What Color Is The Common Wire On A Light Switch? | Simple Wiring Guide

What “Common” Means On Switches

Switch hardware tells the story. Manufacturers mark the common terminal with a black screw or a “COM” label. Travelers ride on brass screws. Ground lands on green. Once you know the terminal’s job, you can match it to the right conductor in your box with confidence.

Common Across Switch Types

Different switch types route power in different ways. The first table shows where the common terminal ties in and how the device marks that screw. This broad map solves most “which wire is which” puzzles before you even reach for a meter.

Why The Common Is Not Neutral

Neutral is the grounded conductor and has its own color rule. In the U.S., white or gray identifies neutral by code. That color is reserved for neutral and should not be used for hot unless it is re-identified. If you see a white on a switch screw, it might be a re-marked hot from an old switch loop. The neutral color rule appears in NEC 200.6.

Identifying The Common Wire On A Switch Safely

Color can help, labels help more, and a short test makes the call. Use this simple process whenever you face an unknown switch box.

Prep And Tools

• Non-contact voltage tester
• Two-lead meter or a small test lamp
• Insulated screwdriver, wire labels, and a marker
• Phone or camera to record the starting layout

Step-By-Step Process

1. Switch off the breaker and confirm the box is dead with the tester.
2. Pull the device forward without unhooking the conductors. Note the screw colors.
3. Find the black screw. That is the common terminal on a single-pole or a 3-way.
4. Restore power for testing only. Keep one hand clear of metal parts.
5. Meter from suspected hot to ground or neutral. The always-hot feed reads live. If the conductor on the black screw is not live, that common is the switched leg to the light.
6. Kill the breaker again, then label the common. If a white served as hot, add tape or sleeving at both ends before you close the box.

When Colors Mislead

Houses change. Painters overspray. Tape falls off. A red may land on the black screw in a multi-wire cable. A white from a loop may sit on common and never got re-marked. None of that blocks you if you test and tag. Terminal markings and a meter beat paint every time.

Common Wire Color On A Light Switch: Regional Variations

Color practice depends on local rules and long-running habits. The device still points you to the common with its black screw, yet it helps to know what colors you are likely to see in your area.

United States And Canada

In many homes the conductor on the common screw is black because black frequently serves as a hot. Red often appears in multi-wire cables and 3-way runs, and it may land on a traveler or the common depending on layout. Neutral is white or gray by rule, and equipment ground is green or bare copper. Makers echo the black screw cue on their sheets; see the Lutron wiring tool for a clear sketch.

United Kingdom And Europe

Harmonised colors standardised brown for line, blue for neutral, and green-yellow for protective earth. Many British switches carry “COM,” “L1,” and “L2” on the body. The conductor on COM is the feed or the switched leg. Where a blue core functions as a switched line, it should be sleeved brown at terminations. A short IET note on color changes is here: IET Wiring Matters.

Australia And New Zealand

Modern work uses brown for active, blue for neutral, and green-yellow for earth. Older work may show red for active and black for neutral. As with every region, the terminal tells the truth: the common screw is the one to follow, not the paint on the insulation.

Hands-On Tips That Save Time

Map The Box

Snap a photo and draw a quick map before you move a single conductor. Sketch the cable sheaths and list which colors live in each sheath. Add the screw colors from the device. That map makes reassembly painless and helps the next person.

Tag The Feed First

Once you find the always-hot, label it. Future upgrades, like a smart dimmer or a timer, become far easier when your feed is already tagged.

Watch For Re-Marked Whites

Many older switch loops brought a white to the box with no neutral present. That white acted as a hot and should be re-identified. If the tape is gone, add new tape or heat-shrink at both ends after you test.

Use The Common Screw To Sort A Crowd

When three or four cables spill into a small box, the black screw gives you an anchor. Trace that conductor back into its sheath so you can match it to its neutral and ground partners in the same cable.

Table: Regional Color Reference

This guide lists common practice for household circuits. Rules can change by jurisdiction and age of the work, so test and label every time.

Mistakes To Avoid

Mixing Up Common And Neutral

Neutral returns current to the source and must follow its own identification rules. The common terminal is part of the hot path. Landing a neutral on the common creates faults and nuisance trips. Keep those roles separate and marked.

Swapping Travelers With Common On A 3-Way

If a 3-way stops working after a swap, the common likely moved to a brass traveler screw. Move it back to the black screw and retest. Hardware sheets show this layout with clear symbols; the Lutron diagram is a handy reference.

Trusting Paint Over Tests

Old pigment fades, new paint covers insulation, and tape dries out. A few quick checks give you certainty. Verify, label, and leave the box tidy for the next upgrade.

Smart Switches And The “Common” Label

Smart dimmers and sensors often keep the same terminal language as standard devices. The common screw stays black. Some models add push-in ports and pigtails, yet the job does not change: the conductor there is either the feed or the switched leg. Read the tiny legend near the screws, match the black screw, and tag the conductor you land there.

Quick Scenarios And Fixes

The Screw Is Black But The Wire Is Red

That can be correct. Red frequently serves as a feed or a switched leg in a multi-wire cable. The terminal’s job is what matters, not the paint on the jacket.

My White Wire Sits On The Common Screw

This shows up in older switch loops. If that white acts as a hot, re-identify both ends with tape or sleeving, then mark the box cover so the next person sees the change.

The Switch Body Has No Clear Markings

Use the test steps provided and tag the always-hot. If the device seems worn or flimsy, replace it with a new unit that has clear markings. Pick a brand with a printed sheet that matches your layout.

Sources And Further Reading

Two items guide fast, safe decisions: manufacturer sheets and national rules. For a clear sketch of screw colors and terminal names, see the Lutron wiring tool. For the neutral color rule in the U.S., review NEC 200.6 on the ICC site. For UK color changes that set brown, blue, and green-yellow, see the IET’s note: All Change — New Colours Agreed.