What Is a Wire Nut?
The twist-on plastic cap that joins two or more wires inside an electrical box. The color tells you which wire sizes it fits.
A twist-on connector with a tapered metal spring inside a plastic cap. Hold the stripped wire ends together, screw the cap on clockwise, and the spring bites into the copper and twists the wires into one joint. The color codes a size range (yellow for two or three 14 or 12 gauge wires, red for bigger bundles), and the package lists the exact combinations. A wire nut is the right way to add a short pigtail to a wire that's too short to reach a switch or outlet terminal.
A good joint passes a firm tug on each wire without moving, and no bare copper shows below the skirt of the cap. If a wire pulls out, cut the ends fresh, strip 5/8 inch, and start over with the next size up. Push-in lever connectors do the same job and are easier for beginners, but a wire nut costs pennies and is what you'll find in most US boxes.
Fixes that use this
Electrical
How to Replace a Light Switch Safely in 20 Minutes
Replacing a single-pole light switch takes a $3 switch, a screwdriver, and a voltage tester. Kill the breaker, prove it's dead, move two wires, done.
Time20–30 min Cost$3–$25 moderate