Some 220/240 outlets are 4-wire: 2 x 110/120v prongs (black, red), 1 neutral, 1 ground. The appliance that plugs in to it combines the two 110/120v currents internally to get the 220/240v.

I've called my step-father who is going to wire this into the circuit breaker box for me, but I think he's used to wiring 4-wire (10-3 or 12-3 wiring) 220 volt systems. This outlet only requires 3 ...