For a two pole 240 Volt 14-50 circuit you should have four wires:
bare copper—ground (the ground wire is sometimes colored green and goes to the green screw on a receptacle, as well as directly to the outlet box to ground it for safety
For an L6-20 receptacle you need hot, hot, ground. The neutral wire is not used. This is also the case when using a 14-50 to L6-20 adapter: the neutral pin can be cut off the plug (it is only used for appliances that need 120 Volts in addition to the main 240 Volts, it is not used for the L2 EVSE).
I am away from home today but my recollection is that the voltages should be:
red to ground (or neutral), 120 Volts
black to ground (or neutral), 120 Volts
red to black, 240 Volts (because the red and black wires are out of phase with each other)
The reason your 20 Amp breaker is double wide is that it is two pole. One side connects to one phase of your power supply and the other connects to the other phase. Between each phase and neutral/ground is 120 Volts. Between each hot wire connected to one phase and hot wire connected to the other phase the voltage is 240 Volts. This is how your panel can supply both 120 Volts for most devices, using single pole breakers, and 240 Volts for high energy requiring devices, using a double pole breaker. For more background on this stuff you might want to read this:
http://www.nojolt.com/Understanding_240 ... uits.shtml
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It is pretty straightforward but if you aren't comfortable with this stuff, you should get the help of an electrician; getting it wrong can be fatal!