Short Answer
Complete Explanation
In residential wiring, the term “common” on a light‑switch device identifies the terminal that serves as the primary connection point for either the incoming power (line) or the outgoing load, depending on the switch’s function. The designation is most commonly seen on multi‑way (three‑way or four‑way) switches, where the common terminal is distinguished from the “traveler” terminals that carry the switched voltage between devices.
- Function of the common terminal:
Acts as the point where the hot conductor from the service panel is attached on a single‑pole switch, or where the load conductor is attached on a three‑way switch configuration. - Identification:
Typically marked with the word “Common”, the abbreviation “C”, or a distinct screw (often black‑colored) that differs from the brass‑colored traveler screws. - Wiring context:
In a standard single‑pole switch the common connects to the line (hot) wire, while the other terminal connects to the fixture (load). In a three‑way arrangement, the common on one switch is connected to the line, and the common on the other switch is connected to the load. - Safety considerations:
Because the common terminal may carry full line voltage, it must be correctly identified and secured before any wiring work is performed, and power should be de‑energized at the breaker.
Common Misconceptions
The “common” terminal is always the ground connection.
The common terminal carries the hot (line or load) conductor, not the grounding conductor, which is attached to a separate green screw.
All switches have a common terminal.
Simple single‑pole switches may label the hot side as “common”, but many decorative or toggle switches omit the label because the wiring is straightforward.
FAQ
What is the difference between the common and traveler terminals?
The common terminal connects to the line (incoming power) on one switch and to the load (light fixture) on the other. Traveler terminals simply carry the switched voltage between the two switches and do not connect directly to the power source or load.
Can I use a switch without a labeled common terminal?
Yes, many simple single‑pole switches do not label the common because the wiring is straightforward: the black (hot) wire attaches to one screw and the load wire to the other. However, correct identification is still essential for safety.
Is the common terminal always black‑colored?
While many manufacturers use a black screw to denote the common terminal, color coding is not universal. Always refer to the device’s labeling or wiring diagram to confirm the correct terminal.
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