Yes—the common is the wire on the dark (black) screw; it carries either the incoming hot (line) or the switched leg (load), not a traveler.
Why The Common Lead Matters
The common terminal is the anchor point that lets a pair of 3-way switches send power one way or the other. One switch puts power onto a traveler; the other switch either passes that traveler through to the light or not. If the common is on the wrong screw, the light may only work from one location, or not at all.
| Terminal / Wire | Role In Circuit | Clues On Device |
|---|---|---|
| Common | Feeds power from the breaker to the loop, or carries the switched leg to the light | Single dark screw (often black); may be marked “COM” |
| Travelers (2) | Provide the two alternate paths between switches | Two brass screws; wires often red and another color |
| Ground | Safety bond | Green screw; bare or green wire |
Understanding The Common Wire On A 3-Way Light Switch
On a standard mechanical 3-way, the common is the single screw with a darker finish. Manufacturers use that darker screw as shown on Lutron sheets to flag the terminal that must take either the line feed or the load to the lamp. The two same-colored screws are travelers; they only run between the switches.
Because color conventions vary by project and by installer, do not trust insulation color alone to pick the common. Instead, find the screw that is different, or look for a “COM” mark. If the old switch is still in place, tag the conductor that was on the odd screw before you remove anything. A wiring reference summarizes that only neutral and ground colors are fixed, which explains why traveler colors can vary.
Line-Side Common Versus Load-Side Common
One of the two switches receives the hot feed from the panel; on that switch, the feed lands on the common. The other switch sends power onward to the fixture; on that switch, the conductor leading to the fixture lands on the common. The travelers connect between the two remaining screws on both switches.
How To Identify The Common In An Existing Box
Start with the breaker off. Confirm with a tester. Pull the switch forward with the wires still connected and find the single dark screw. Tag that conductor. If a previous installer mixed things up and you cannot rely on the screw color, turn the breaker on only long enough to test. The wire that stays live in one box no matter the traveler positions is the line. In the other box, the wire that goes up to the lamp is the load. Both of those are commons at their switches.
Non-Contact Tester Method
With power restored briefly, probe each conductor on the suspect switch. The one that reads live in all toggle positions is the line feed; shut power off and tag it as the common for that switch. Move to the other box, restore power, then toggle while probing. The one that goes live only when a traveler from the other box lines up with it is a traveler, not the common.
Two-Lead Voltage Test
Use a meter only if you know how to handle the probes safely. Test hot to ground to find the feed, then hot to neutral at the light to confirm the load lead. Shut the breaker off before moving any wires.
Where The Common Terminal Connects On A Three-Way Switch Circuit
When the feed enters at Switch A, its hot must sit on Switch A’s common, and the load to the fixture must sit on Switch B’s common. When the feed enters at the light, the switched leg returning from the travelers must land on the fixture and the remaining switch’s common takes the feed. The rule stays the same: feed on one common, load on the other.
Traveler Behavior In Plain Terms
Each switch selects between traveler 1 and traveler 2. If both switches select the same traveler, power reaches the lamp. If they select different travelers, power stops. That is why a 3-way has no fixed “on” or “off” mark on the handle.
Common Mistakes To Avoid
Placing a traveler on the dark screw stops the two-location control. Swapping neutral and hot at the fixture can trip a breaker and create shock risk. Leaving the ground off removes a layer of protection. Twisting travelers together in a wirenut and landing that bundle on the common is another frequent error.
Safety Steps Before Any Switch Work
Flip the breaker off, then confirm the loss of power at the switch box. Tag the breaker so another person does not restore it. Use a non-contact tester and confirm with a meter when available. An ESFI safety workbook outlines these steps and a few added reminders such as avoiding damp floors during any wiring task.
Smart Controls And The Meaning Of “Common”
Many smart dimmers still have a dark screw that acts like a common for the line or the load, but they may add a neutral terminal and a special terminal for a companion. Read the sheet for your model and match the marked terminals, not just the colors. The presence of a neutral in the box often decides which models will work.
Adapting From Old Cable Colors
In older homes, a white in a switch loop may have been repurposed as a hot. If so, it should be taped to show that it is not a neutral. Travelers may be any color that is not white, gray, or green. Label conductors as you find them, and update any missing tape at the end.
Field Tips That Save Time
Before you pull a single screw, snap a photo of each switch showing which conductor sits on the odd screw. Tag those conductors with tape. When replacing a bad switch, move the tagged conductor to the new dark screw first, then land the two travelers on the paired screws in either order.
