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Thread: 3 phase vs. 2 phase?

  1. #1
    DIY Member SD44's Avatar
    Join Date
    Feb 2008

    Default 3 phase vs. 2 phase?

    Guys, my dad is sort of a jack of all trades, and alot of times I just have to take his word on things because I'm not, especially with electricity.

    I live in a rural area about 10 miles out of town. We obviously have 2 phase at our house. We just moved here, and I have a shop with no electricity. I have a couple old cars that I am going to restore, and my dad is going to help run electricity to it. He came over yesterday and said he noticed out on the highway that there is 3 phase on the poles (There is a factory down the road). He said he has a friend at the power company, and is going to speak to him about possibly getting my house transformer changed over to 3 phase, so I'll have 3 phase in my house and out at my shop. He has a huge compressor that he is going to give me to do paint/bodywork with, and he said it needs 3 phase. He says it will cut my lightbill almost in half. I'm about to get a new central unit, and he told me to wait on that until we know if we can get the 3 phase.

    Is he right? If I could get it changed, would I be better with 3 phase?

  2. #2
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Aug 2005
    Columbus, OH


    Generally residential electric supply is not 3-phase, or 2-phase for that matter. . 2-phase does exist, but it's rate. You probably have single phase service with a center tap to give you 240/120.

    You're probably going to pay a lot to bring in three phase service--cost in the thousands. What is the voltage of that compressor? If it's 480V then you'll probably have to add the 480/240 3p to your current service. If it's 240V then you may be able to get by with 240V 3p and center tap one transformer for your lighting load and not have to change too much, maybe add a new panel.

    Your lighting bill will most likely be the same unless your rate changes.

    I would look into using a phase converter or changing out that motor in the compressor to a 240V single phase motor.


  3. #3
    In the Trades Bob NH's Avatar
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    Oct 2005
    New Hampshire


    Is your dad going to rewire your service and install a 3-phase breaker panel? If you have an electrtic water heater, range, and oven that operate now on 240 Volts, they are probably going to operate on 208 Volts with the 3-phase Wye connection. That will result in 3/4 of the power for resistance heating elements.

    It is possible to get 240 3-phase and 120 Volt single phase in a 3-phase setup but some utilities discourage it. It is usually used where the 120 Volt loads are small compared to pure 240 Volt single phase and 3 phase loads.

  4. #4
    Electrician Chris75's Avatar
    Join Date
    Aug 2007
    Litchfield, CT


    Quote Originally Posted by SD44 View Post
    Guys, my dad is sort of a jack of all trades, and alot of times I just have to take his word on things because I'm not, especially with electricity.

    Tell your Dad to stay out of the electricity part of the trade...

  5. #5


    You will not lower your wattage used with 3 phase. It takes a certain amount of power to pump air or water or run lights and as long as the motors, etc. are in equally good condition they could be 120 or 240 or 208 or 480 and they will use the same amount of power. As mentioned above, only if you can get a special rate on the 3 phase will it reduce your bill.

  6. #6


    Tell your Dad to keep his day job

    The cost of three phase equipment far outweighs the miniscule "savings".

    We convert old resi 3 phase to single phase here all the time. Single phase for residential electrical is the way to go.

  7. #7
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default 3 phase

    There are advantages to 3 phase in certain situations, but a lower electric bill is not one of them. In your case, the only reason to get 3 phase would be to run that compressor, and since the electric bill might have a minimum usage under 3 phase your bill could increase.

  8. #8


    Electric motors which run on 3 phase will run more efficiently than a single phase electric motor. And that is about it so far as an advantage for your house...


  9. #9
    General Contractor, Farmer HandyAndy's Avatar
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    Apr 2007
    Haxtun, CO


    I have what is called delta three phase, and it is 240 with one leg of it center tapped for single phase loads, it works nice in the shop and for what I need,
    (actually mine is a open delta three phase, and only uses two single phase transformers).

    but I would not go out looking for it unless you really need it, the boxes are very expensive, the good thing is if your a large motor user many times one can get surplus motors for a good price,
    it would probly be cheaper to either use a rotary phase converter for the air compressor (with a time delay) for running the compressor, and any other loads you may have that are three phase unless there need continuisly,

    (phase converters, on compressors and refrigeration one needs a rotor converter , any thing that is going to run at rated amps)

    one can get by with a static converter, for loads that do not pull max amps,
    like lathes, mills, saws, Down rate the motor power by 1/3, as what is happening is your using capacitors to shift the phase to start the motor, and then it runs on single phase,

    a rotary converter is a static converter with a rotary generator, (a 3ph motor) that actually generates the missing third phase,

    but on heavy loads the rotary converter needs to up and running for 5 to 15 seconds before the loaded motor kicks in,

    it may be worth it depending on the type of shop you want, but check into all the costs first,

  10. #10
    Engineer Furd's Avatar
    Join Date
    Jul 2007
    Wet side of Washington State


    phase converters, on compressors and refrigeration one needs a rotor converter, any thing that is going to run at rated amps
    Not necessarily. I have made several static phase converters that ran air compressors and they worked just fine. It IS possible to get rated horsepower using a static converter as long as you properly connect the motor. You need to use only wye (star) connected dual voltage motors and it is necessary to internally break the "star point" and add three more leads so that the motor now has 12 leads instead of nine.

  11. #11
    General Contractor, Farmer HandyAndy's Avatar
    Join Date
    Apr 2007
    Haxtun, CO


    I am not saying there may not be other ways to do single phase to three phase,

    but when using most manufactures equipment, the majority will recommend rotary converters over static, on hard starting equipment.


    with the following motor and non-motor loads:

    motor loads:
    - 2 or 3 speed motors
    - lathes (non-clutched)
    - pumps
    - CNC
    - fans
    - refrigeration equipment
    - unattended equipment
    - welders
    - air compressors
    - frequent-start loads
    (more - than 6x's/hr)
    - hard-starting applications
    - multiple motor applications
    - equipment run at more
    than 2/3 rated horsepower non-motor loads:
    - ovens
    - heaters
    - lasers
    - battery chargers
    - resistive or
    - rectifier loads
    Please consider a rotary phase converter
    for these applications.
    http://www.phase-a-matic.com/StaticA...ionNotes.htm#1. WHAT TYPE OF CONVERTER

    The most important question is whether or not the motor is heavily loaded. You must keep in mind the fact that the motor will produce approximately 2/3 of its rated horsepower when running on the static converter. If the motor is loaded beyond that point, you must use our Rotary Converter, or use method number 2 as explained in the Installation Instructions. Otherwise, you must unload the motor by reducing the motor pulley diameter by 1/3, or increase the motor horsepower size by 50% to accommodate the loss of power.

    For further information or verification of proper sizing, contact Phase-A-Matic, Inc.

    NOTE: For the following outlined applications you MUST use a Rotary Converter:

    Battery Charger Plasma Cutter Some CNC Equipment
    EDM Machine Radar Transmitter (Radio & Television)
    Heating Elements Rectifier Variable Frequency Drive
    Laser Rheostat Welder


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