The term "common" (often labeled COM) on a light switch refers to the central pivot terminal that connects the incoming hot power feed to the outgoing switched load, or vice versa. On a standard single-pole switch, both brass screws act as common terminals and are interchangeable. On a 3-way switch, the common is the uniquely colored screw (usually black or dark brass) that must receive either the line hot or the load leg, while the remaining two brass screws handle the traveler wires. Misidentifying the common terminal is the number one cause of 3-way switch failure.
What "Common" Actually Means on a Light Switch
To wire a switch correctly, you have to understand the electrical path. The common terminal is the gateway. In a mechanical switch, it is the point where the internal lever either makes or breaks the connection to the traveler or load terminal.
- Single-Pole Switches: Standard $1 toggle switches don't usually label a specific "common" screw because the internal mechanism simply bridges the two brass terminals. Polarity doesn't matter; you can land the line hot on either brass screw and the load on the other.
- 3-Way Switches: This is where the "common" designation is critical. A 3-way switch has three terminals: one common (black/dark screw) and two travelers (light brass screws). The common screw must always terminate the continuous hot feed from the breaker panel OR the continuous hot leg going up to the light fixture. The two traveler screws connect to each other across the two switch boxes.
- Smart Switches: Modern Wi-Fi or Zigbee switches explicitly label a "COM" or "Line/Load" terminal. Unlike dumb mechanical switches, smart switches have internal PCBs that require strict line/load polarity to power their internal radios.
Tools, Materials, and Device Ratings
Before pulling wire or terminating connections, match your device rating to your breaker. Installing a 15A switch on a 20A circuit is a code violation and a fire hazard.
Required Tool List
- Non-contact voltage tester (NCVT) rated for CAT III 600V
- Digital multimeter (DMM) with CAT III rating
- Wire strippers (calibrated for 14 AWG and 12 AWG solid copper)
- Lineman's pliers and needle-nose pliers
- Phillips #2 and flathead screwdrivers (insulated preferred)
- Wire nuts (yellow for 14 AWG, red for 12 AWG) or WAGO 221 lever connectors
Mains Safety Protocol: De-Energize and Verify
- Locate and Trip the Breaker: Identify the exact breaker feeding the lighting circuit and switch it to the OFF position. Apply a lockout/tagout device if others have access to the panel.
- Initial NCVT Sweep: Before removing the wall plate, hold your Non-Contact Voltage Tester against the switch cover. It should remain silent and unlit.
- Verify Dead with a Multimeter: Remove the wall plate and switch mounting screws. Pull the switch out gently. Set your DMM to AC Voltage (V~). Place one probe on the bare copper ground wire and the other probe on the black wire on the switch. Expected reading: 0.0V. If you read 120V, you have the wrong breaker. Stop immediately.
Decision Path: Identifying Your Switch Type and Common Screw
Use this decision table to identify exactly which screw is your common terminal based on the physical switch in your hand.
| Switch Type | Total Screws (Excluding Ground) | Identifying the "Common" Terminal | Wire Landing Rule |
|---|---|---|---|
| Single-Pole (Standard) | 2 (Both light brass) | Neither is uniquely marked; both are common. | Black (Line) to one brass, Black (Load) to the other. |
| 3-Way | 3 (1 dark/black, 2 light brass) | The dark/black screw (often marked "COM" on the back). | Black (Line Hot OR Load Leg) MUST land on the black screw. |
| 4-Way | 4 (2 pairs of brass/black) | No single common; acts as a crossover bridge. | Travelers from Switch A to one pair; travelers from Switch B to the other. |
| Smart Switch (Wi-Fi/Zigbee) | 3 or 4 (Labeled Line, Load, Neutral, Ground) | Explicitly labeled "LINE" or "COM". | Line Hot to LINE, Load to LOAD, White to NEUTRAL. |
Step-by-Step Wiring: Landing Wires on the Correct Terminals
The following procedure assumes a standard 120V AC, 15A single-pole switch loop with a 14/2 NM-B feed, compliant with recent NEC requirements for a neutral at the switch box.
- Terminate the Ground (Bare Copper): Loop the bare copper ground wire clockwise around the green grounding screw on the switch yoke. Tighten firmly. If the box is metal, you must also bond the box using a grounding pigtail.
- Handle the Neutral (White Wire): Standard mechanical single-pole switches do not use the neutral wire. Cap the white neutral wires together with a yellow wire nut or WAGO connector, tucking them neatly into the back of the box. Exception: If installing a smart switch, connect the white neutral wire to the switch's "NEUTRAL" terminal.
- Identify Line and Load (Black Wires): If you have two black wires and aren't sure which is the feed (line) and which goes to the light (load), use the NCVT (with power temporarily restored and extreme caution) or trace the cables. For a standard single-pole switch, this distinction doesn't matter.
- Land the First Black Wire: Strip 3/4 inch of insulation from the first black wire. Form a J-hook and loop it clockwise around one of the brass common screws. Tighten until the yoke is snug; do not over-torque and strip the screw.
- Land the Second Black Wire: Strip and loop the second black wire clockwise around the remaining brass common screw. Ensure no bare copper is exposed outside the terminal housing (a sign of over-stripping, which creates a short-circuit hazard).
- Secure and Close: Carefully fold the wires into the box (ground in back, neutrals in middle, hots on sides). Mount the switch with the provided 6-32 machine screws, ensuring the yoke sits flush against the plaster ears. Attach the wall plate.
Verify and Test: Expected Meter Readings
Do not just flip the breaker and hope for the best. Verify the circuit integrity before applying full load.
Pre-Power Resistance Check (Power OFF)
Set your multimeter to Continuity or Ohms (Ω). Place probes across the two brass screws on the switch.
- Switch ON: Expected reading: < 1.0 Ω (or audible continuity beep). The circuit is closed.
- Switch OFF: Expected reading: OL (Over Limit) or infinite resistance. The circuit is open.
Live Voltage Check (Power ON)
Restore power at the breaker. Set DMM to AC Voltage.
- Line to Ground: Probe the black wire and the bare ground. Expected reading: 120V AC (±5%, so 114V–126V is acceptable).
- Switched Load (Switch OFF): Probe the load-side black wire (at the fixture or second terminal) and ground. Expected reading: 0V.
- Switched Load (Switch ON): Probe the load-side black wire and ground. Expected reading: 120V AC.
The Most Common Botch and Its Symptom
The most frequent error DIYers make occurs when upgrading a standard 3-way switch. They assume the physical position of the screws on the new switch matches the old switch. They do not. Manufacturers place the common (black) screw in different locations depending on the brand (Leviton vs. Lutron vs. Legrand).
The Botch: Landing the continuous line hot wire on a light brass traveler screw instead of the dark common screw.
The Symptom: The light behaves erratically. It may only turn on when the other switch in the 3-way pair is held in a specific position, or it may work from one location but fail to toggle from the second location. This happens because the internal lever is attempting to route power from a traveler back into the line feed, breaking the logical path to the load.
The Fix: De-energize the circuit. Identify the single wire that is hot when all switches are disconnected (using an NCVT or multimeter against ground). That wire is your Line Hot. It must land exclusively on the uniquely colored black/dark COMMON screw on both 3-way switches in the circuit.
For authoritative guidance on switch loop wiring and grounding requirements, always consult the NFPA Electrical Safety guidelines and verify your local Authority Having Jurisdiction (AHJ) rules regarding neutral conductors at switch boxes, as outlined by the Department of Energy's wiring standards.
