Replacing a 3-Way Switch: Which Wire Goes on Which Screw

A replacement 3-way switch can leave a circuit working in only one position, or not at all, when the wires land on the wrong screws. A helper trimming out a house swapped an older device for a new one, and the 3-ways stopped working. Lights in a different room were switching, even though none of the wiring had changed.

What each screw does

The device has a black screw, two gold screws and a green ground screw. The black screw takes the incoming hot, or the conductor that carries power on to the next device. The other two conductors are the travelers. When the switch is flipped, power moves from one traveler to the other, and that is how a 3-way works.

If the incoming hot is put on a gold screw, the other black feeding the switch will not pass power through. Because the hot is then on a traveler, the switch only works in one position.

Taking the old device out

Power comes off first at the breaker. Some devices have a screw with a hook bent around it, while others, such as many Decora devices, have pressure plates that are tightened down. With plates, no hooks are bent.

The box in the demonstration also served as a junction box, with many other wire nuts inside. The advice is to leave those alone and take apart nothing but the device connections. Four wires should end up on the four screws, with the ground going on the green screw.

Finding the travelers if you lose track

The two travelers always come in the same sheathing. In the example, a 12-3 cable carried the travelers, which went on the two gold screws. A conductor coming from a different direction, running down to the bottom of the box in a 12-2 cable, is the hot or the leg, and it goes on the black screw. This works even when drywall hides the cables.

Putting it back

The black screw is wired first, so it does not get mixed up again. It does not matter which traveler goes on which gold screw. A screw gun does the initial tightening, followed by a final pass with a screwdriver. A torquing screwdriver is needed only if the device has torque specs, because code requires installing per manufacturer's instructions when those exist.

The ground conductor must not touch another terminal, or it causes a ground fault. A ground fault is a hot touching grounded metal. A short circuit is two hots, or a black and a white, shorting each other. After the device is back in, the breaker goes back on.

Some unusual 3-ways, such as dead-end setups, may be wired differently, but most problems with 3-ways come from the mix-up described here.