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In the diagram below, a 2-wire NM cable supplies power from the panel to the first switch box. The black (line) wire connects to the common terminal of the first 3-way switch.
A 3-wire NM connects the traveler terminals of the first and second 3-way switch together. Traveler wires are interchangeable on each switch.
The common terminal of the second 3-way switch connects to the light fixture(s).
The white neutral wires are connected together in each switch box.
Diagrams shown on this page are simplified for clarity. Electrical outlet boxes can have numerous NM cables going in and out. See Actual Switch Box Wiring Diagram.
Ground connection diagram is shown separately.
With conventional wiring, line voltage enters the first 3-way switch box
Line voltage enters the light fixture outlet box. No longer allowed after 2011 NEC (if no neutral wire in switch boxes)
Line voltage enters the first 3-way switch outlet box, light fixture is located between switch boxes. No longer allowed after 2011 NEC (if no neutral wire in switch boxes)
Black common wire is usually wrapped around the two traveler wires on a 3-way switch.