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How do I Wire a 120V Indicator Lamp?

Jan Benschop

Indicator lamps exist to tell you whether something is either on or has power going to it. When you deal with 120 volts AC, all outlets built into houses since the 1950s are polarized and have a "hot" leg and a neutral leg. The energized "hot" leg is the narrow blade on polarized plugs. Some plugs are not polarized and have two narrow blades. In-line switches and wall switches always interrupt the hot leg when turned off, if they are wired correctly. The best place to connect an indicator light is between the switched side of hot and neutral.

Indicator lamps come in many shapes and sizes
  1. Use a commonly available neon induction probe to test the AC source for which you will install an indicator light. Test which leg is actually wired "hot" (the probe will light), and make a note.

  2. Unplug the device you are wiring with the indicator, or if you're working with a hard-wired connection like a wall switch, turn off the breaker or take out the fuse for the circuit.

  3. Cut the hot-side wire between the switch and the device you are powering. Strip back 3/4-inch of insulation on both sides of the cut. The indicator light fixture usually comes wired with stripped ends, but if not, strip those, as well. Wire-nut the cut ends together with one indicator wire.

  4. Repeat Step 3 with the neutral return wire and the other indicator lead. If you are wiring the indicator to a wall switch, wire one wire end under the neutral wire screw and the other under the switched-side screw, along with the wires that are already there.

  5. Flip the circuit breaker on or reinsert the fuse. Test the indicator by flipping on the switch. If it does not light, try changing the bulb. When everything works, mount your indicator light in place.