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Two Phase - Three Phase - Whats the scoop?

  1. Nov 23, 2004 #1
    I am a little embarassed asking this, as I was an aircraft electrician at one time, but have since forgotten the answer. :bugeye:

    When dealing with house electricity, you commonly hear the terms two phase and three phase. Now I understand this to be that the 'waves' are 'out of phase' by a certain degree.

    But why do we have them? Why multiple phases? Simple so we can tie two together to get 220V for certain appliances? Is it simply a matter of convience that we can bring in more on the same line?
  2. jcsd
  3. Nov 23, 2004 #2


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    There really is no such thing as two phase. Have a look at the earthing alternatives thread and you will find some good info.
  4. Nov 24, 2004 #3
    This is actually the info I was looking for....

    http://ths.gardenweb.com/forums/load/wiring/msg112302526261.html [Broken]
    Last edited by a moderator: May 1, 2017
  5. Nov 25, 2004 #4


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    I know he was corrected, but in that thread one guy is bleating on about 2 phase systems. Ignore him!
  6. Nov 25, 2004 #5
    Last edited: Nov 25, 2004
  7. Nov 28, 2004 #6
    There IS such a thing as two-phase AC. It's used with servo motors, where the phase angle determines position. But it's not used for AC power.
  8. Nov 28, 2004 #7

    Ivan Seeking

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  9. Dec 1, 2004 #8


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    Domestic or house current in US is single phase (line-to-neutral) 120 V, 60 Hz.

    IIRC, 240 volt comes from 2 line to neutral phases.

    T&D systems use 3 phase, each 120° out of phase to the others. This also helps to reduce inductive line losses.
  10. Dec 2, 2004 #9
    i was the one that started the "Earthing alternatives" thread and i now realise that what peope are referring to as 3 phase and 2 phase (which i was referring to myself), is the misuse of a term as i understood from Averagesupernova. What they are referring to as the different phases is just the difference in the voltage (and current) but as i learnt it is much more than that.
  11. Dec 3, 2004 #10
    >Domestic or house current in US is single phase (line-to-neutral) 120 V, 60 Hz.
    >IIRC, 240 volt comes from 2 line to neutral phases.

    Actually domestic 240 volt is single phase with a center tap typically grounded and identified as neutral.
    The confusion stems from the assumption that 3 live lines equals 3 phases, so 2 live lines are 2 phases. Not so.
    If we label the 3 lines A, B, and C, we create the phase relationships of AB, AC, and BC. If one fuse blows, say line B, we lose AB *and* BC, so we're down to single phase of AC only.
    Last edited: Dec 3, 2004
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