No set color—find the common on the dark “COM” screw; in U.S. homes it’s usually the black hot feed or the switched leg.
Heard the phrase “common wire” while staring at a switch box and wondered which color to trust? You’re not alone. The term “common” describes a function on the switch, not a paint code for the insulation. Reading color helps, yet the safest way to spot the common is to identify the terminal marked or colored for it and then verify with testing.
This guide clears up the naming, shows you how color conventions work in the U.S., and walks you through quick checks for single-pole, 3-way, and 4-way setups. You’ll also see how dimmers and smart switches label their leads, when a white wire can be hot, and the steps to tag and move wires cleanly.
Common Wire Color On A 3-Way Switch: What To Look For
On a standard 3-way switch, the common terminal is the odd one out. Two screws match for the travelers; the remaining screw has a darker finish or is stamped “COM.” The wire on that dark screw is your common. Manufacturers show the different-colored COM screw in their sheets and often show the dark screw position in diagrams, so match your switch body before moving a single conductor.
What lands on that common? Either the incoming hot feed (line) or the outgoing hot to the light (load). The travelers shuttle power between the two switches over the other two screws. Color helps you guess, but it isn’t definitive: many homes use black for the common, red and a re-identified white for travelers, while remodels and conduit runs might use blue or yellow for switched legs.
How “Common” Differs By Switch Type
| Switch Type | What “Common” Means | Usual Conductor(s) |
|---|---|---|
| Single-Pole | No “common” label; two hot terminals pass the same ungrounded conductor | Black to brass on both screws; white neutral not on the switch |
| 3-Way | Odd-colored screw marked COM; either hot feed or switched leg | Often black on COM; travelers are red and re-marked white |
| 4-Way | No COM; device only flips traveler pairs between two 3-ways | Two traveler pairs, often red/white and black/white (re-marked) |
| Dimmer/Smart | Leads labeled LINE and LOAD; some models need NEUTRAL too | Black lead to line (common), another lead to load; white neutral if required |
Which Color Is The Common On A Light Switch? Practical Checks
If you’re swapping a 3-way or adding a dimmer, use a process that doesn’t rely on assumptions. Here’s a fast, low-stress way to find the common and move it correctly.
Step-By-Step ID
- Kill power at the breaker. Tape the handle so no one flips it back mid-job.
- Pull the switch gently and take a clear photo of the wiring. Labels help if the box is crowded.
- Look for the odd screw. The darker screw or the one stamped “COM” holds the common. Tag that conductor with tape so you can put it back on the right point of the new device.
- If color is confusing, test. With power restored for a moment and the light removed or off, a non-contact tester or a two-lead tester can show which wire is the constant hot feed. Return to safe and de-energized work before moving any wire.
- Move the tagged common to the new device’s common or LINE lead. Then move travelers to the remaining traveler screws or leads in any order. Ground to green, neutral stays tied to neutrals unless your new control needs it.
White can be repurposed as a hot in older switch loops or inside certain cable assemblies. When that happens, the code requires re-identification with tape or other marking at every visible point so no one confuses it for a neutral later. You’ll often see black or red tape on the white traveler in a 3-way.
U.S. Wire Color Rules You Can Trust
Green or bare goes to the equipment grounding conductor. White or gray is reserved for the grounded conductor (neutral). Black and red are typical ungrounded (hot) conductors in 120/240-volt branch circuits; blue and yellow show up as hots in conduit runs and control legs. Since remodels, repairs, and creative swaps exist, always test rather than trusting color alone.
Travelers Vs. Common: Don’t Mix Them
Travelers only run between the two 3-way switches. They do not connect to the light or the panel directly. The common is the odd terminal and handles either the feed or the switched leg. If you accidentally put a traveler on COM, the light may work from one location and misbehave from the other. Tagging before removal prevents this headache.
Single-Pole, 3-Way, 4-Way: Quick Scenarios
Single-Pole Switch Replacement
Two insulated conductors on two brass screws, plus ground. Move one wire at a time from old to new. If your new device has pigtails, connect black to the pair and cap with a listed connector. Neutral stays in the splice; it does not land on a standard single-pole switch body.
3-Way Switch Replacement
Find the dark screw first and tag that conductor. Move it to the new switch’s COM or LINE. Move the other two to the traveler screws. If the light only works in one toggle combination after you’re done, swap the two traveler wires; leave the common where it is.
Adding A 4-Way Between Two 3-Ways
A 4-way sits between the two 3-ways. It has two pairs of traveler terminals. Bring the pair from the first 3-way to one side and the pair to the second 3-way on the other side. There is no common on a 4-way; it only reroutes traveler pairs internally.
