To wire a standard single-gang double switch (often called a duplex or twin switch) controlling two separate 120V loads from one location, connect the incoming hot black wire to the black LINE terminal, the two outgoing load wires (typically red and black) to the two brass LOAD terminals, and the bare copper ground to the green ground screw. The white neutral wires bypass the switch and are spliced together in the back of the box. For a standard 15-amp lighting circuit, use a 15A rated double switch like the Leviton 5622-W and 14 AWG copper wire.

The Decision Path: Which Double Switch Do You Actually Need?

The term 'double switch' causes massive confusion at the hardware store. Before you strip a single wire, you must confirm you are buying the correct device for your circuit topology. A true 'double switch' for DIYers is a Single-Pole Duplex Switch (two independent toggles/rockers on one single-gang strap). It is entirely different from a Double-Pole Switch (one single toggle that breaks two hot legs simultaneously for a 240V appliance).

Your ScenarioDevice RequiredConcrete Part Pick
Controlling a vanity light AND an exhaust fan from one single-gang box (120V)15A Single-Pole Double RockerLeviton 5622-W (15A)
Controlling two separate light fixtures from one single-gang box on a 20A circuit20A Single-Pole Double ToggleEaton 1877W (15A/20A rated)
Controlling a 240V baseboard heater or well pump (requires breaking TWO hot lines)Double-Pole Single Switch (NOT a duplex)Leviton 3032-PL (30A Double Pole)
Default Recommendation: If you are replacing a standard bathroom or hallway switch that controls two separate 120V lights or a light/fan combo, stop here. You need a Single-Pole Duplex Switch. The rest of this guide is built specifically for wiring the Leviton 5622-W (15A) or Eaton 1877W (20A) single-pole double switch.

Tools, Materials, and Device Ratings

Matching your wire gauge to your breaker and switch rating is non-negotiable. A 20A breaker requires 12 AWG wire and a 20A-rated switch. A 15A breaker requires 14 AWG wire and a 15A-rated switch. Never install a 15A switch on a 20A circuit.

Materials List

  • Double Switch: Leviton 5622-W (15A) or Eaton 1877W (20A).
  • Wire: 14 AWG solid copper NM-B (for 15A circuits) or 12 AWG solid copper NM-B (for 20A circuits). Reference the Southwire ampacity charts to verify temperature derating if your ambient temperature exceeds 86°F (30°C).
  • Wire Nuts: Yellow or red Ideal Wire-Nuts (rated for 3x 14 AWG or 12 AWG solid conductors).
  • Pigtails: 6-inch lengths of 14 AWG or 12 AWG bare copper and white THHN for grounding and neutral splicing.

Tool List

  • Non-Contact Voltage Tester (NCVT) (e.g., Klein Tools NCVT-2)
  • Digital Multimeter (Fluke 117 or equivalent)
  • Wire strippers (Klein 11063W for 14/12 AWG)
  • Lineman's pliers and needle-nose pliers
  • Phillips #2 and flathead screwdrivers

Mains Safety Protocol (Do Not Skip)

WARNING: LETHAL VOLTAGE. Working inside a switch box exposes you to 120V AC mains voltage, which can cause fatal electrocution or arc flash burns.
  1. Turn OFF the circuit breaker at the main panel that supplies this switch.
  2. Apply a lockout/tagout device or tape the breaker in the OFF position so no one accidentally flips it while you are working.
  3. Test your NCVT on a known live circuit first to verify the tester's battery is good.
  4. Scan the wires in the switch box with the NCVT. It must remain silent and unlit.
  5. Verify dead with a multimeter: measure between the black hot wire and the bare ground wire. You must read 0.0V AC before touching any bare copper.

Step-by-Step Wiring Procedure

For this procedure, we assume you have an incoming line cable (14/2 or 12/2: Black, White, Bare) and an outgoing load cable (14/3 or 12/3: Black, Red, White, Bare) feeding two separate fixtures. Always leave 6 inches of wire length extending past the front edge of the drywall.

