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With conventional dimmer wiring using NM cable, a NM cable supplies line voltage from the electrical panel to the dimmer outlet box.
Another NM cable connects from this dimmer box to the light fixture box. Both line and neutral travel from the switch box to the light fixture 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.
Ground connection diagram is shown separately.
In the diagram below, a 2-wire NM cable supplies power from the panel to the dimmer box. The black (line) wire connects to the common terminal of the 3-way dimmer.
A 3-wire NM connects the travelers of the dimmer to the travelers of the 3-way switch. Traveler wires are interchangeable on each switch. The 3-way dimmer and 3-way switch are also interchangeable.
The common terminal of the 3-way switch connects to the light fixture(s).
The white neutral wires are connected together in each switch box.