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First off ---- I am by no means an electrician.

That said, I have a dimmer switch that needs to go..and be replaced by a new one. However, upon searching the internet and reading the instructions several times over, I am stuck. The dimmer being replaced is from 1978 (original to the house). I have attached photos of the new one, the old one (still attached the house) and the instructions. All the help they have been was saying, "consult an electrician if what is on the wall doesn't match the paper..."

Please note that the wires attached to the switch are all black, they just have over-spray from painting on them. Additionally, there is a ground in the back but I don't know which is the positive or which is the negative.

I appreciate any and all help! I can easily provide more pictures if needed or information.

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J Crosby
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  • How many fixtures does the existing dimmer control? – ThreePhaseEel Jun 06 '19 at 01:48
  • Looks like you are in luck in that you have neutral available in the switch box. The three white wires in the same wire nut appear to be neutrals. Your old switch did not require neutral, but your new one does. – Jim Stewart Jun 06 '19 at 01:50
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    @Jim Stewart, I just answered assuming no neutral needed. Where on the new dimmer do you see a neutral terminal? – Jimmy Fix-it Jun 06 '19 at 02:00
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    There is only one fixture on the circuit. – J Crosby Jun 06 '19 at 02:07
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    @JimStewart the new one does not need neutral. – Harper - Reinstate Monica Jun 06 '19 at 06:51
  • If you're feeling like this is beyond you, remember there is **NO** shame in getting a pro to do the work. Electricity done wrong can go very wrong. – Criggie Jun 06 '19 at 09:19
  • Yeah, I saw the instructions mentioning the neutral so without reading further I just assumed it was being used (confused with modern *timers* and *smart* switches which *do* require a neutral). – Jim Stewart Jun 06 '19 at 13:30
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    @Criggie, it's not about "shame" in asking for a pro to do it. It's about wanting to learn. I have changed out switches before and outlets too. But those were always in newer builds (like no older than 2013) so I have simply not seen the way this box was tied in before. I have no problem calling a pro when I need it. But I know with a little guidance (which numerous people have provided here) I can do it safely and not burn down my house. – J Crosby Jun 06 '19 at 14:31
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    Just a quick note, in AC (alternating current) wiring, you don't really use the terms "positive" and "negative". It's normally talked about in terms of hot, neutral and ground. Reason being is that the current switches between positive and negative 50-60 times per second (depending on where you are in the world). – JPhi1618 Jun 06 '19 at 16:03
  • @JPhi1618 Thanks for that insight! I appreciate it. As I prefaced in my original post I am by no means an electrician and appreciate all the help and insights I can get. Seeing as the current switches constantly how does a guy go about figuring which is which? Wouldn't a contact tester show both as hot? – J Crosby Jun 06 '19 at 17:01
  • Think of it as a loop where electricity comes from the hot side and really wants to go to the neutral side. When no electricity is flowing (nothing plugged in, no light on) then the "hot" side is where electricity would come from. Neutral (in many systems) is the same potential as ground so there's just nothing there if no power is flowing. Positive and negative don't matter and a device that uses AC current can be hooked up either way. For safety they are hooked up a specific way (polarized plugs), but its not because it cares about + and -. – JPhi1618 Jun 06 '19 at 18:25

2 Answers2

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You have a "single-pole" arrangement, i.e. one switch controlling the load (the light). You simply need to turn off the circuit breaker controlling this circuit, remove the two wires from your existing dimmer switch and place them on the two wire terminals on the new dimmer switch (the terminals that are NOT green). It does not matter which wire goes to which terminal.

There does appear to be a ground wire (bare) under a screw in the back of the box; if possible you should attach a short green or bare wire from the green terminal on the new dimmer to that same screw (or alternate ground screw) in the back of the box. This is not required for the new switch to work; it is a safety feature.

Attach the dimmer and cover plate to the box, then turn the circuit breaker on. Done.

