I'm attempting to install a smart light switch in an existing light switch box. There's isn't a ground wire I can see, nor does my voltage detector light up when I touch one end to hot and the other to the box. My outlets are all grounded, so could it be that my light switch box is not? This condo was built in 2004 in the Chicago area, so I imagine a ground of some kind must exist.

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    @GlenYates, not sure if you're suggesting that tongue-in-cheek or not, but to clarify for others finding this question: don't do this! A safer way is to test continuity to ground with an ohmmeter.
    – spuck
    Commented Mar 12, 2020 at 17:00
  • If you are doing the hot-to-box test properly, and it doesn't light up, then the box is not grounded.
    – Hot Licks
    Commented Mar 12, 2020 at 17:11
  • I haven't been able to follow much of what's been said, however my contribution is this: if a wierd voltage is being read it may be a 'ghost voltage' arising from unintentional inductance of multiple a.c. conductors.
    – user113301
    Commented Mar 12, 2020 at 19:46

4 Answers 4


You have metal conduit connecting metal junction boxes.

All of it is ground. The conduit, the boxes, all of it. This is the ideal system.

Switches ground through the yokes and mounting screws. They don't need ground wires.

Receptacles are not allowed to do that unless they yoke has hard clean flush metal-metal contact with the box, and yours won't because the box is painted. However, in the back of the junction box will be a hole that is tapped #10-32 for a ground screw. Any #10-32 screw will do, but they sell little green ones very cheap. Then you can just run a recep ground to the green screw.

  • Is it normal that I'm able to observe some voltage on the black wires even when I've turned off the circuit at the breaker? Commented Mar 12, 2020 at 4:27
  • @dfitzgerald possibly, there are 2 potential reasons for that, one harmless, one pretty bad. Can you find which breaker de-energizes the black? Is it right next to this breaker? Commented Mar 12, 2020 at 8:05
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    So why did the empirical test fail? What are the chances of a botched installation where the conduit does NOT provide a good earth?
    – MikeB
    Commented Mar 12, 2020 at 12:09
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    @MikeBrockington I'm guessing he was testing a switched hot tht was off. Also, there's a question about which breakers de-energize the box so he might have been testing a dead circuit.
    – JACK
    Commented Mar 12, 2020 at 12:33
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    @TTT I don't like guessing about research I haven't seen myself, but my wild-guess is a receptacle has to handle ground current for anything plugged into it which could be anything. A switch does not. Commented Mar 7, 2021 at 23:16

Some of those wire compo's look strange so I'm guessing this in a conduit installation. the box should be grounded. Make sure you're going from box to always hot and not switched hot when testing for ground. Try reading from some of those black wires back in the box.

  • Yep, the block wires to the box is showing voltage, so the box is grounded. The black is showing voltage even after I turn off the power at the breaker, is that expected? Does that mean this is line? Or is it load? Commented Mar 12, 2020 at 2:39
  • What kind of voltage are you reading? Full 120V or somewhat less? Commented Mar 12, 2020 at 2:52
  • @Harper I only have a detector that shows either 120V or 220V (it was very cheap), but I'm seeing 120V Commented Mar 12, 2020 at 2:54
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    @dfitzgerald: If a hot wire has voltage in it after the circuit breaker is off then either you're switching off the wrong breaker or the breaker has failed into the unsafe mode due to an internal fault. Either way, figure out what is going on before you electrocute yourself. Commented Mar 12, 2020 at 22:11

What is the third wire entering at the top left? Is that uninsulated copper? If so, it's a ground wire.

  • If you zoom in, it looks insulated, tan or brown. Love the cat.
    – JACK
    Commented Mar 13, 2020 at 13:07

It appears that the device you took out had more than two wires on it. Is /was it a three way switch or a switch plug combination?? How many screws (for wires) are on your new device? From my 40 years at that trade, i would fire whoever made up that box.. The bare copper is surely grounded -- I just wonder where. Jim.

  • If you zoom in, that wire in the upper right isn't a bare copper ground and only a white and red wire have been disconnected. There are some shadows that look like wires.
    – JACK
    Commented Mar 12, 2020 at 22:39

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