The electricity in your house wouldn't be useful without the wall sockets that provide access to it. Whether you call them sockets, receptacles or outlets, most of those in your house probably look alike, with the exception of those you use for large stationary appliances, like your dryer or stove. They all have terminals for hot, neutral and ground wires, and they only complete a circuit when you plug something in and turn it on.
The earliest wall sockets were not grounded and had two vertical slots of the same size. To prevent the frequent shocks these outlets delivered, Philip F. Labre invented the three-prong outlet in 1928, and the use of this plug eventually became mandatory for standard 120-volt circuits. The National Electric Manufacturers Association (NEMA) has approved a large variety of pin configurations. They vary according to the maximum current draw a circuit can support and the voltage of the circuit. The two-prong configuration for 120-volt circuits has returned, but the pins are different sizes so that plugs can be inserted only one way to maintain polarity.
Standard 120-Volt Socket Wiring
Two- and three-prong sockets for standard 120-volt circuits each have the same number of terminals, and you follow the same procedure for wiring them. They have a pair of brass terminals on one side of the receptacle, a pair of silver ones on the other, and a green terminal for ground wires. You connect the hot circuit wire, which is black, to one of the brass terminals, and the neutral wire, which is white, to the corresponding silver terminal. If you want to extend the circuit, you connect the outgoing wires to the unused set of terminals.
240-Volt Socket Wiring
The 240-volt sockets, such as the one into which you plug a clothes dryer, have an extra brass terminal. This is necessary because a 240-volt circuit uses both hot wires that enter your house through the main service panel. The extra hot wire, which is red, connects to one of the brass terminals; the black wire connects to the other one. As in 120-volt sockets, the white wire connects to the silver terminal and the ground wire to the green one. Some older 240-volt sockets don't have a ground terminal. They are legal because they are grounded through the neutral wire, but they are gradually being phased out.
Ground Fault Circuit Interrupter (GFCI) outlets are required in locations where you might contact both the circuit and water, such as in bathrooms or kitchens. You wire them like a standard 120-volt socket with one important difference: One set of terminals, usually the top one, is for the incoming power, or the Line, while the other set is for extending the circuit to other devices, or the Load. When a GFCI is wired properly, it provides ground-fault protection by tripping if it detects a change in current, such as sudden grounding through your body while you are standing in water.