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My bathroom has a double switch plate: one for a fan and one for the light. I've replaced the light switch with a dimmer (Lutron Maestro Model# MACL-153M.) And I've also replaced the incandesent light with a 40" LED by Commercial Electric which is dimmable. The light won't power off, nor will it dim. It just stays on. Worth mentioning…there was no visible ground wire when I installed the switch. Only two wires were available to connect to my dimmer. So I capped the green ground wire coming out of the Lutron. Could this be why? (BTW, is this still considered a single pole even though the fan is attached to the same switch?)

This is the inside of box with my attempt at connecting the Lutron switch. enter image description here

this is the metal face of switch: this is the metal face of switch.

Under ‘Important notes’ in the Lutron instructions, it mentions: When no ‘grounding means ‘ exists in the wall box, the…code allows a control to be installed as a replacement if 1) a non-metallic, non-combustible faceplate is used with non metallic attachment screws, or 2) the circuit is protected by a ground fault circuit interruptor. When installing a control according to these methods, cap or remove green wire before screwing control into wall box.

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  • Can you post photos of the inside of the switch box please?
    – ThreePhaseEel
    Aug 12, 2020 at 11:47
  • Does the Lutron's instructions say you must hook up ground? Is the box made of metal and are there ground wires in the back of it?
    – Harper - Reinstate Monica
    Aug 12, 2020 at 14:02
  • These switches do require a ground. Sounds like a switch loop no neutral or ground.
    – Ed Beal
    Aug 12, 2020 at 14:16
  • Note that there IS a ground wire bundle tucked into the back of your box, even if it isn't apparent at first glance, so you'll need to clean up your grounding in any case.
    – ThreePhaseEel
    Aug 12, 2020 at 23:06

2 Answers 2

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This dimmer's instructions endorse the idea of just capping off the ground wire if the switch outlet has no access to ground. However it also requires non-metallic faceplate and also non-metallic faceplate screws, so you'll need to take care of that.

That means this dimmer is NOT a modern type (those either need neutral to power themselves, or a special UL variance to bootleg ground as neutral. Those won't work without ground, but that is not your dimmer, so not your problem). What you have here is the "old school" classic dimmer that powers itself by leaking power through the [incandescent] bulb. Only certain LEDs will tolerate this; the rest need the modern dimmer type that self-powers.

Lutron has a list of recommend LED bulbs; a few bulbs from Commercial Electric are on it. This one is not.

I would start by rolling it back to incandescent (or just temporarily hooking up an incandescent light for testing purposes). If the dimmer now works properly, you know the dimmer is reliable.

Next, I would try temporarily wiring up BOTH the incandescent and the LED fixture. This is a trick we use a lot for diagnosis. Now, see if the LED fixture dims correctly with the incandescent in parallel with it.

If it does, then the dimmer is using the traditional strategy of leaking current "through the bulb" to power itself. Of course you can't very well go leaving an incandescent bulb in your lighting; those things burn out! And when they do, your LED will be back to the bad behavior, and it won't be obvious why. Lutron actually has an answer for that; the "LUT-MLC" incandescent bulb simulator module. This can be wired in parallel with the LED and left up in the junction box.

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  • Thanks for your amazing response, although I'm afraid its over my head. I'll try calling Lutron to see if they can talk me through it. (I really don't want to disconnect the LED fixture as it has those new clear plastic thingys that seem to be one-use only connectors. If there's an easy way to 'release' those, I would do as you're suggesting.)
    – lesleycasson
    Aug 12, 2020 at 16:20
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    Well you could try to add the LUT-MLC, but I don't see any reason to be held hostage by some thingies just because they came with the fixture. Wire nuts are cheap. By the way, in the future when the system asks "Are you sure you want to answer your own question?" always answer NO. This site uses a Q&A format that keeps answers separate from discussion (which has the advantage that answers stay clear and pure). If you prefer a discussion forum try diychatroom.com.
    – Harper - Reinstate Monica
    Aug 12, 2020 at 16:41
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It sounds like a switch leg to me I regularly used to run 12-3 or 14-3 from bathroom fixtures with the white being hot and marked black then the fan was black and the red was the light this is quite common so you only have a hot and a switched hot available if no ground the switch won’t work.

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