220 volt electrical outlets are specially wired outlets that provide additional voltage for appliances that demand large amounts of power, such as air conditioners, electric furnaces, water heaters and dryers. A 110 volt appliance is incompatible with 220 volt service and vice versa.
In the United States, power companies provide a split-phase 240 volt service to the home. Most household wiring only uses one of the two provided phases, and with the resistance of the wiring inside the walls, the effective voltage at the outlet is around 110 to 115 volts. With an effective 220 to 230 volts at the outlet, 220 volt wiring utilizes both phases simultaneously.