Jimmy Fix-it
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    The ground is not necessary. A switch can ground via the metal box and mounting screws. – Harper - Reinstate Monica Jun 06 '19 at 06:48
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    From the looks of that box there is no hope of pigtailing onto that ground. Also looks like the grounds are not tied together but connect only through the box - that's definitely not allowed anymore. Drives me mad, mid-century electricians... never leave enough wire in the box and cut the grounds down to little stumps. If you're lucky you can sometimes pull enough staples to fish yourself an extra length from the wall. In this case it's not strictly necessary, but it kills me when crap workmanship turns a five minute job into a weekend teardown. – J... Jun 06 '19 at 11:44
  • @J... so you are saying that I shouldn't try to tie in some extra ground wire (which I happen to have) into the existing ground? Other than the obvious (the set screw may not be long enough) why is that? – J Crosby Jun 06 '19 at 14:11
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    @JCrosby No, you'll get sufficient grounding from the metal screw attachment. It's not ideal, but it's allowed and reasonably fine. Trying to mess with those ground stumps will just be more headache than it's worth. Critically, it looks like downstream grounding is relying on those back-box connections so if they're firmly in place as it is I wouldn't risk breaking those connections only to re-make them worse than they already are. To fix it properly you would need to pull more wire into the box (if there's enough in the wall) so that you had enough ground wire to pigtail them all together. – J... Jun 06 '19 at 14:18
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Based on the instructions and Single Pole installation (i.e., one switch rather than 2 switches), this should be quite simple:

Line - Hot

This is one of your black wires. Hard to tell from the old dimmer *and it may not have made any difference on the old dimmer which wire was hot and which wire was switched hot. It might not matter on the dimmer, but you would have to read the rest of the instructions to figure that out. If it does matter, you can tell by turning the old dimmer off and then checking with a non-contact tester. Whichever wire lights up is hot and the other is switched hot.

Neutral

The installation instructions mention neutral but don't actually use it. So you can ignore that. As it turns out, you do appear to have neutral (the whites all together), which matches the diagram, but it actually doesn't matter in this particular case.

Ground

If you don't have any ground wires then you can, provided everything else was done right (ha ha ha!), ground to the metal box using a ground screw with a pigtail like. However, with switches (as I understand it, for some reason, not with receptacles), you can ground from the metal switch yoke to a metal box by simply screwing it in place, provided there is no paint on the box or paper spacer on the screws or anything else that would prevent a solid, electrically conductive, connection.

Load - Switched Hot

This is the "other" black wire.

As far as "all black": Hot, switched hot and traveler (for 3-way switches) can be any colors except: White (allowed in certain situations), Gray, Green or bare wire. Black + White is the most common type of cable, so "all black" is typical, albeit confusing. But even if the colors were Blue & Red, that wouldn't actually be any more informative.

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manassehkatz-Moving 2 Codidact
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    No need to run a ground wire, it will ground via the mounting screws to the metal box. – Harper - Reinstate Monica Jun 06 '19 at 06:49
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    There does appear to be a ground-wire connected to the box. You can barely see it in the 4th picture. – JimmyJames Jun 06 '19 at 15:59
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    In fact, there appears to be a second ground screw in the back of the box next to the one that's tied into the house ground. I suppose you could run a green or bare copper pigtail from that to the dimmer. Given the apparent corrosion on the screw tabs, I'd be in favor of that over relying on the dimmer's metal plate to make good electrical contact with the box. – Adrian McCarthy Jun 06 '19 at 19:34
  • @Harper: that's true *if and only if* you remove the insulating paper washers, but it's still better to attach a ground wire. – Tim Sparkles Jun 06 '19 at 19:50
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    @Timbo in that installation the drywall ears will hold the yoke proud of the box, and the washers will be irrelevant. (And the grounding path unsuitable for a receptacle, regardless). Metal-metal contact yoke to box is not required for switches. – Harper - Reinstate Monica Jun 06 '19 at 19:59