Dimmer switches usually have two black wires protruding from the back of the switch. One wire connects to the light (s), the other connects to the line. The green wire connects to ground. If a dimmer has one black wire and one red wire, usually the black wire connects to the line and red wire to the load.

A dimmer switch needs three wires for a single-pole switch: one black, one black or red, and one green (ground). The remaining dimmer switch wire (usually, red/white striped) is not needed for single-pole operations, so it is capped off.