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Thread: converting air compressor 220v motor to 110?

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  1. #1

    Default converting air compressor 220v motor to 110?

    Just moved to a new place...moved my fairly large Sears upright 60 gal air compressor to the new garage and just noticed that there is no 220 in the garage, only 110 outlets. Checked my panel for the house and it is slam full, no extra slots for a 220 breaker and an additional line to the garage.

    Looking for advice on my options:
    1) Replace or re-wire the motor? I wonder at the cost of replacing the motor with a 110 v motor. Should it be three-phase? I need to look at the info on the siide fo the motor more closely. The current motor is 220v, single phase. I think the compresor label says it is a 7 hp? I had it wired in my previous home detached shop which had a sub panel with plenty of extra slots for a 220 breaker

    2) Couldn't I get a step up transformer to plug into an outlet in the garage? Looking at a 1000 watt tranformer was only like $60 online.

    This compressor is not used for commercial duty, just home/hobby use.

    Any advice would be appreciated!

  2. #2
    DIY scratch-pad engineer leejosepho's Avatar
    Join Date
    Nov 2006
    200 miles south of Little Rock


    Depending a bit upon the distance and any obstacles involved, I would get an electrician to install a disconnect next to your meter base and run 240 to the compressor. I do not recall ever seeing a 7hp motor run on 120, and I would doubt any of your existing circuits could handle the demand placed upon them by any kind of transformer driving your compressor.

  3. #3
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona


    If you do not have 220/240, then you do not have 3 phase available at the motor, and it would be a rare house that has 3 phase available anywhere in i, (although we do have many older areas here that do have it). Three hp is about the largest 120v motor you will find. A 7 hp would require at least 40 amps and that would tax the average 120 circuit. A step up transformer does NOT change the amperage requirements and the 1000 watt transformer would be completely inadequate for it. Your motor should require about 20 amps and that means at least a 5,000 watt transformer.
    Last edited by hj; 07-06-2010 at 06:43 AM.

  4. #4
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    Check the label plate on the motor. It would not be unusual for that type of equipment to be dual rated for 110/220. Basically the motor is set up with two windings, and to run 220 they connect in series, and to run 112 they connect in parallel. The amp draw will be double on 110.

    If the compressor is a good unit, it might be worth replacing the motor if necessary, but you have to find out what your whole unit would cost new, vs the fairly high cost of a new motor. Remember, don't buy anything electrical made in China. Motors and such is something they still don't do well.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Sears likes to rate things at peak, not constant. It is highly unlikely that it is really a 7hp motor. Power = current * voltage. Or, current = power/voltage. 1 hp = 745w. So, 7*745=5220W. 5220 w (power)/220 (volts) = 23.7 A (current). You'd double that to 47.4A if you ran it at 110vac. That would take some really large wire. As I said, I doubt it is really a 7hp motor. 15A circuit at 110 vac = 1650w and at 120vac = 1800w, so a 2hp motor would work on a 15A breaker with little else on it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    Electrical Contractor/Instructor jwelectric's Avatar
    Join Date
    Jun 2007
    North Carolina


    Let’s look at this from the front to the back.
    First the 7hp is the rating of the compressor not the electric motor driving the compressor.
    Second it is very unlikely that you have or can obtain three phase power at your home.
    Third if the electric motor was 7 hp at 120 volts then we would use 80 amps to start the math. The conductor that supplied the motor would be required to be 80 times 125% or a #3 copper conductor.
    Forth the breaker would be a 200 amp breaker at 120 volts.
    At 240 volts the conductor would be #10 and the breaker would be a 60 amp

    The above information is more than enough to figure out that the electric motor is not a 7 hp.

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