Question about USA electrical system

  1. Hello,
    I live in Europe in house we use 3 phase (230 V every phase, 400 V voltage between phases) and null. I heard that in USA there are two phases (120V phase, 240V between two phases) and I wonder if that is true.
    Do they use 3 phase power transmission to homes or what?
  2. jcsd
  3. Danger

    Danger 9,878
    Gold Member

    I don't know about the US, but you have to pay extra to get 3-phase in Canada. Standard household input is 240VAC 2-phase. That's only used for heavy appliances such as driers. The normal outlets are 115VAC.
  4. Micko, I'm not sure I understand what you are talking about concerning how Europe is wired. I would assume that the voltages you mention are in a wye configuration and not delta? The null being the center of the wye?
    Here in the U.S. it is typical to have SINGLE phase running into your house. It is NOT 2 phase. This is a common misconception. There is a single pair of wires feeding the primary side of the transformer. The secondary side has a center tap which is referred to as neutral. It is bonded to the earth with a ground rod. Each end of the secondary winding is obviously 120 volts to neutral and 240 volts between themselves. 3-phase power costs more to have run to a building. Commercial buildings such as offices usually have 3-phase wye connected transformers and industrial buildings which use heavier machinery usually have delta connected transformers.
  5. Yes, exactly that was what I meant. If I measure voltage in my wall connector (sorry, don't know exact english word) voltmeter shows 230 V and in three phase wall connectors voltage between any two phases is 400 V.
  6. That ratio correspondes to the 208/120 volt wye setup in the U.S.
  7. Astronuc

    Staff: Mentor


    GENIERE 288
    Science Advisor

    Give it a little thought Astronuc. I think you'll find, as averagesupernove stated, the typical residential voltage in the USA is single phase 120/240vac.
  9. NateTG

    NateTG 2,537
    Science Advisor
    Homework Helper

    The feed to the house is typically two-phase 240. However, the outlets are typically wired with one-phase+neutral rather than both phases.
  10. Nope. It is SINGLE phase. There is such a thing as 2-phase power but it is very uncommon. Someone on this forum has mentioned it in previous threads. True 2-phase power has the phases out of phase by an odd angle. Just by using a transformer to invert 180 degrees does not count as 2 phase.
  11. FredGarvin

    FredGarvin 5,087
    Science Advisor

    You guys are a bit confused by the term "phase." The basic input to a house is two legs (two hot wires) of a single phase. You have a double tap on the transformer with the ground in the center. There should be no phase difference between the two AC forms in your electric panel.
  12. It would be really nice if some post picture to see clearly and therefore avoid this confusion...
  13. Technically there is a phase difference of 180 degress between the 2 legs of a residential service. Otherwise, there would not be a voltage between them. It is still single phase though. If there is a single transformer involved and just a pair of wires feeding the primary side then it is single phase. More than 2 wires feeding the transformer(s) is polyphase of some sort.
  14. FredGarvin

    FredGarvin 5,087
    Science Advisor

    I guess that makes sense. I was thinking in different terms, i.e. 120 degree phase shift. The 180 degreee would give you the double voltage. Good catch.
  15. Astronuc

    Staff: Mentor

    Ah, that seems right and along the lines of Fred's comments. I just checked the transformer outside and it has one tie to the distribution line, but it has two hot lines out. So I would it would seem there would be two coils (windings) coming off - and with opposite polarity which puts them 180┬░ out of phase.

    I seem to remember the electrician talking about two phases, but then that is a different reference to the 3-phase distribution system.

    Adding to the confusion I found this:
    This statement from the electric company infers two phases. We have a three wire system. But it appears they mean two hot lines and a neutral, with the hot lines coming from two coils (windings) on the same core as one of the 3 T&D phases.

    Sorry for the confusion.
    Last edited: Jul 3, 2006
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