A split outlet is a duplex outlet, or receptacle, with one half of the outlet that has power all the time and one half that is controlled by a switch. If you have a bedroom that has no overhead light, chances are at least one of the receptacles in the room is a split receptacle. The National Electrical Code requires that bedrooms have a light fixture controlled by a wall switch, and when the construction has not included an overhead light, the standard solution is a plug-in light controlled by a wall switch. The bottom receptacle on the outlet remains constantly hot for other uses.
Most split receptacles are fed by a single circuit, but it's possible to wire a receptacle to be fed by two different circuits, as is often done in older installations where a single outlet serves a dishwasher and garbage disposer.
How to "Split" a Receptacle
Standard duplex receptacles have two halves (each with a set of slots for a plug), and each half has a hot and a neutral wire terminal. The two receptacle halves have a continuous electrical pathway connected by metal strips, called connecting tabs. When the tabs are intact—as they are when receptacle comes from the factory—you can connect one hot wire to either hot terminal and connect one neutral wire to either neutral terminal, and both halves receive power. To convert the receptacle to a split duplex receptacle (also called a split-tab receptacle), you simply break off the tab between the hot terminals. This is easy to do with needle-nose pliers.
With the tab removed, you must connect a different hot wire to each of the hot terminals in order to supply power to both halves of the receptacle. Because the neutral tab remains intact, you can connect a single neutral wire to either neutral terminal so that the two outlets share a neutral. However, with some configurations, an additional neutral wire is used as a hot wire for the switch; in this case, the neutral should be labeled with a band of black or red tape, indicating that it is hot.
Spit Receptacle Circuit Wiring
In the standard wiring configuration for a switched split-wired receptacle, a two-wire cable (with a hot, neutral, and ground) supplies power to the switch or receptacle, and three-wire cable (with red and black hot wires and a white neutral) then brings power from the switch to the split receptacle, where the tab between the hot terminals on the receptacle is removed.
At the switch box, the black wire from the two-wire feed cable connects to one of the screw terminals on the switch, using a pigtail, and to the black hot wire in the three-wire cable leading to the receptacle. This wire will provide constant power to the non-switched half of the receptacle. The red-hot wire from the three-wire cable connects to the other switch terminal. This will power the receptacle half controlled by the switch.
The white neutral wire from the feed cable connects to the white neutral wire running to the receptacle box (there is no neutral connection on a standard switch).
At the receptacle box, the black hot wire from the three-wire cable (coming from the switch) connects to the hot screw terminal on the non-switched half of the receptacle. The red-hot wire from the three-wire cable connects to the hot screw terminal on the switched half of the receptacle. The white neutral wire connects to either of the neutral screw terminals on the receptacle.
The bare copper ground wires in both boxes are connected to the green screw terminals on the receptacle and switch, and to the electrical boxes (using pigtails) if they are metal.
It's important to note that this is the standard configuration and that other configurations are common and will affect the nature of the wire connections. For example, when the switch is at the end of the loop (when the power source passes through the receptacle box before reaching the switch), the wiring will look different. And when a wall switch controls multiple receptacles, the connections will vary slightly.
Split Receptacle with Two Circuits
As mentioned, a receptacle can be split and receive power from two circuits. Such a configuration is often used in kitchens, where two circuits power several split receptacles above the countertop, with one circuit powered the top halves, the other circuit powering the lower halves.
Either circuit can be switched or not switched, as applicable. In such a situation, the receptacle can be wired with a single 3-wire cable so that a single neutral wire serves both circuits, while the red and black wires each control separate circuits, each powering one-half of the split receptacle. The black and red hot wires of the cable each connect to one of the hot terminals on the receptacle, which has had its connecting tab removed.
The important thing to remember is that even though there are effectively two 120-volt circuits in this scenario, they must be protected by a single double-pole (240-volt) circuit breaker in the service panel. Here's why: If you connected each of the circuit's hot wires to a different single-pole breaker, you (or someone else) might turn off just one of the breakers before working on the receptacle. This would leave half of the receptacle live, a very dangerous situation. By connecting both hots to a double-pole breaker, you can't shut off one circuit without shutting off the other.