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What happens if you wire a light switch wrong?


Depends on what you do wrong. Worst mistake would be hooking the hot wire to a ground screw. If you did that without a functioning ground wire you could get a bad shock—and the light wouldn’t work.

Most likely case for a mistake is in a three way system where the common and a “traveller” gets mixed up. Then that switch needs to always be in one position for the light to work, instead of always being able to switch the light on or off from any switch.

On a basic single pole switch the terminals are interchangeable, so as long as you don’t include a neutral wire or swap in a ground wire you can’t really go wrong.

On a motion sensor or other “smart” light switch you need to keep track of the Line and Load wires, or it won’t have power to sense that it should switch the light on. Some of those also have a neutral connections which you need to keep straight.

You could put the switch on the neutral wire and everything would work, but it would leave voltage potential to ground in the light when it is switched off. That makes it a bit hazardous for changing light bulbs. If you became an electrical path to ground through a wet floor or something you could get a shock. Some old houses have switches on the neutrals.

Or you can just damage the wires somehow and one could break or short out or come loose.

Or you could just have all the wires crossed up and have a short circuit. That will pop the breaker or fuse right away. Or melt down some wires if the breaker sticks.