Wiring an outlet to a switch—specifically creating a "half-hot" or split-receptacle where the top plug is controlled by a wall switch and the bottom plug remains always-on—requires a 3-wire cable feed. The direct answer for terminal mapping is: the black wire (always-hot) lands on the brass screw of the unbroken side, the red wire (switched-hot) lands on the brass screw where the connecting fin has been snapped off, the white wire (neutral) lands on the silver screw, and the bare copper wire (ground) lands on the green screw. You must use a duplex receptacle rated for your circuit amperage and break the brass fin on the hot side before installation.
Switched Receptacle Sizing and Cable Matrix
Before stripping any wire, you must match your cable gauge, breaker size, and receptacle rating. The National Electrical Code (NEC) dictates strict ampacity limits based on the 60°C or 75°C column of NFPA 70 (NEC) Table 310.16. For standard residential 120V split-receptacles, you need a 3-wire cable (two hots, one neutral, one ground) to carry both the constant and switched power to the outlet box.
| Circuit Breaker | Wire Gauge (Copper) | Receptacle Rating | Cable Type Required | Max Continuous Load |
|---|---|---|---|---|
| 15A | 14 AWG | 15A | 14/3 NM-B | 12A (1440W) |
| 20A | 12 AWG | 20A | 12/3 NM-B | 16A (1920W) |
| 20A | 12 AWG | 15A (Standard) | 12/3 NM-B | 12A per plug (1440W) |
| 15A (MWBC)* | 14 AWG | 15A | 14/3 NM-B | 12A per leg |
*MWBC (Multi-Wire Branch Circuit): If your black and red wires are fed from a 2-pole breaker with a handle tie, they share the white neutral. This requires a 2-pole breaker per NEC 210.4 to ensure simultaneous disconnect.
Tools, Materials, and Mains Safety Protocol
Working with branch circuit wiring demands exact tooling and strict adherence to de-energization protocols. Do not rely solely on a wall switch to isolate power; the line-side wires in the switch box remain lethal even when the switch is off.
Before opening any junction box, receptacle, or switch enclosure, you must de-energize the circuit at the main service panel. Turn off the specific branch circuit breaker, apply a lockout/tagout (LOTO) device if available, and verify the circuit is dead using a Non-Contact Voltage Tester (NCVT). Follow up by testing a known live circuit to confirm your NCVT battery is functional, then test the target wires. Local AHJ (Authority Having Jurisdiction) codes may require a licensed electrician for any new circuit runs or panel work.
Required Tools and Materials
- Cable: 12/3 NM-B (Romex) for 20A circuits or 14/3 NM-B for 15A circuits.
- Devices: One 15A or 20A duplex receptacle (e.g., Leviton 5252 or 5362) and one single-pole 15A/20A toggle or rocker switch.
- Testers: NCVT (e.g., Klein NCVT-2) and a digital multimeter (CAT III rated).
- Hand Tools: Wire strippers (calibrated for 14 and 12 AWG), lineman's pliers, needle-nose pliers, flathead and Phillips screwdrivers, and a utility knife.
Step-by-Step: Wiring a Half-Hot Switched Outlet
This procedure assumes you are running a new 3-wire feed from a switch box to an outlet box. The switch box receives a 2-wire (12/2 or 14/2) line feed from the panel.
- Prep the Cable and Box: Strip the outer NM-B jacket back 3/4 inch past the box clamp using a utility knife. Strip 3/4 inch of insulation from the black, red, white, and bare copper wires. Ensure no copper scoring is present; nicked wire creates a high-resistance hot spot under load.
- Break the Brass Fin (The Hot Side): Locate the small metal connecting tab (fin) between the two brass screws on the receptacle. Using lineman's pliers, grip the tab and snap it off completely. Do not break the silver fin on the neutral side. This physically separates the top and bottom receptacles on the hot side.
