Replacing a switch with a dimmer

[SarahFair]

I'm replacing my dining room switch to a dimmer switch.

Currently there are 2 switches, if youre looking at the switch, the right one turns the light on and off and the left one does nothing. The right one will work regardless of the position of the left switch.

I'm guessing at one point they had a light fixture with a fan as the dining room was treated as a 2nd livingroom.

I opened it the box to install the dimmer switch and noticed that the right switch had 2 black wires and a grounding wire and the left switch has 2 red wires and a grounding wire.
The black wire is labeled neither positive or negative, just black wire, no writing, no groves on the wire.
The switch is the type you just put the wire directly into the back of, it's not labeled either.

Sorry for the odd photos, the room is dark and I had to use the flash, which created shadows.

What now?

[Gothichome]

Sara, good to hear from you. It sounds like you have a three wire cable between the switch and the fan. It would seem the power is sourced in the ceiling box and the each switch just complete one leg of the circuit, either fan on/off and the light on/off. I bet you will find the red wires disconnected in the ceiling box as they abandoned the fan. If not it should be properly abandoned. It might be wise to get up there and have a look. Once you assured yourself the reds have been abandoned properly do the next person in there a favour and mark them abandoned. Your dimmer just replaces the light switch assuming it a simple push on/off and turn to dim. Do you have a multimeter?

[SarahFair]

I got it figured out. I realized I had grabbed the wrong type if switch, so I went back to home depot to swap it out and saw an employee in the aisle and asked him.

He said all I should need was a single pole switch and the black to black was fine, which was what the hookup was on the back of the switch.

Now Im battling fitting the new faceplate to fit.
The wall is paneling over plaster and lath and the box is a little squeezed in there so the dimmer switch is sitting slightly crooked and the more you straighten it, the deeper it "sinks" backwards

I'll figure it out

[1918ColonialRevival]

They make extensions for switch boxes to bring them flush with the surface of the wall. Or, you could get longer screws for the faceplate.

[Gothichome]

Sara, glad you got it worked out.

[PaulJohnson]

Sara - you figured it out perfectly!

Typically, only the black wires connect to the switch. When the switch is in the off position it breaks the circuit of the black wire. When it is in the on position it connects the circuit.

The earlier post re: the red wire and labeling it for the next person who visits the box is a great habit.

[phil]

Its normal for the white to be neutral and black to be hot, in the case of a light switch the electrician needs to run a piece of romex to the switch so in that instance one of the wires is used as a hot. usually a good electrician will wrap it ( probably the white) with colored tape to signify that the color isn't being used as "normal" .. so if you see colored tape on some of the wires don't just remove it because it is there to signify to another electrician that the white wire may be hot in that application.

this is normal practice because the electrician cant; open up the romex and change colors and in that application one needs to be hot when the switch is on.

the electrician needed to feed the box plus have 2 returning ( switched wires)
to do that I think he just needed three wires. Im not sure why he would run two 3 wire cables. maybe his thinking was that he would use the blacks for one and the reds for the other application and he may have fed one red and one black and returned the other red and black and maybe just clipped the white lead short? if the wires are run in conduit then the electrician can choose the colors.

the red wires come about when using 3 wire, then there is a neutral and a black and a red. he likely ran two pieces of 3 wire down to the switch. You can see the wire where it enters each box and confirm that's what he did. I cant; guess but I would wan to know why so many red wires appear, maybe it is feeding something else or being backfed from something else? maybe one of the switches was involved in a three way situation like in a stairway where you have 3 wires connected to each switch? maybe it is switching an outdoor plug for xmas lights ?

If you dont understand how the wire is being used or why you have extra circuits you should stop and consult an electrician. don't just guess.

if the three wire is connected to a double breaker back at the panel then the white neutral may be shared between the two power circuits. each is on a different "side" of the panel so this is possible, basically they are out of phase and can share the neutral and that saves wire. the inclusion of two reds and two blacks means there is likely more involved and it might take some looking to figure out for certain where the wires in the box you are working on all terminate.

you can remove the switch and put a blank plate rather than a switch doing nothing. if you do it do it correctly. If you are stuck there is no dishonor in finding out whats going on, but Id' encourage you to at least know for sure that what you are doing is correct, and if you are confused, or unsure just stop.

some boxes can be screwed to the studs from inside the box some nail outside the box. youll find a bunch of types. use the type you can screw in through the opening so you dont damage the drywall surrounding it. If it's difficult to position as the screws want to move it I'd put a dab of epoxy on the outside of the box and clamp it in place. the epoxy will just help it not shift so much when you return and tighten the screws. You can use screws with countersunk threads at an angle even though that seems weird because they aren't really countersunk , but they enter at an angle because your screwdriver needs to approach the screw at an angle. you can anticipate that the screw will want to pull the box back a little when it tightens up. you can predrill with a small long bit if it helps.

in the above the bare grounds were not mentioned , it is understood that they exist and are bonded to each box. these are for safety and should never be part of the power circuit.

if old screw holes interfere then plug them with toothpicks ( or splinters of wood ) and glue so you can reposition as you like.

i wouldn't leave a dead switch, it doesn't meet code. If you are saving cost by doing your own wiring I'd take the time to do it so it does meet code. I know its a small thing and not particularly dangerous if it's safely capped off.

is the breaker controlling it a double breaker or a single?

there is a handy tool called a sniffer. its about 10 bucks and the size of a sharpie, if you put it near a hot wire it beeps. You should have one in your pocket and then you can check if each wire is live or dead to confirm because sometimes people make errors and so it's a confirmation , for your safety and of course you should also have the breaker off and locked out particularly if there is any chance someone could turn it on while you are working on it for 10 bucks if it stops you from touching a hot, even once, it's money well spent.

a meter does more, and the sniffer isn't a substitute for a meter Its only a handy gadget. If you dont own and know how to use a voltmeter and have the ability to perform basic simple checks for continuity , presence of power or resistance checks then you shouldn't be messing with wiring. technically if you dont have an electricians ticket you are already doing things you shouldn't I think many here would argue that a homeowner who is fairly well educated with house wiring may be able to do things like replace a worn part. Not my point to argue where these lines should be drawn, just be aware that if you do wiring and then if there is a fire trhere will be all sorts of questions.

I know some because I did wire my own house and I do electrical work on machinery but for example if a neighbor asks me to help even replace a outlet or add a light, I wont do that because it can get me into a tricky legal position, so I'll just say flat out, no you need a qualified electrician for that.

if things get really confusing the electrician will likely have a tool that emits radio waves into a wire and that's handy for tracing where they go when they can't be seen. he can then detect when he is near or far from the wire because it is transmitting a signal that can go through walls. he could disconnect the breaker and connect that then trace where that wire goes.