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 usually 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 (without insulation), but in some cases, the ground wire has green insulation.
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 receptacle (outlet) box, it's usually because one half of the receptacle is operated by a wall switch, while the other half is always powered, or hot. The red wire is for the light switch, and it controls power to the switched half of the receptacle. This configuration, sometimes called a split, half-hot, or split-tab receptacle, is most commonly used in bedrooms, as it allows you to plug in a lamp and control it with a switch next the bedroom door.
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What Different Electrical Wires Mean by Their Color Codes
- Red wire: In a three-conductor cable feeding an outlet box, the third insulated wire is red and is usually a hot wire that connects to a switch. A 240-volt circuit may also have a red wire that is hot.
- White wire: In a two-conductor or three-conductor cable, the white wire is typically neutral but not always. A white wire used as a hot conductor should be labeled with a band of black or red electrical tape to indicate it is hot and not neutral.
- Black wire: The black wire in a two-conductor cable is always the hot wire. In a three-conductor cable for a 240-volt appliance outlet, the black wire and the red wire are hot.
An Overview of Split-Receptacle Wiring
You can use split-receptacle with any standard duplex receptacle; you cannot use this configuration with GFCI or AFCI receptacle. Examine a standard duplex receptacle, and you'll see that the two brass (hot) screw terminals and the two silver (neutral) screw terminals are joined by metal conducting plates, each with a small tab. If you bend and break off the tab between the two brass (hot) screws, the two halves of the outlet will function independently. You can power one half with a hot wire from a wall switch and the other with a hot wire that is always live.
The two halves will share a common return path on a single white neutral wire because the connecting tab between the silver terminals is left intact. The outlet also needs just one ground wire, connected to the ground screw.
The standard way to power a split-tab outlet is to run a three-conductor cable between the outlet and the wall switch. The cable has a black, a red, a white, and a ground wire. At the switch box, the black wire connects to the source cable and the switch. The red wire connects to the switch only. The ground wire connects to the source cable and the switch. The white wire connects to the source cable only and bypasses the switch.
This wiring configuration applies to a switch-controlled receptacle with the switch at the start of the run; other configurations have different wiring connections.
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 connecting tab between the two brass (hot) screw terminals. After modifying the receptacle to split the outlet, make the following wire connections:
- Attach the black wire to the brass terminal of the outlet half you want to always be on (usually the top one). Connect the red wire to the brass terminal of the switched half.
- Connect the white wire to either of the silver terminals (remember, they are still joined) on the outlet. Connect 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 — the feed cable with a black, white, and ground wire and the cable going to the outlet, which has a black, red, white, and ground wire. Join the black wires from both cables together, add a 6-inch pigtail wire, twist them all together, and screw on a wire nut. Do the same with the ground wires. Join the white wires with a wire nut but omit the pigtail.
- Connect the black pigtail to the top screw terminal on the switch. Connect the red wire coming from the outlet to the bottom screw terminal. Connect the ground pigtail to the ground screw on the switch.
The two cables must be of the same wire gauge: 12 AWG for 20-amp circuits and 14 AWG for 15-amp circuits.
If either electrical box is metal (not plastic), add an additional ground pigtail and connect it to the metal box.
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 of a suitable wire gauge for the circuit ampacity. In this case, both the red and black wires are hot and connect directly to the circuit breaker in the main electrical panel. Connect the black wire to either of the brass hot screw terminals on the outlet — it doesn't matter which one — and connect the red wire to the other brass hot terminal. Connect the white wire to the silver neutral terminal and connect the ground wire, which is bare or green, to the ground terminal.