I'm trying to change my light switch to a Leviton dimmer switch (4).

I checked with a voltage detector pen...

  1. Both bottom two wires connected with my old switch has power when switch is off. Does that mean there are 2 hot wires? How do I connect it with the dimmer?
  2. The upper wire connected with old switch has no power when switch is off but has power when switch is on. So I assume it is the load wire.

I can't find any ground wire connected to old switch, but there is a red wire in the wire box connected with nothing. Should the ground wire to connect with Leviton green wire?

enter image description here

  • Are you sure both bottom black wires are separately hot? I'd have expected an incoming hot and a downstream circuit load. The red wire might indicate the former presence of a three-way switch, or it could be a disused multi-wire branch circuit, or it could have been a switched outlet load.
    – isherwood
    Dec 11, 2019 at 18:07
  • Can you post a photo clearly showing the inside of the back of the box please? Dec 12, 2019 at 3:23

3 Answers 3


Those two bottom wires are connected internally in the switch, so if one is hot they both would read hot while connected to the switch. One of the wires daisy chains the hot to other outlets. Both of those wires need to be connected with the hot to the dimmer switch. You may need a larger wire connector than one that might have come with the switch. I know this is a DIY advice webpage, but if those two bottom wires are both hot when disconnected from the switch then it isn't a DIY problem.

Back some time ago in the previous century it used to be accepted practice to not ground light switches, those days are gone. Some dimmers require a ground to function properly, try folding out the wires in the box to locate bare wires that should be connected together and to the box. If you cant find a ground wire in the box then the ground might be accomplished by a cable assembly or raceway system that grounds the box. If you can identify that one of those systems are used you will need to find a 10-32 green ground screw to put into threaded hole in the back of the box.

Also it looks a little like the glue on the tape on the red wire may be failing, you might want to replace the tape or put a correctly sized wire connector on that before you tuck it back into the box.


There's no correllation at all between "the wires in the box" and "the wires that you need".

The red wire is useless; we don't know what it is, and it won't be used. It needs to be safely capped off and pushed into the back of the box.

The type of switch you have uses "screw-and-clamp" terminals, which allow 2 wires per screw. That simply means the screw is also being used as a splice between those two wires. These should be wire-nutted together, with a pigtail to the switch. Note that the new switch has pigtails naturally.

The single wire on its own screw is the switched-hot to the lamp. I would mark that wire with blue tape, and also mark the pigtail on the new switch that is for the switched "load" side. Now, hooking up inside the box is as easy as matching colors.

On the ground wire, the universal colors for grounds are green, yellow-green, or bare. All grounds must be joined together in the box. Most likely the ground is mandatory here, so you need to find it in the box.


Thanks for all your advice. As both of you said, I looked at the wire box again and I found 2 bare wires screwed at the very back of the wire box. So I connected my green wire to it.

Now it's time for me to connect all the wires. And it works! Yeah!

enter image description here


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.