3 phase 3 wire help??

Discussion in 'General Electronics Chat' started by electicamateur, Jul 29, 2013.

  1. electicamateur

    Thread Starter New Member

    Jul 29, 2013
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    Ok, first off be gentle. I will be the first to say I really don't understand 3 phase systems. My question is....Can I pull off two legs of a 3 phase 3 wire system to have a single phase service?? I have a 100A irrigation pump panel that is 3 phase 240v with only 3 wires from my meter to the 100A disconnect in the pump panel. All voltage measured between any two phases is 240+ volts. Voltage measured between phases A & C to ground measure 120+ Volts and between phase B & ground is 208+volts. My first thought is I can come off phases A & C to a single phase breaker panel, but I don't know what to do since there is not a neutral wire in the system??? Any Help would be much appreciated. Thanks.
     
  2. donpetru

    Active Member

    Nov 14, 2008
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    Single phase service = Voltage between any phase and neutral.

    Phases are usually symbolized by R, S, T. In addition there are three phase and neutral conductor. For example, if the voltage between phases R and S is equal to 240V, the voltage between R-null (R-0), ie between S-neutral voltage is equal to:
    V_R-0 = V_S-0 = V_R-S / 1.73 = 138V.

    There are three-phase systems that do not have the neutral conductor. For example, these systems supplying consumers or loads that the three windings connected in delta.
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    What sort of loads do you want to run off the single phase service you want to implement? Perhaps a transformer across two phases with the right turns ratio might do the job. You may need to drive an earth stake at the remote point. This is a job for a qualified electrician.
     
  4. electicamateur

    Thread Starter New Member

    Jul 29, 2013
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    I would like to have 100a or less single phase panel. Would be nice to be able to plug in an RV or whatever. A couple 20 amp outlets would work.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    To use a single phase voltage to ground requires a neutral wire. If you can't run the neutral wire then a transformer across one of the phases to convert the voltage to 110V would work.
     
  6. electicamateur

    Thread Starter New Member

    Jul 29, 2013
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    Obviously I can't just pull off of phase A or phase C even though voltage ( to ground) on those phases is already 120ish. The transformer must provide me with a neutral somehow???
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The transformer simply provides a secondary floating voltage. Depending on your local code requirements you would probably need to earth one side of the secondary via an earth stake. A single phase 100A supply is significant. What do you need to run? A transformer of that capacity won't be cheap. Plus you will have additional installation labor & hardware costs. Might be easier to run a neutral to the site from your main board.

    You aren't planning to do the work yourself are you?
     
  8. alim

    Senior Member

    Dec 27, 2005
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    1
    Hi not want to put a damp squib on your search for help, but a couple of statements you have written suggest you are ill equipped to accomplish what you desire.Firstly what you have is called a high-leg delta, secondly you do have a neutral -that is how you got 120 volts BETWEEN A to GROUND or BETWEEN C to GROUND. From B to GROUND IS 208 VOLTS that is the high leg. Your question if you can pull two legs to have a single phase suggest a deficiency in knowledge I would humbly suggest you get a qualified electrician to carry out the job and you position yourself to learn from him. 100 amps is a lot of current, if you were to do things incorrectly you can cause great harm to yourself and those around.
     
  9. electicamateur

    Thread Starter New Member

    Jul 29, 2013
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    I'm confused how you say I have a neutral when I only have three leads from the meter?
     
  10. donpetru

    Active Member

    Nov 14, 2008
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    If you don't have neutral wire, it can be created by at least two methods:
    1) using a three-phase transformer separation, adequate power, with delta connected primary windings and secondary windings connected in star (it is a rather expensive option especially if the required power is high);
    2) the neutral conductor can be drawn from earth connection (if you are nearby, if not, will have to undertake). It is a cheaper solution than the first but the price difference is small if you need to design and build ground socket.

    Another question: you want to have available 100 amp single phase? If you use electrical outlets 16 or 20 amps, then you need to use multiple outlets, and their protection circuits to be made ​​more breakers.

    LATER EDIT: Some specialty acts, in some countries, do not allow a single phase load more than 6kW (230VAC). For higher power used only three phase consumers (or loads).
     
    Last edited: Jul 30, 2013
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I'm 100% with alim on this..

    You should stop immediately and hire a qualified electrician to come wire your system properly. You are NOT qualified to perform the modifications safely.
    This thread IMO should be closed immediately.
     
  12. donpetru

    Active Member

    Nov 14, 2008
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    You are right. It is the best option for him.

    electicamateur, I would have helped you, because I'm an qualified electrician (maximum working voltage 400kV and any value of installed power), but unfortunately, you do not live in Romania.
     
  13. alim

    Senior Member

    Dec 27, 2005
    113
    1
    Hi coil A-C is centre tapped for you to get 120 volts to ground, and that is what is referred to as the neutral.
     
  14. electicamateur

    Thread Starter New Member

    Jul 29, 2013
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    Let me first say...thanks for the knowledge. I will take the advice to hire an electrican. Mcgyver I will NOT take your advice to STOP because all I started was a conversation.
     
  15. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    obviously by stop I meant to stop attempting the rewiring without a qualified electrician which you have decided to do...
     
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