New compressor is 240v, 20amp, 3 wire (white, black, ground). Existing 250 volt outlet & plug are L14-30, 30amp, 4 connectors (2 hots, neutral, and ground). How do I wire the plug? If I have to change the plug and outlet to L6-30, what do I do with the 4th wire in the wall?

Use an L6 30 receptical and plug, cap the neutral in the box. Make sure the new breaker is 30 Amp double.

If this compressor is "out of the box, new" then I am surprised that the manufacturer is using a three wire connection. NEC code now calls for a four wire connection. However, if the existing receptacle is thee wire, then you can convert the cable from the device to the receptacle.

Check with the manufacturer to make sure you have a device that is listed for the U.S.A.

If this compressor is "out of the box, new" then I am surprised that the manufacturer is using a three wire connection. NEC code now calls for a four wire connection. Don, seriously?

You should know that this only applies to 120/240V household cooking appliance and electric dryers. A compressor does not and never did apply, mainly because it is a straight 240V load which only requires two hots and a ground.

Use an L6 30 receptical and plug, cap the neutral in the box. Make sure the new breaker is 30 Amp double.

Thank you all for your replies. I was thinking I could still use the L14-30 plug and receptacle by using the 2 hot leads (brass screws) in the plug and the ground (not use the silver screw connection at all); then cap the neutral in the box - 3-wire to 3-wire?

Thank you all for your replies. I was thinking I could still use the L14-30 plug and receptacle by using the 2 hot leads (brass screws) in the plug and the ground (not use the silver screw connection at all); then cap the neutral in the box - 3-wire to 3-wire?Using the plug is fine without the neutral hooked up, but keep it hooked up at the receptacle.

Using the plug is fine without the neutral hooked up, but keep it hooked up at the receptacle.

Okay great - just so you know, I had the shop wired single phase, not 3-phase 250V circuits.

"Existing 250 volt outlet & plug are L14-30". Sorry, missed that. Yes, just disconnect the neutral and cap off.