strapper

A strapper is a conductor, usually a single solid copper wire or a cable run, that interconnects the traveler terminals of two three-way switches so that a light can be controlled from either location. In a standard three-way installation, the strapper carries the switched leg signal between the switches; it is not a live wire itself but rather a messenger between two control points. The strapper is essential to the logic of three-way switching and has no role in a simple on-off switch pair.

The term arises from its literal function: it "straps" or binds the two switches together electrically. On each three-way switch, there are two traveler terminals (sometimes marked as traveler 1 and traveler 2, or simply labeled on the switch body). The strapper connects the first traveler on switch A to the first traveler on switch B, and another strapper connects the second traveler on switch A to the second traveler on switch B. These two conductors work in parallel, and at any moment only one of the two paths is active, depending on the position of the switch toggles.

Installation and Materials

Strappers are typically run inside the same conduit or cable as the hot and neutral wires serving the light circuit. In residential work, they are often part of a three-conductor plus ground (3/C+G) cable run between the two switch boxes. The gauge matches the circuit; a 15 amp lighting circuit uses 14 AWG (or 12 AWG if required by local code), while a 20 amp circuit uses 12 AWG. The strapper must be the same gauge as the other current-carrying conductors in the circuit, not undersized.

Common mistakes include breaking or pinching a strapper during installation, which leaves one switch unable to control the light from that location. A severed or corroded strapper creates an open circuit in one of the two paths, making the light uncontrollable from one switch. Testing with a multimeter across the traveler terminals will confirm continuity. The strapper cannot be omitted or replaced with a different conductor type; the wiring diagram for three-way switching depends on having two distinct parallel paths.

The strapper sits in the middle of a larger switching chain: the source hot wire enters the common terminal of switch A, the strappers carry the switched logic to switch B, and a final wire from switch B's common terminal feeds the light. Four-way switches, used when a light must be controlled from three or more locations, also employ strappers but use different internal wiring logic. For troubleshooting or renovation work, identifying the two strappers and verifying their connections to the correct traveler terminals is the first step in diagnosing three-way switch failure.