Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A question about AC power

  1. Aug 16, 2010 #1
    Regarding this topic,


    a question sprung into my mind.

    How many conducting cables go to my home circuit to deliver the power from the grid?

    If heard these electrician's jargon words:

    1) phase (up to 3)
    2) null
    3) ground (earth)

    Does anyone care to comment?
    Last edited by a moderator: Apr 25, 2017
  2. jcsd
  3. Aug 16, 2010 #2


    User Avatar

    Staff: Mentor

  4. Aug 17, 2010 #3
    I suppose it depends where you live.

    The minimum is 2 wires - (single) phase and neutral. You have to provide your own earth. This situation tends to occur in isolated / rural place.

    It should be noted that in Europe at least the Electricity Supplier is under no obligation to provide an earth, although they usually do. Usually the mechanical outer protective metal sheath of the incoming cable is used for this purpose, not a separate wire within the cable.

    The above situation actually also covers the case stated by Russ since the power comes from the grid as a single phase (2 wires) and is split at a local transformer, which may be yours.
    The centre of the transformer is then earthed.
    The correct term for this mode of supply is not 2 phase but split phase.

    If you live in a large block of flats, the block may receive a 4 wire, 3 phase supply. That is 3 phase wires plus neutral again perhaps plus an earth connection via a metal sheath.

    Individual flats may receive 1, 2 or all three of these phases. In Europe certain high power fixed appliances such as water heaters are powered by the voltage between two such phases, rather than the voltage between any single phase and neutral. This is because the interphase voltage is higher (415 volts) than the phase to neutral (230 volts).
  5. Aug 17, 2010 #4
    So, if I understood the article about split phase correctly, the effective voltage between the two phases is actually 240 V in the US!
  6. Aug 17, 2010 #5
    Correct. They are 2 of the 3 phases provided by the substations. The typical delta configuration from a generator doesn't have a ground, merely 3 wires, each hot and 120 degrees out of phase. The Y configuration allows for different neighborhoods to use different sets of 2 phases (at 240vac) and a ground so as to evenly distribute the load. Some older places might still be using the delta configuration and only 2 wires (stepped down to 120vac) which produces the same results but is more susceptible to single phase imbalanced loading and ground currents. Isolation has to be provided and ensured from the source so as to not fry the customers.
  7. Aug 17, 2010 #6
    It may well be that some parts of the US has 240 volts generated between 2 phases 120 degrees apart. I am no expert on US custom and practice.

    If this is the case then they cannot be 120 volt ac supply phases.

    However I understand the general 240 supply in the US to be the result of two phases that are 180 out of phase, not 120.

    post #7 here says it all

  8. Aug 17, 2010 #7
    I think what James was trying to say is that even in US, the power plants would produce 3-phase current (each 120o out of phase from the other two) with no neutral wire. This is transmitted by power lines to local distribution points. At those points, depending on the power consumption, you might be given either 3 or 2 of the power lines.

    If you are given 2 lines, this is single phase. Then, you have a transformer of the split-phase type: there is a neutral wire in the middle of the secondary of the transformer. The (effective) voltage between each end of the secondary and the middle is 120 V, but the ends are at a phase difference of 180o, so the voltage between them is 120 - (-120) = 240 V. You are still connected between two phases of the grid though.
  9. Aug 17, 2010 #8
    I know what James was trying - and did - say.

    Unfortunately he was in error, which you have continued.

    The relationship between phase to neutral and interphase voltage, in a three phase 120degree system, is

    Vinterphase = [tex]\sqrt{3}[/tex]Vline

    not, as you claim 2Vline.

    For the interphase voltage between two supplies from a three phase star connection to be 240, the phase or line voltage has to be 141 volts.
  10. Aug 17, 2010 #9
    You forget the transformer. We are simply saying that at the secondary end of the transformer, the (effective) voltage between each end and the middle is 120 V. I don't know what is the interphase voltage on the primary.
  11. Aug 17, 2010 #10
    Extract from my post#3

    I stand by everything I've said. I even linked (post#6) to an earnest discussion on this selfsame subject by a bunch of american electrical engineers.

    It was also said that the phases are 120 degrees apart.

    They are not they are 180 degrees apart, otherwise it is impossible for the voltages to add to twice their individual value.
  12. Aug 17, 2010 #11
    This might be true, but irrelevant to what me and James are saying.

    Look at this setup:

    This is what I was trying to say.

    Attached Files:

  13. Aug 17, 2010 #12
    You have just described exactly one of the situations I outlined back in post#3, so we are in now agreement.

    But I just checked post#5 and I couldn't find a single reference to a transformer.
  14. Aug 18, 2010 #13
    Sorry for the confusion guys. There are 2 methods that are common in the US to get to a residency. If it is a low occupancy home, it is probably fed by a distribution split-phase transformer as Dickfore showed;
    But if you are in a larger apartment complex having anywhere from 50 to 500 apartments (also used in commercial buildings), things are done a little differently as they use a delta-wye transformer to provide the variety of power voltage needs as shown here - http://en.wikipedia.org/wiki/Delta-wye_transformer" [Broken].

    It just depends on where you are in the world.
    Last edited by a moderator: May 4, 2017
  15. Aug 28, 2010 #14


    User Avatar
    Gold Member

  16. Aug 28, 2010 #15


    User Avatar
    Gold Member

    Yes, but 240 is usually found on only one or two circuits - dryer, water heater, maybe your electric car plug in few years. The majority of the circuits in the house run off one phase and 'ground' from that center tap, i.e. 120 VAC.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook