220V 4-wire to 3-wire connection?


Old 05-03-07, 06:48 PM
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Question 220V 4-wire to 3-wire connection?

I need to assemble a short extension cord in order to temporarily plug my 220V 5-horse air compressor into my clothes dryer wall plug. I have a length of 10/3 wire, a new 6-ft dryer cord (4 wires- red, green, black, & white), and the proper female connector (a 5-20R, matching the factory compressor plug which looks like a standard 110V plug, except one "blade" is rotated 90 degrees) which has 3 terminals. What is the proper safe way to hook this together?
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Old 05-03-07, 07:05 PM
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it is common for mfgs of things like air compressors to rate their equipment on HP of the air, instead of HP draw electrical.

Before we can help, we need the hp electrical, or volts and full load amps for the compressor motor.
Old 05-03-07, 07:06 PM
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The problem you will have with this setup is you are protecting a 20 amp compressor with a 30 amp breaker. That is why the plugs don't match. Connecting the pig tails in a junction box will suffice for the connection, but I doubt you will get the 10 gauge stranded wire under the screws of the 20 amp female plug. If this is a temporary setup, why not use a proper 20 amp double breaker, and 12 gauge wire directly from the breaker to the female plug. Or permanently wire up another receptacle dedicated to the compressor.
Old 05-03-07, 07:34 PM
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Unhappy 220V 4-wire to 3-wire?

I closely examined the Sears single-stage oil-lubricated 20-gallon tank, compressor, and motor, and amazingly even located the original papers that came with it, and nowhere does it state any electrical info other than a 5-horse 20-amp 220V motor. I was hoping to avoid a permanent installation since our home will be forcibly aquired & bulldozed to the ground by our state highway department in a few weeks (LONG story), and I wanted to use the air tools to get my old Mustang ready to transport. The power box is on the opposite end of the house, and to make a proper new installed circuit would take time & money I do not have at the moment, thus I had hoped this rigged extension cord would get me by in a pinch, since the dryer is within a few feet of the car. My luck continues... Thanks anyway, fellas. You help a lot of folks with your wisdom, experience, and common sense. I salute you all.
Old 05-03-07, 08:54 PM
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The experts will have to wade in on code issues but since the dryer plug is four wire you could probably make a temp portable power panel that plugs in to the dryer plug and has the correct receptacle for the air compressor. You would use a small subpanel with isolated neutral and a 20 amp 240 breaker for the air compressor receptacle . Power to the subpanel would be via a standard 4 wire dryer cord set. (Note: Neutarl of course wouldn't be used in this case but only mentioned because of standard wiring for subpamel.)
Old 05-03-07, 08:56 PM
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A NEMA 5-20 is a 120V 20A device. Are you sure the compressor isn't a NEMA 6-20? They look the same, except it is the other prong which is rotated.

But there is no problem with the pigtail. The breaker is designed to protect the premisis wiring, not the compressor. What you are wanting to do is common for powering welding machines and other 240V equipment in a small garage shop.

You have a 4-wire dryer receptacle? If so, it'll be a NEMA 14-30. If you have a prewired dryer cord, connect only the black and red to the hots on the compressor and then connect the grounds. You will not connect the white wire to the compressor.

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