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Thread: Old style 220 (3 wire) to new style (4 wire) connection

  1. #1
    DIY Junior Member grassy's Avatar
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    Default Old style 220 (3 wire) to new style (4 wire) connection

    Borrowed an older 2 hp craftsman air compressor to do some sand blasting. Thought I could run it via my 4000 watt generator....had to play with the wiring because the gen has the new style plug. Unfortunately, the gen does not have enough power to get the motor over the initial power up surge.

    BTW, I have rented to 2 (large and larger) gas / desiel compressors but all they can produce is large quantities of water that overloads my moisture catching filters and clogs the blasters...this at a 57% humidity.

    I ma thinking that if I have my own compressed air..I can work on my project for short periods w/o having the rental charges pile up.

    I am getting deserate to complete this job so I have my eye on our dryer plug. I would like to know how to wire the two systems together or should I cut the existing wire and put in a junction box but as still left with the delema of connecting 3 wires to 4 and making sure I don't elecute my kinds and me nor burn down the house.

    Have I missed anything ?

    Any suggestions would be helpful.

    Thanks,
    Grassy

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    In theory, you may only need two wires! ON a four-wire plug, you have two hots, neutral and a safety ground. On a 3-wire, you usually (I think) eliminate the safety ground and neutral and internal ground a tied together in the device. The neutral is required on some devices to provide both the 120 vac (for say for the electronic controls or lightbulbs). All newer devices are required to include the safety ground, so it needs to be a 4-wire cable.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Moderator & Master Plumber hj's Avatar
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    This is the type of job, that if you have to ask directions on the Internet, you should NOT be doing it. Connecting the dryer and compressor together is NOT a good idea, especially if the compressor could start automatically while the dryer is running, or vice versa. If the compressor has an "unloader" valve the starting power on initial startup, and under load, should be minimal.
    Last edited by hj; 09-03-2011 at 03:03 PM.
    Licensed residential and commercial plumber

  4. #4
    DIY Senior Member dlarrivee's Avatar
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    Quote Originally Posted by hj View Post
    This is the type of job, that if you have to ask directions on the Internet, you should NOT be doing it. Connecting the dryer and compressor together is NOT a good idea, especially if the compressor could start automatically while the dryer is running, or vice versa. If the compressor has an "unloader" valve the starting power on initial startup, and under load, should be minimal.
    He was planning on plugging the compressor into the dryer PLUG, not tying the two together.

  5. #5
    Test, Don't Guess! cacher_chick's Avatar
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    Which plug belongs on something is determined by the rated load of the device.

    If the load is less that the receptacle rating, one "could" make up a cord with different ends on it, but it would need to be sized appropriately for the load and length of the cord.

  6. #6
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by grassy View Post
    Borrowed an older 2 hp craftsman air compressor to do some sand blasting. Thought I could run it via my 4000 watt generator....had to play with the wiring because the gen has the new style plug. ........................
    ................

    I am getting deserate to complete this job so I have my eye on our dryer plug. I would like to know how to wire the two systems together or should I cut the existing wire and put in a junction box but as still left with the delema of connecting 3 wires to 4 and making sure I don't elecute my kinds and me nor burn down the house.
    You are completely off base as to how this all works.
    There is NO "new style" and "old style" plugs and receptacles. They have all been around pretty much always.
    You need to determine your voltage and amperage needs, then determine if what you have available is what you need.

    For clarity sake I am not going to be talking about anything other than 120/240v single phase residential systems, because beyond that you should not be messing with this stuff.

    There is 120v, 240v, and 120/240v.
    A 120v 3-wire circuit is one hot, one neutral and a ground.
    A 240v 3-wire circuit is two hots and a ground.
    A 120/240v circuit is two hots, a neutral and a ground.
    The only thing that is "old style" is an older 120/240v range or dryer circuit, which if existing is still completely legal to use in the right application. This is one with two hots and a neutral, but no ground. The neutral serves as both neutral and ground.

    Provided the amperage of the circuit or receptacle is proper you CAN use a 240v load on a 120/240v circuit. Just omit the neutral connection.
    That said, the opposite is NOT TRUE, you CANNOT use a 120/240v load on a straight 240v circuit.

  7. #7
    Moderator & Master Plumber hj's Avatar
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    quoyte; would like to know how to wire the two systems together or should I cut the existing wire and put in a junction box

    That statement does NOT imply that he is just going to use a single receptacle and switch devices as needed.
    Licensed residential and commercial plumber

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