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  1. #1
    Join Date
    May 2005
    Posts
    1
    I have looked through the board and still am not sure about my situation. Namely how to deal with existing 3 phase power to my current unit.

    I have a 5 ton 15 yo Trane AC with 3 phase AC. I know that is uncommon for residential, but maybe its because this is an older house in Phoenix? I cant tell you the colors of the wires right now but there are three contacts in the disconnect box. I have not put my volt meter on it either.

    I live in Phoenix so the efficiency of a new unit makes sense at the higher SEER ranges. We plan to live here for 10+ more years.

    I have three bids, all with load calcs and two of the firms seem to be excellent contractors. All three recommended the XL19i.

    Of the two remaining contractors, one wants to have a new single phase line run from across the yard (no attic access) which means trenching and alot of labor. The other company feels they can convert the existing 3 phase supply to single phase.

    Has anyone had experience with this? I guess I will ask for my own electrician to look at this as well, but if I can avoid thousands of dollars to run a new single phase line it certainly would be nice.

  2. #2
    Join Date
    Dec 2002
    Location
    Tampa, Florida
    Posts
    706

    if you already...

    if you already have three power, switching to single phase would cost you more money. in three phase power, your unit does not require starting or running capacitors for your compressor, condenser motor may still be single phase but don't sweat the small stuff. what kind of three phase do you have? delta or wye? what is the voltages between L1+L2, L2+L3 & L1+L3. keep in mind the equipment manufacturer may not warranty the unit if it is a single phase unit rigged up to work on 3-phase. the price difference between a 1-phase & 3-phase is roughly the same, can order either one. by all means, forget about switching to single phase. if 3-phase, i would want phase monitor installed as well. have it monitor the front side phase, as well as, the back side phase. get digital monitor. better than a phase monitor is a "Heinemann breaker" . phase monitors interupt low voltage but if the contactor is stuck shut, no way to stop it from running. "Heinemann breaker" will kill the high voltage and save the unit. reason for all this is, power company's power is not always perfect on 3-phase, if one legs drops power, your unit starts to single phase and over-heat or burn out. we install monitors or "Heinemann breaker" on all of our 3-phase equipment. truely equipment savers for 3-phase.

  3. #3
    Join Date
    Nov 2002
    Location
    emerald city, sc
    Posts
    1,469
    i have swapped a single phase for a 3 phase unit on more than once. the single phase unit you are wanting to install is much more efficent than a new three phase. if the wiring that is there now is large enough, then there is know reason to change it. the single phase unit will run fine on 2 legs of the 3 phase system.
    i wanted to put a picture here

  4. #4
    Join Date
    Nov 2004
    Location
    Austin, TX
    Posts
    888
    It is my understanding that 3 phase power is preferred by industry because it is cheaper. No power factor losses translates into lower operating cost. All the start and run capacitors in a single phase system try to emulate 3 phase current. Other than the dropping a phase, what problems do 3 phase systems create?
    “I am for doing good to the poor, but...I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. I observed...that the more public provisions were made for the poor, the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer.”
    ― Benjamin Franklin

  5. #5
    Join Date
    May 2005
    Location
    Grapevine, Texas
    Posts
    82
    If I had 3 phase power to my ac my bill would be cheaper. I would pay the extra money for the 3 phase unit any day. A 3 phase monitor would be cheap insurance in case of a leg of power loss. A 3 phase unit will pull about 1/3 less amps.
    Me:here's your invoice. The total is $$.. Customer:Oh would you check...while your here?

  6. #6
    Join Date
    Apr 2004
    Posts
    7,680
    Then why are the EER's so low?

    Three phase, especiailly in a residential application has a chance of single phasing (blown fuse, or main). As long as the phase that drops is on one leg of the transformer your ok, if not, then...well its an expensive replacment compressor not covered under warranty.

  7. #7
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    Simple change. Leg-leg voltage on a 3 phase 120 volt service is 208 volts. Most residential units are rated 208-240 volt operation. Running on 2 legs for power is a standard practice.

    A competant electrician/installer should have no issue with this.

    FYI for the other guys, 3 phase is of greatest advantage in industry because it provides motors with inherent starting capability. There is no need for any auxiliary starting equipment (like caps or relays) and starting torques of 400% of running are typical.

    paul

  8. #8
    danger is offline Professional Member - bad email t member
    Join Date
    May 2003
    Posts
    26
    Without a doubt, spend the money and replace with a new 3 phase unit.

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