The "common" terminal on a standard US 3-way light switch is the single dark-colored screw (usually black or dark bronze), distinctly separate from the two lighter brass traveler screws. This common screw must connect to either the LINE (hot power source) or the LOAD (the wire leading to the light fixture), never to a traveler wire. If you are working in the UK, AU, or EU, the "COM" terminal serves a similar purpose, accepting the permanent live wire (brown or red). For smart switches, "common" is frequently misused by manufacturers to mean the neutral wire (white).
Getting the common terminal right is the single most critical step in multi-location switching. Miswiring it is the number one reason a newly installed 3-way circuit fails to operate correctly from both locations. Below is the complete, bench-tested procedure for wiring a US-style 3-way switch with a common terminal, adhering to NEC-style guidance for 120V/60Hz residential copper circuits.
Tools, Materials, and Device Ratings
Before opening the wall box, ensure your materials match the circuit's ampacity. Do not mix 14 AWG wire with 20A breakers, and do not use 15A switches on a 20A circuit.
- Switch Device: 15A or 20A 3-way toggle or rocker switch (e.g., Leviton 5603-2W 15A Decora or Eaton 1873).
- Wire Gauge: 14 AWG NM-B (Romex) for 15A circuits; 12 AWG NM-B for 20A circuits. Assumes 60°C/75°C ampacity column, copper conductors.
- Wire Nuts: Yellow or red wire connectors (e.g., Ideal 33 or 341) rated for the specific wire combination.
- Testers: Non-contact voltage tester (NCVT) and a digital multimeter (DMM) capable of measuring AC voltage and continuity.
- Hand Tools: Wire strippers (calibrated for 14/12 AWG), lineman's pliers, Phillips and flathead screwdrivers, and a torque screwdriver if the switch specifies inch-pound ratings.
Understanding the Common Terminal (and the Most Common Botch)
To wire a light switch with common terminal accuracy, you must understand its role in the circuit. In a 3-way setup, the common terminal acts as the "bridge" between the power source and the traveler network (at the first switch), or between the traveler network and the light fixture (at the second switch).
| Terminal / Screw Color | Function | Wire Color (Typical US) |
|---|---|---|
| Common (Black/Dark Screw) | Line (Hot) or Load (Switched Hot) | Black (Line) or Black/Red (Load) |
| Travelers (2x Brass Screws) | Carries current between switches | Red and Black (in 3-wire cable) |
| Ground (Green Screw) | Equipment grounding path | Bare copper or Green |
The Most Common Botch: The most frequent DIY error is landing a traveler wire on the common screw. The Symptom: The light will only turn on and off from Switch A if Switch B is left in one specific position (e.g., UP). If Switch B is flipped DOWN, Switch A ceases to function entirely. This happens because the traveler network is broken, forcing current through a single static path rather than alternating between the two traveler wires.
Step-by-Step Wiring Procedure
This procedure assumes you are replacing an existing 3-way switch. If you are pulling new wire, consult NFPA 70 (NEC) Article 404 for switch loop and neutral requirements.
- De-energize and Verify: Turn off the breaker. Test the wires at the switch box with your NCVT. Confirm zero voltage by placing your multimeter probes between the suspected hot wire and the bare ground wire (should read 0.0V AC).
- Identify the Line and Load Commons: Before removing the old switch, note which wire is on the dark common screw. At the first switch box, this is your LINE (always hot). At the second switch box, this is your LOAD (goes to the light). Tag these wires with black electrical tape. The two remaining wires in the 3-wire cable are your travelers.
- Prepare the Wires: Strip exactly 3/4 inch of insulation from the 14 AWG or 12 AWG wires. If the old copper is heavily oxidized or deeply grooved from previous screw terminals, snip the end and re-strip to ensure a clean, high-conductivity connection.
- Terminate the Ground: Loop the bare copper (or green) ground wire clockwise around the green grounding screw on the switch. Tighten firmly. If the metal box is grounded, also run a 6-inch bare copper pigtail from the box's ground clip to the switch's ground screw.