- Terminate the Ground (Bare Copper): Form a J-hook in the bare copper wire. Loop it clockwise around the green ground screw on the receptacle. Tighten until the screw head is flush and the wire is pinned securely. If metal boxes are used, pigtail the ground to the box as well per OSHA and NEC 250.148 bonding requirements.
- Terminate the Neutral (White): Form a J-hook in the white wire. Loop it clockwise around either of the silver screws. Because the silver fin remains intact, this single white wire provides the neutral return path for both the top and bottom receptacles. Tighten firmly.
- Terminate the Always-Hot (Black): The black wire carries constant panel power. Loop the black wire clockwise around the brass screw on the side where the fin was not broken (usually the bottom half). This makes the bottom plug always-hot.
- Terminate the Switched-Hot (Red): The red wire carries power only when the wall switch is closed. Loop the red wire clockwise around the remaining brass screw (the isolated side, usually the top half). Tighten securely. Ensure no bare copper is exposed outside the terminal plate.
- Wire the Switch Box: At the switch, the incoming black (line) lands on one brass switch terminal. The red wire (load/switched-hot returning to the outlet) lands on the other brass switch terminal. The white wires from the line and the 3-wire cable are spliced together with a wire nut and pushed to the back of the box (they do not land on the standard single-pole switch). The bare grounds are spliced and pigtailed to the switch's green ground screw.
- Secure the Devices: Fold the wires neatly into the back of the boxes (grounds first, then neutrals, then hots). Screw the devices to the box, ensuring the plaster ears are flush with the wall surface to prevent the cover plate from cracking.
Verify and Test: Meter Readings and Botch Prevention
Never energize a newly wired circuit without a systematic verification sequence. Set your digital multimeter to AC Voltage (V~) and insert the black probe into the ground slot (the U-shaped hole) of the receptacle.
Expected Meter Readings
- Switch OFF: Insert the red probe into the bottom hot slot (smaller vertical slot). Reading should be 120V (±5%). Insert the red probe into the top hot slot. Reading should be 0V.
- Switch ON: Insert the red probe into the top hot slot. Reading should now jump to 120V (±5%).
- Neutral Verification: Measure between the hot slots and the neutral slot (larger vertical slot). You should read 120V on the always-hot, and 0V/120V on the switched-hot depending on switch state.
The Most Common Botches and Their Symptoms
When a split-receptacle fails to operate correctly, it almost always traces back to one of three installation errors:
Symptom: Both the top and bottom outlets are always hot. The wall switch does absolutely nothing.
Fix: De-energize the circuit, pull the receptacle out, and snap off the brass fin. The red and black wires are currently bridged together internally.
Symptom: The breaker trips instantly with a loud bang the moment you turn it on, or the bottom outlet has no power while the top works.
Fix: If you break the silver tab but only connect one white wire, half the outlet loses its neutral return. If you somehow wired a hot to the isolated neutral side, you created a dead short. Replace the receptacle; the internal neutral bus is compromised.
Symptom: The outlet works initially but fails months later under load (e.g., when a vacuum or space heater is plugged in), often accompanied by a burning smell or melted plastic faceplate.
Fix: Backstab connections rely on a tiny spring clip that grips 14 AWG wire poorly and cannot legally accept 12 AWG wire. Always strip the wire and loop it clockwise around the side terminal screws, torquing them until the wire is pinned flat. See Leviton's installation guidelines for proper termination methods.
Fully Switched vs. Split-Receptacle: Which Do You Need?
While the half-hot split receptacle is the standard for living rooms and bedrooms (allowing a lamp to be switched while a vacuum or phone charger remains always-on), some applications require a fully switched outlet.
If you are wiring a switched outlet for a garbage disposal under a kitchen sink, or a dedicated workbench tool, you do not need 3-wire cable. You can use standard 2-wire cable (12/2 or 14/2). In this scenario, you do not break the brass tab. The black switched-hot wire from the switch lands on one brass screw, the white neutral lands on the silver screw, and the entire receptacle turns on and off with the wall switch. Choosing the wrong configuration leads to either wasted cable or a disposal that only works on one half of the duplex outlet.
