less puts the last piece of "my" puzzle together. I really appreciate your time in staying with me!
    Thank you,
    Mike
     
  9. foolios

    Active Member

    Feb 4, 2009
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    Within the house Mains Panel branch circuits the current flow alternates from phase to phase for 240v and from phase to phase through the neutral bus for 120v.
    It is 240v (hot) to (hot), Yes. It is not phase to phase through neutral; it is phase(hot) to neutral that equals 120v.

    Also, current isn't alternating from phase to phase in the 240v. Current is traveling back and forth throughout the phase. This is a single phase service, not two phases. The single phase, same phase, is phase shifted 180' to give you two hot legs that can be connected together without bucking(with connected appliance added, of course).

    It seems there might be some confusion when you mention the current flow in the neutral wire as if you were considering both scenarios of voltage; both the 120v and 240v.
    In hopes of clarifying and maybe even learning something myself:
    There isn't any imbalance when you are considering the 240v appliances that are using both hot legs. The current is staying within the hot legs, traveling back and forth, and is not going to travel in the neutral.
    The imbalance occurs with your 120v appliances as they are using the neutral return.
    So let's say your neutral disconnects; now there will be an imbalance of resistance with your 120v appliances and lights will be dimmer on circuits on one side of the breaker box(every other breaker) and lights on the other side(every other breaker) will be brighter.
    That's a decent guess that may help. Hopefully someone can chime in and improve my understanding on this as well.
     
    Last edited: Jan 17, 2015
  10. twodotmike

    Thread Starter New Member

    Jan 16, 2015
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    folios,

    Thank you, too, for your input. I believe I understand the 240v/120v setup and that the unbalance, if any, is in the 120v circuits. As I said in an earlier reply to "MaxHeadRoom", it was my poor choice of words to use the phrase "phase to phase". Again, I appreciate your information as it all helps me see the system more clearly.
    MaxHeadRoom's response, mentioning that once the neutral return current gets back to the transformer center tap then that circuit is complete, helped me quite a bit. For some reason I felt that the "unbalanced" current had to go somewhere else.

    Mike
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Regarding the grounded neutral, it is arbitrary which conductor is grounded to make a neutral.
    For this discussion the earth ground can be ignored.
    Max.
     
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