How to Find a Live Wire

Kevin McDermott

Wiring projects in old buildings can be maddening because you cannot be sure what those old-time electricians were thinking. The black wire is supposed to be live and the white neutral, but did the person who originally did the wiring project know that? What if the white wire is wrapped in black tape?

Find a Live Wire

What if there's a red wire? Don't take chances. Before replacing a switch or doing any other wiring project, know exactly where the electricity is flowing.

  1. Turn off all power at the fuse box.

  2. Go to the electrical box. Pull out and separate all the wiring, but keep the wires within a few inches of the metal of the electrical box. One exception: If there's a copper grounding wire connected to a metal electrical box on a grounding screw, leave it there.

  3. Turn the power on at the fuse box.

  4. Return to the electrical box, taking care not to touch the wires. Touch one prong of the tester to the metal of the box. Carefully touch the other prong to each of the wire ends, one after another. Note which ones set off the tester light and which ones don't.

  5. Immediately turn the power back off. Cap the live wire(s) with wire nuts and mark them with tabs of electrician's tape on the insulation to indicate electricity.

  6. Tip

    If your electrical box is not made of metal or does not appear to be grounded with the bare copper grounding wire from the electrical cable, use the bare copper wire as your ground, putting one prong on that wire and the other on each of the other wires, one at a time.


    Never touch the tester to two potentially live wires at once.

    Make sure children or unaware adults are not near the open electrical box when you leave the room to turn the power on or off.