Smart And Dimmer Switches: Where The Common Goes
Most smart controls and many dimmers label their leads instead of using screw colors. The black lead is usually LINE (your common in this context), another colored lead goes to LOAD, and a white lead ties to the neutral bundle if the model needs a neutral. Some also include a separate traveler lead for multi-location control. Always match the labeled function, not the insulation color from your wall.
If your old 3-way used a switch loop with no neutral, pick a model that supports that wiring or add a neutral to the box under permit. Many manufacturer sheets show the dark screw as common on legacy devices and label LINE on newer ones. That mapping helps you move wires with confidence.
Function And Color Cheatsheet (US vs IEC)
| Function | U.S. Typical Color | UK/EU (IEC) Typical Color |
|---|---|---|
| Equipment ground | Green or bare copper | Green/yellow |
| Neutral | White or gray | Blue |
| Line / hot | Black; red for second hot | Brown; black or gray for other phases |
| Switched leg / travelers | Red, black, or re-identified white | Brown (often sleeved or tagged) |
Colors can vary by era and method. Never rely on color when a label, diagram, or test can confirm the function.
Safety Steps Before You Touch A Switch
- Turn off the correct breaker and verify with a tester at the box.
- Use a listed voltage tester and treat every conductor as live until proven safe.
- Cap loose conductors while you work and keep grounds clear of the hot screws.
- Use the right size wire connector and box fill that meets code.
- If you open a box and find aluminum branch wiring, stop and call a licensed pro who is trained in the proper repair methods for that material.
A quick pause to check labeling and test points prevents nuisance tripping and keeps people safe—see the ESFI wiring safety tips.
Common Mistakes And How To Fix Them
Assuming Color Equals Function
Color helps, but remodels and re-identification rules allow exceptions. Verify with the dark screw, labels, and tests.
Moving Wires By Position, Not By Terminal
Switch brands locate the common screw in different spots. Move wires by terminal function: COM to COM or LINE, travelers to traveler screws, ground to green.
Forgetting To Tag The Common
A tiny piece of tape saves you from flipping travelers around later. Tag first, then move.
Leaving A White Hot Unmarked
If a white insulator carries an ungrounded conductor in a cable assembly, it needs a marking that circles the insulation at each visible point. Tape or permanent dye works.
Landing Neutral On A Standard Switch
Except for smart controls that need neutral, a basic toggle only switches the ungrounded conductor. Neutral stays in the splice.
Mini Walkthrough: Replacing A 3-Way Switch
- Power off and verify.
- Tag the wire on the dark screw as common.
- Move that tagged wire to the new switch’s COM or LINE.
- Move the two remaining conductors to the traveler screws.
- Attach ground, tuck wires neatly, and mount the device.
- Restore power and test from both locations; if behavior seems odd, swap the two travelers.
This routine works regardless of which brand you install, as long as you match the terminal function names.
When To Call An Electrician
Bring in a pro when the box is overfilled, conductors are too short to splice, a metal box lacks a bonding path, or the layout includes shared neutrals, multi-wire branch circuits, or mystery cables. Those cases can be solved cleanly, yet they call for tools and training that go beyond a quick swap.
For any device that needs a neutral or a traveler where none exists, plan a new cable run under permit. That path keeps your installation safe and inspection-ready.
How To Read The Box And The Cable
Boxes and jackets often reveal more than color alone. Nonmetallic cable sleeves print legends like “NM-B 12/2 with ground,” telling you conductor count and size for matching parts. The outer sleeve color hints at gauge in many modern homes: yellow often houses 12-gauge branch circuits and white often houses 14-gauge lighting runs. Printed legends beat sleeve color when in doubt, so read them before you pick parts.
Clues Inside The Box
Look for a bundled group of whites tied together; that is the neutral splice. A lone white on a switch that is taped or dyed likely carries hot. A bare or green conductor attaches to the metal box or the green screw on the device strap.
Testing Without Guesswork
A two-lead tester or a meter set to volts can confirm which conductor is live relative to neutral or ground. Place one probe to the suspected hot and the other to the neutral splice or to the box if it is bonded. A steady 120-volt reading marks the feed. De-energize again before making changes, then confirm operation with the cover back on.
Bottom Line
The “common” on a light switch is a role, not a color. On a 3-way the common lands on the dark screw and handles either the feed or the switched leg. Single-pole and 4-way switches don’t use a COM terminal the same way. Trust labels, the dark screw, and testing; then move conductors by function and tag any white that carries hot. Do that, and the switch will work from every location the first time you flip it.
NEC 200.7 requires re-identification when a white in a cable serves as a hot; mark it at each visible point so the next person knows it isn’t neutral.