Reading The Device Body
Most 3-way switches print small hints on the strap. Look for “COM” near the dark screw, and “TRAV” or a symbol near the pair of brass screws. Some brands place the dark screw on the opposite end from ground. Read markings before you move wires from the old device.
Core Takeaways For Safe Wiring
- Find the odd screw and put the feed or the load there. That is the common.
- Travelers land on the two like-colored screws; their order does not matter.
- Kill power, test for zero, and tag wires before you move them.
- Do not rely on insulation color by itself; confirm by screw color and testing.
- Match device sheets when installing smart controls.
Quick Troubleshooting For 3-Way Problems
Use patterns to steer your checks. If the light only works from one spot, look for a traveler placed on the dark screw. If the light never turns off, the feed and load may have been tied together. If a smart control will not sync with a companion, the special lead may be on the wrong terminal.
| Symptom | Likely Cause | Fast Check |
|---|---|---|
| Light only switches from one location | Common and traveler swapped on one switch | Find the dark screw and move the feed or load there |
| Light stuck on | Feed tied to load, or both switches selecting the same traveler all the time | Separate splices; verify travelers land only on brass screws |
| Light stuck off | Open traveler, loose wirenut, or lamp failure | Meter continuity of traveler pair; replace lamp |
| Breaker trips when toggled | Neutral and hot crossed at the fixture or in a box | Trace neutral path; fix any white used as hot without tape |
| Smart companion will not work | Special lead on wrong terminal or missing neutral | Match labels on device sheet; verify neutral presence |
Three Common Wiring Layouts, Mapped In Words
Feed At Switch A, Lamp After Switch B
A cable brings the hot and neutral into Switch A. The hot lands on Switch A’s common. A 3-conductor cable runs between Switch A and Switch B to carry the travelers and a neutral if present. From Switch B a 2-conductor cable runs to the lamp. At the lamp, neutral from the feed meets neutral to the lamp, and the switched leg from Switch B’s common meets the lamp’s hot. This is the layout many installers meet in modern work because it keeps all switching functions in the boxes.
Feed At Lamp, Switch Loop Down To Switches
The hot and neutral arrive at the fixture box. The neutral ties to the lamp. A 3-conductor cable drops from the fixture to Switch A, then continues to Switch B. One conductor in that drop brings constant hot down to the common of Switch A; a different conductor brings the switched leg back up from the common of Switch B to the lamp. The remaining two conductors serve as travelers. Whites in those drops can be re-marked when used as hots so that a later helper does not misread a neutral where there should be none.
Feed At Switch B, Lamp Before Switch A
Sometimes the feed enters at Switch B, and the lamp sits between the switches in the cable path. In that case the common at Switch B takes the feed. The common at Switch A carries the switched leg back toward the lamp. The travelers still link the like-colored screws. Keeping the mental rule “feed on one common, load on the other” helps in any of these wiring paths.
Safe Testing Checklist
- Open the breaker feeding the circuit. Lock or tag it when possible.
- Confirm dead at the box with a tester on each conductor and at the device screws.
- Pull the switch forward without detaching wires until you tag the odd-screw conductor.
- Use insulated tools and keep one hand clear of metal parts around the box.
- When you restore power for tests, keep conductors spaced and capped.
- Shut the breaker off again before moving any conductor from one terminal to another.
Neutral Paths And Old Switch Loops
Homes built in some eras ran a two-wire loop to the switch, sending power down on a white that had been re-purposed and bringing the switched leg back on a black. That loop did not include a neutral at the switch box. Many smart devices need a neutral, so that older loop may not work with them without extra work. If you meet a white on a switch screw, it may be a re-purposed hot; add a wrap of tape so the next person sees its true role.
Pro Tips For Reliable Results
Label every conductor you move. Work with good light and take clear photos before any disassembly. Replace any chipped devices or cracked plates. When a box is shallow, a switch with side-clamp terminals can reduce stress on the copper. When you join copper of different gauges, use a wirenut rated for that mix, and tug each splice to be sure it holds.
When To Call A Licensed Electrician
Bring in a pro if a box shows signs of heat, if aluminum branch wiring is present, or if you open the box and find more cables than will fit back safely. A pro will also sort out multi-way setups that involve travelers crossing between floors or unusual conduit colors that don’t line up with common patterns.