replacing a dryer outlet with a L6-20

Advice greatly appreciated. I have a dryer outlet that i no longer need. It is a 30-R-30A non-grounding receptical. It has 2 hot, 1 common wire connected to it. The breaker is 2 pole 120/240V 30 amp.
Instead of using the dryer outlet, I would like to put in a L6-20-R. We are plugging to a device that requires 240V. But it only has, what appears to be 2 hots, 1 ground.
So I am confused .. how can this outlet work w/out a common wire? Can I just hook up the 2 hots, 1 ground, leave out the common and it work or is it more complicated than that?

 

Is there a ground to the receptacle? If not you can not use it for a 240v only equipment. If it does not have a bare wire or green wire or metal conduit continuous to the breaker or fuse box it is not grounded. If it has old style NM cable with only white, black, red and no bare wire it is not grounded. The white wire by code can not be used as a ground.

If it is grounded then the breaker will need to be changed to 20 amps.

Your house is supplied with 240 volts. There is no common. There is a center tap from the transformer that supplies the 240v that is called the neutral (AKA grounded conductor). The 120v used in your house comes from one leg of the 240v and the neutral (center tap). Appliances like Dryers and stoves have both 120v equipment* and 240v heating elements so need the neutral. An electric heater or air compressor usually only has 240v devices and needs no 120v. Old style dryers used a combined ground and neutral but that is less safe then separate ground and neutral and is no longer code compliant.

*Note: Basically you can think of stoves and dryers as 120v gas appliances that have had the gas burners removed and replaced with 240 volt electric heat sources. This saves the manufacturer money since both gas and electric use the same parts except heat source. There are in some cases exceptions but that is a general explanation.

 

Yes, there is a ground wire where they dryer receptacle is currently located - just not used. The current setup is that the white, red and black wires are currently the only wires hooked up to the 30-R-30A w/ the ground not hooked up at all.

So should/can I just hook in the new L6-20-R w/ the white wire unused? So I should just use the ground wire that is already there?

Is changing the breaker to 20 amps absolutely necessary? This is only for temporary use. The wires from the laundry room to the panel are rated for 30amps, I just need a 20 amp plug to power up a system for a few hours - nothing else would be plugged in.

Thanks again.

 

What are you trying to power from the dryer circuit?

 

I am trying to power up some computer equipment that requires a 20 amp 240v twist lock - only need for short duration to configure system.