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Voltage
Single-phase, 220-volt power really consists of two different power forms, each with a nominal potential of 120 volts. These two voltages combine to make 240 volts. The voltages on the pins may vary, so the combination of the two could actually range from about 220 to 250 volts.
Three-wire Outlets
All 220-volt plugs and outlets with three conductors have the NEMA 10 classification. NEMA size 10-20, 10-30 and 10-50 outlets are capable of supplying 20, 30 and 50 amperes, respectively. The 30 and 50 sizes are commonly used for electric dryers and stoves. The three pins on these connectors carry two 110-volt power forms and a neutral return. Connections between either of the two power pins and the neutral line deliver 110-volt service, and connecting across the two power pins produces the full 220 volts. The different NEMA 10 outlets all share the same pin configuration, but they are slightly different in size and pin shape to prevent inadvertently using one with an incompatible current rating.
Four-wire Outlets
Outlets with the NEMA 14 designation have four pins, which include the two 110-volt power pins, the neutral and a ground pin. The different sizes are NEMA 14-15, 14-20, 14-30, 14-50 and 14-60, with the dash numbers indicating the current ratings. As with the three wire outlets, the pin configurations and sizes are all slightly different to avoid mixing devices with different ratings. Four-wire outlets are usually required in mobile homes and recreational vehicles because of the added safety afforded by the ground connection. NEMA 14-30 and 14-50 outlets are the most common sizes in use.
Locking Pins
In industrial settings, some outlets and the plugs that mate with them have a locking feature to prevent accidental disconnection. The male pins of the plugs all have a right-angle bend at the end so that once plugged in, the user can twist and lock them into place. These devices have the same designations as the straight pin varieties except for a preceding L, as in NEMA L14-50.