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Thread: converting air compressor 220v motor to 110?

  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
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    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
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    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
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    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
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    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
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    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.

  7. #7

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    thanks for all the great replies ! If the motor could be converted to 110v, is that something I (average DIY) could do? And if it could be done, would the normal 110 circuit to the garage (15 or 20 amp breaker) be sufficient to run the compressor intermittently (homeowner/hobby) use?

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    The manual that came with the thing (often available online if you don't have it) will show you how to convert it, if it can be converted. It will also tell you everything you've asked, including how to do it. If possible, it's usually just remove a cover, move a jumper wire(s), install a proper plug and wire, and plug it in. It would also tell you how big of a circuit it needs. it might need a 20A, but the manual would tell you. If it does require 20A, you'd need a 20A plug on the power cord.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    DIY Senior Member CarlH's Avatar
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    I doubt that this is a unit that allow conversion to 120V. Like Jim said, check the owners manual. Otherwise check the motor nameplate. If it is rated for 120 volts, it would have a voltage rating of 120/240V on the nameplate (if so, check the owners manual on how to convert). I really doubt 7HP motor is going to have a 120V option. Forget about a step up transformer since one to fit your needs will be very large and expensive plus some other issue to worry about. While your motor is most likely rated at 7HP by NEMA standards, the load of the compressor is not demanding that much under normal conditions. The compressor likely requires a motor that size just to get things started (like when you first turn it on).

    Your options are:
    1. Hire an electrician to install a 240 line.
    2. Replace the compressor with one that is rated to run at 120V.
    3. If you have a pulley driven unit, Replace the motor with one that is designed to run on 120 and replace the pulleys to reduce the torque requirements of the motor.

    Of those, the best option in my opinion is to hire an electrician to install a 240V line.

    I have a 5HP compressor and it does have a dual voltage option and I have thought seriously about running a 240V line to the garage. For my 5HP unit 120V barely cuts it. On those cold days where the oil in the compressor is cold, it will trip the 15Amp breaker when starting up.

  10. #10

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    thanks to everyone with the replies and advice. After looking at the motor on the compressor and calling around, I think that replacing the motor is just not the best option here.
    Soooo, looks like I will be focusing on how to get 220 from my full panel in the basement up into the garage.
    I am thinking:
    1) Can I jump off of the 220 coming into the top of my existing full service panel, run appropriate size wire (approx. 25') over to a small sub-panel box with appropriate size breaker,
    then up the wall into the garage to my compressor? If this is what the electrican is probably going to do, I think I could probably do this myself.

    If I am missing something with this plan, please advise. Any comments are appreciated.

  11. #11
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by dabiz7 View Post
    1) Can I jump off of the 220 coming into the top of my existing full service panel, run appropriate size wire (approx. 25') over to a small sub-panel box with appropriate size breaker,
    then up the wall into the garage to my compressor?

    If I am missing something with this plan, please advise. Any comments are appreciated.

    I think that there might be a little more to it than just that

  12. #12
    Retired Defense Industry Engineer jadnashua's Avatar
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    Do you have a subpanel in the garage now? While less convenient, you might be able to just put a new breaker in the main panel and run the 220vac to the garage. Is the garage attached, or detached? That can make a difference. You could put a two-pole on/off switch in the garage as a disconnect (if you get one rated for the specified load).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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