- Terminate the Travelers: Connect the two traveler wires (typically red and black from a 14/3 or 12/3 NM-B cable) to the two brass-colored screws. The order of these two wires does not matter; swapping them simply reverses the physical toggle direction (UP for ON vs. DOWN for ON) but does not affect circuit operation.
- Terminate the Common: Connect the tagged LINE wire (at the first switch) or LOAD wire (at the second switch) to the single black/dark-colored common screw. Ensure the wire loop is tight and no bare copper is exposed outside the terminal pad.
- Secure and Fold: Carefully fold the wires into the back of the box. Push the switch flush against the drywall or plaster ring and secure it with the provided 6-32 mounting screws. Do not over-torque, as this can crack the switch yoke.
Verify and Test Procedure
Do not skip the testing phase. A visual inspection cannot confirm internal switch continuity or proper traveler routing.
- Pre-Power Continuity Test (Breaker OFF): Set your multimeter to Continuity (the diode/beep symbol). Place one probe on the common screw and the other on one of the brass traveler screws. Flip the toggle. You should hear a beep in one position, and silence in the other. Move the probe to the second brass screw; the continuity states should reverse. This confirms the internal mechanical wipers are functioning. Expected reading: < 1 ohm when closed, OL (Open Loop) when open.
- Live Voltage Test (Breaker ON): Restore power at the panel. Set your multimeter to AC Voltage (V~). Carefully place one probe on the common screw terminal and the other on the bare ground wire. At the LINE switch, you must read a steady 114V–126V regardless of the toggle position. At the LOAD switch, you will read 120V only when the switch is energizing the fixture.
- Operational Test: Flip Switch A. The light should toggle. Flip Switch B. The light should toggle. If the light only responds to Switch A when Switch B is in the UP position, immediately turn off the breaker and swap the common and traveler wires on the misbehaving switch.
Frequently Asked Questions
What if my smart switch asks for a "common" wire?
This is a widespread terminology trap in the smart home industry. When manufacturers of Wi-Fi or Z-Wave smart switches (like GE Enbrighten or TP-Link Kasa) refer to a "common" wire in their manuals, they are almost always referring to the Neutral wire (the white wire bundled in the back of the box). Smart switches require a neutral to complete the 120V circuit for their internal Wi-Fi radios and LED indicators. A standard mechanical 3-way switch does not use a neutral. If your box lacks a neutral wire (common in pre-1985 switch loops), you must purchase a "no-neutral required" smart switch, such as the Lutron Caseta line, which operates via a proprietary low-voltage signaling protocol rather than drawing standby power through the load.
Can I use a standard single-pole switch if I only have a common and one traveler?
No. If you are converting a 3-way circuit to a single-location switch because you removed the second switch, you cannot simply cap a traveler and use a single-pole switch without reconfiguring the wiring at both boxes. To single-pole a 3-way circuit, you must wire-nut the two traveler wires together at the removed switch location, effectively turning the 3-wire cable into a standard 2-wire run. At the remaining switch location, you then install a standard single-pole switch, connecting the LINE to one brass screw and the newly joined LOAD (formerly the common) to the other brass screw. The dark common screw on a 3-way switch cannot be used in a single-pole application.
Why does my UK/AU "COM" terminal look different from the US common?
International wiring standards (IEC 60446 and AS/NZS 3000) use different physical architectures. In the UK, Australia, and New Zealand, a standard 1-way or 2-way light switch features a terminal explicitly labeled "COM" (Common). This COM terminal is where the permanent live wire (Brown in modern IEC, Red in legacy) connects. The switched live (going to the light) connects to "L1" (or "1-Way"). In a 2-way setup, the travelers connect to "L1" and "L2". While the physics of the "common" connection remain the same—it acts as the primary feed or output point—the physical labeling, wire colors, and voltage (230V/240V) are entirely different. Never apply US 120V wire color assumptions to an IEC/AU 230V circuit.