  1. Prepare the Ground Wires: Strip 3/4 inch of insulation from the bare ground wires of both the incoming line cable and the outgoing load cable. Add a 6-inch bare copper pigtail. Twist the three bare wires together and secure with a wire nut. Fold the bare pigtail into the back of the box for now.
  2. Splice the Neutrals: Strip 3/4 inch from the white neutral wires of both cables. Add a 6-inch white pigtail (only required if you are also wiring a smart switch or GFCI downstream; otherwise, just splice the two whites together). Twist and cap with a wire nut. Do not connect white neutral wires to a standard single-pole double switch. Push them deep into the back of the box.
  3. Terminate the Line (Hot) Wire: Identify the incoming black hot wire from your 14/2 or 12/2 line cable. Strip 3/4 inch of insulation. Form a J-hook with your needle-nose pliers. Hook the Black Line Wire clockwise around the Black (or Dark Common) LINE Screw on the double switch. Tighten firmly until the brass washer bites into the copper (approx. 10-12 in-lbs of torque).
  4. Terminate Load 1: Identify the black wire from your outgoing 14/3 or 12/3 load cable. Strip 3/4 inch. Hook this Black Load Wire clockwise around the Top Brass LOAD Screw. Tighten securely.
  5. Terminate Load 2: Identify the red wire from your outgoing load cable. Strip 3/4 inch. Hook this Red Load Wire clockwise around the Bottom Brass LOAD Screw. Tighten securely.
  6. Terminate the Ground: Take the bare copper pigtail you created in Step 1. Strip 3/4 inch. Hook it clockwise around the Green Ground Screw at the bottom of the switch strap. Tighten securely.
  7. Dress the Box: Carefully fold the wires in a Z-pattern. Push the ground and neutral bundles into the deep corners of the box first. Tuck the hot and load wires behind the switch body. Push the switch into the box and drive the two mounting screws into the drywall ears, alternating turns to keep it level.

Verification and Testing

Do not just slap the wall plate on and flip the breaker. Verify your work with a meter to prevent a dead short.

Pre-Power Continuity Test (Breaker OFF)

Set your multimeter to Continuity (the diode/beep symbol). Place one probe on the brass Load 1 screw and the other on the green ground screw. You should read OL (Open Loop) or infinite resistance. If it beeps, you have a dead short to ground—stop and check for stray copper strands touching the ground terminal. Repeat for Load 2. Toggle the switches; the meter should remain OL regardless of switch position (since we are testing load-to-ground, not load-to-line).

Live Voltage Test (Breaker ON)

Restore power at the breaker. Set your multimeter to AC Voltage (V~).

  • Measure between the Black LINE screw and the Green GROUND screw. Expected reading: 120V AC (±5V).
  • With Switch 1 turned ON, measure between Brass Load 1 and Ground. Expected reading: 120V AC.
  • With Switch 2 turned ON, measure between Brass Load 2 and Ground. Expected reading: 120V AC.
If readings are correct, turn the breaker back off, install the wall plate, and restore power for final use.

Troubleshooting the Most Common Wiring Botch

The single most frequent mistake DIYers make when wiring a double switch is swapping the LINE (incoming hot) and LOAD (outgoing to fixture) wires. Because a double switch has three dark/brass screws clustered together, it is easy to put the incoming black wire on a brass load terminal instead of the black common terminal.

Symptom

Switch A does absolutely nothing. Switch B turns on both the light and the fan simultaneously, but only if Switch A is also toggled to the 'ON' position.

The Physics of the Botch

If you land the incoming hot on Load 1, and your two fixture wires on Line and Load 2, you have essentially wired the two switches in series rather than parallel. Power cannot reach the second switch until the first switch is closed. Furthermore, the internal metal fin/tab on some older or specific duplex switches connects the two load terminals; if you break the wrong tab (or fail to break a required one on older models), the loads will bridge together.

The Fix

  1. Turn off the breaker and verify dead.
  2. Pull the switch out. Look closely at the manufacturer stamping on the back of the strap.
  3. Locate the terminal explicitly marked LINE or COMMON (often distinguished by a black screw or a darker colored strap around the screw on Leviton models).
  4. Move the incoming hot black wire to the LINE terminal.
  5. Ensure your two outgoing fixture wires (red and black) are on the two brass terminals marked LOAD.

NEC Box Fill: The Hidden Code Violation

When upgrading to a double switch, you must calculate box fill per NEC Article 314.16. A standard single-gang switch counts as one device. A double switch (duplex) counts as two devices for box fill calculations because it houses two independent internal switching mechanisms.

If you are using 14 AWG wire (2.0 cubic inches per conductor volume allowance):

  • Incoming 14/2 cable (Black, White, Ground) = 3 conductors
  • Outgoing 14/3 cable (Black, Red, White, Ground) = 4 conductors
  • All grounds combined count as 1 conductor volume = 1 x 2.0 = 2.0 cu in.
  • All neutrals combined count as 1 conductor volume = 1 x 2.0 = 2.0 cu in.
  • Current carrying hots/loads (Black, Black, Red) = 3 x 2.0 = 6.0 cu in.
  • Double Switch (counts as 2 devices) = 2 x 2.0 = 4.0 cu in.

Total Required Box Volume: 14.0 cubic inches. If your existing single-gang box is a shallow 12.5 cu in 'old work' box, you are violating code and risking heat buildup. You must upgrade to a deep single-gang box (minimum 16 cu in, preferably 20.25 cu in) to safely accommodate a double switch with 14/3 and 14/2 cables.