According to standards in use since the mid-1900s, a wire color code identifies the purpose of each wire in an electrical circuit. The white wire is always neutral, and when the cable has only two conductors, as most 120-volt cables do, the hot wire is black. The ground wire is usually left bare, but in some cases, the ground wire color is green.
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In a three-conductor cable, the extra wire is red, and it's almost always used as a hot wire. When you see a red wire in a 120-volt duplex outlet box, it's usually because one of the outlets is operated by a wall switch, while the other is always on. The red wire is for the light switch. This configuration, sometimes called a half-hot or split-tab outlet, is becoming increasingly popular, so it's good to know how to wire it.
An Overview of Split-Tab Wiring
You can wire a split-tab circuit with any duplex outlet rated for the ampacity of the circuit except for a GFCI or AFCI outlet because you can't modify either of these to separate the outlets. Examine a conventional duplex receptacle, and you'll see that the two brass screws and the two chrome ones are joined by conducting plates. If you break or remove the plate connecting the brass screws, the two outlets will function independently. You can power one with a hot wire from a wall switch and the other with a hot wire that is always on. The two outlets will share a common return path on a single white wire, and they will share a common ground.
The standard way to power a split-tab outlet is to run a three-conductor cable to a wall switch. The cable has a black wire, which connects directly to the circuit, and a red wire, which connects to the switch. The ground wire also connects to the switch, but the white wire gets spliced to the circuit neutral wire and bypasses the switch.
How to Wire an Outlet with Red, Black and White Wires
Before you make any wire connections to the outlet, you must break or remove the bonding plate between the two brass terminals. The safest option is to remove the screws, lift out the plate and replace the screws. That way, you eliminate any possibility of arcing between the two terminal screws.
After modifying the receptacle to split the outlets, make the following wire connections. Attach the black wire to the outlet you want to always be on (usually the top one) and the red wire to the switched outlet. Connect the white wire to either of the chrome screws (remember, they are still joined) and the ground wire to the green ground screw. You can now screw the outlet to the box and proceed to the switch box.
You'll have two cables in the switch box – a live one with a black and white wire and the one going to the outlet, which has a red, black and white wire. They should both be of the same wire gauge: 12 AWG for 20-amp circuits and 14 AWG for 10- and 15-amp circuits. Join the black wires from both cables together, add a 6-inch jumper cable of the same gauge, twist them all together and screw on a wire cap. Join the white wires and cap them in the same way, but omit the jumper wire.
Connect the black jumper wire to the top terminal screw on the switch, and now both the switch and the top receptacle in the split-tab outlet have power. Connect the red wire coming from the outlet to the bottom switch terminal, and now the lower receptacle on the split-tab outlet will get power only when the switch is on. Finish up by twisting the ground wires together and connecting them to the ground screw on the switch.
240-Volt Circuits Also Have Red Wires
If you're wiring an outlet for a 240-volt appliance, such as a stove or air conditioner, you'll also need three-conductor cable with a wire gauge of at least 10 AWG. In this case, both the red and black wires are hot and connect directly to the circuit breaker in the main panel. Connect the black wire to either of the brass screws on the outlet – it doesn't matter which one – and the red wire to the other brass screw. Connect the white wire to the chrome terminal and the ground wire, which is bare or green, to the ground terminal.