Explain: neutral wire in split-phase system

Discussion in 'General Electronics Chat' started by jasonosaj, Dec 30, 2009.

  1. jasonosaj

    Thread Starter New Member

    Dec 23, 2009
    2
    0
    Hello,
    I am having trouble understanding the neutral wire in a split-phase system. In the All About Circuits E-book chapter about single-phase power systems (http://www.allaboutcircuits.com/vol_2/chpt_10/1.html), it states "...the neutral wire only has to carry the difference of current between the two loads back to the source." I have attached a diagram from the E-book, which I have marked up to illustrate my question. Following the red path, which goes from the source through load #2, I would assume that the electricity would take the path of least resistance, and travel along the neutral wire back to the source, as opposed to traveling through load #1. However, it appears that the electricity takes the harder path through load #1, and I am confused as to why.
    Thanks.[​IMG]
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,535
  3. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    I'm not sure I understand your question, but the current does follow the red path as drawn; it does not return via the other source's hot path.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,535
    If both loads are equal, the voltage between the loads is 0V. This is not the normal state of affairs though, usually there is only one load, and neutral is not used.
     
  5. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    As I understand it, Bill, this is true for 240 v loads only. Otherwise they use the neutral to ground path.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,535
    In most breaker box situations, there are three wires coming from the breaker box. You generally treat each 120VAC line as separate, with it's neutral carrying full current.

    Overall, it might balance from the pole, but this is highly unlikely, and doesn't really apply for the outlets in the house.

    Houses use 240VAC all the time, for dryers and air conditioners.

    Personally I feel this illustration is a bit off, for the reasons I stated above. I've put a comment in the feedback forum.
     
  7. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    Jason, the only way to develop 120 volts is to use a neutral which is attached to ground. The 240 volt derived current travels from hot leg to hot leg, but the 120 volt derived current goes from hot leg to ground via the neutral conductor.

    This happens because outside your house there is a transformer on a pole with 240 volts and a center tap. The center tap is connected to the ground (real earth ground) right there at the transformer. This creates 120 volts. Then the two hots (each 120 volts) are sent to your house and connect to your service panel. The service panel is itself connected to ground and all neutrals are connected to it.

    The reason the neutral current subtracts from each leg (hot) is because they are 180 degrees out of phase. I hope this clears up your question.
     
Loading...