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I have a new Legrand dimmer switch. It has a connection for a ground.

However the box it needs to go into appears not to be grounded.

The dimmer is model LSCL453P. The load is an LED light about 30 watts. This is not in an office (not bathroom or kitchen). The switch cover is plastic.

The voltage between the two wires is about 100V AC. The voltage between one wire (left in the picture) and the box is about 50V AC. The voltage between the other wire and the box is about 40V AC.

Legrand dimmer switch Legrand dimmer switch Legrand dimmer switch the box

Is it going to work ok if I just connect the two wires?

Is that going to be any less safe than the alternative of just putting back the old on-off switch?


Edit: Some more investigating, it seems that the box is at about 40 volts AC relative to ground. So the basic premise of the question ( that the box is not grounded) is correct, but not in the way I assumed (i.e. that the box is not electrically connected to anything else in the world). I fear this has gone beyond a DIY project and calls for on site help from a qualified electrician.

Another edit. Putting an ammeter between the box and a trusted ground gives 0.2mA at about 40V. Resistance between the box and a trusted ground is very high. So I'm fairly sure the 40V is a parasitic voltage caused by capacitative coupling (and maybe some inductive coupling too) between the BX armour and the hot wire running through it. Likewise, past a certain point everything else on that circuit that should be grounded isn't. So now I'm back to where I started. The box simply isn't grounded. The lesson for me is to check both the voltage and amperage against a trusted ground. Just for safety's sake, not to mention getting the dimmer to work, I'm going to ensure that everything that should be grounded is.

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    actually it looks like your house uses conduit, which is an acceptable grounding path. there might be a threaded hole in the back where you can attach a grounding wire. Nov 25, 2020 at 15:51
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    Use a multimeter to check (carefully!) voltage between the hot wire and bare metal of the box. If it shows a steady 120ish then box is grounded. Model number of the dimmer? Nov 25, 2020 at 16:13
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    Usually the smallest holes in the box are threaded 10-32 for grounding screws.
    – Ecnerwal
    Nov 25, 2020 at 16:16
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    @isherwood they've been threaded in every box I've ever seen. Nov 25, 2020 at 16:54
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    That looks like a clamp and standard cloth to me , you really can’t say conduit with what can be seen. I have run into several all plastic switches lately even the yoke is plastic in this case the appropriate thing would be to pigtail a ground to the box. I tried searching for LSL453P to see if the ground was required (it may be) but could find no information on the Legrand web site or google searching.
    – Ed Beal
    Nov 25, 2020 at 17:38

2 Answers 2

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The OEM instructions say to use the green grounding screw. With that said it is irrelevant if the screws are grounded.

My guess is that a small current Is dumped on the ground to make the dimmer function properly this is not unusual, if the box is grounded through a conduit the switch may work, if it is not grounded and the switch doesn’t work it is now legal to pull a new ground from an existing circuit or going back to the panel as long as the ground is pulled from a device fed from the same panel. If you only have a main panel you could tag any other circuit that was grounded and be code compliant.

I have run into a few of this style switch that uses the ground as a return for the switch. This is rated for small loads of cfl or led with 10w being the minimum load for proper operation.

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  • Thanks. After more looking into it and making more measurments, I'm not fairly certain that the switch box is floating and has a 40V parasitic voltage on it. It should be grounded via armoured cable, but that armour itself isn't grounded. I'm getting an electrician in to figure out why that cable isn't grounded. Once that's fixed, the dimmer switch should be straight forward. Nov 27, 2020 at 16:24
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If it's there, the switch will pick up ground via its mounting screws.

The rule of grounding is if there's a metal box, the wiring must ground to the box as its first priority. Ground to the box first, only then have pigtail grounds to receps and switches if they're needed (which they're not on switches; certain receps need that).

Keep in mind, some modern dimmers require a ground to perform properly. They have gotten permission from UL to use ground as a current return, because their device inherently will be leaking too little current to possibly matter, and has internal safeguards so no foreseeable damage or failure will cause more. The upshot is that if ground is no good, the device might not function.

I agree that it looks like metal conduit or possibly AC cable, which provides a ground path to the box. (and again the rule is "must ground the box first").

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    If the plate is plastic the screws won’t do anything and a pigtail would be required to ground. I have run into this several times lately and I thought the yoke was required to be metallic but not true. In both cases the ground was required for the switch to operate
    – Ed Beal
    Nov 25, 2020 at 17:41
  • Thanks Harper and @EdBeal . The plate seems to be metal painted with insulating black paint. So I'll use a pigtail if the box turns out to be grounded. My main concern is what to do if the box isn't grounded. Nov 25, 2020 at 19:16

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