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Disclaimer: I definitely do not know what I am doing and my skill level here is minimal.

Background: I want to install a smart dimmer switch for a light fixture in a house originally built around 1907. The switch box contains the three wires shown at the bottom of this post.

Initial testing with a voltage detector seemed to indicate that the white, black, and plain wires were hot, load, and ground, respectively, with no neutral. The plain wire was in any case connected to the green screw on the current light switch, so it presumably it is ground (right?). I wired up the switch based on that assumption and it did not power on. I procured a $60 Klein Tools auto-ranging multimeter to investigate further - I have no context about the quality of this device. This tool says that with power on, the voltages on these wires are as follows:

  • White -> plain 53V
  • Black -> plain 45V
  • White -> black 116V

Assuming the plain wire is ground, or supposed to be ground, of course the smart switch refused to work (it expects ~120V from hot to ground, not 53). Obviously something is wrong here. If it's my understanding of what these voltage readings are telling me, what am I missing? If it's the wiring (specifically "ground" being nowhere close to ground), are there additional actions I can safely attempt to try to determine what's going on with this wiring, or should I consult an electrician? Given this situation, how safe or unsafe should we consider use of this switch (and by extension the wiring in the home as a whole) to be?

Wiring box

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  • Normal/expected readings are hot(black) to ground ~120v, white(neutral) to hot ~120v, white to ground 0v. 116v is in range but a bit low. H to G at 45v is a bad ground to panel connection. Ground is just for your safety, every device smart or not will work without it, you just might die without it.
    – crip659
    Dec 18, 2022 at 0:00
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    @crip659 The problem is that some smart devices rely on ground instead of neutral because they (a) don't use much power in standby mode, (b) got UL/ETL/NEC to sign off on it and (c) intend to be a solution for old no-neutral-available switch loops. For those devices, ground must be close to neutral or it won't work. Dec 18, 2022 at 0:26

1 Answer 1

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Based on your voltage measurements (Klein is a good brand), it sounds like you have a bad ground. Since the cable here looks good, that could be anywhere from the other end of this cable (probably at the light fixture as this is a switch loop, more on that below) or the main breaker panel or anywhere in between.

I would start by checking voltage at some grounded (3-prong) receptacles. If you get similar voltages (hot to neutral ~ 120V, varying numbers around 1/2 of that for hot to ground and neutral to ground) then the problem is likely at the main panel. If most receptacles give better values (close to 120V for hot to ground, close to 0V for neutral to ground) then the problem is at the fixture or some other junction in this circuit.

The main panel is where it gets more complicated than just "a loose wire". The main panel should have:

  • A connection between neutral and ground - often this is just one special screw to connect the neutral bar to the back of the panel case.
  • A ground wire going to one or more ground rods.
  • A ground wire going to a metal water pipe (not as common in new installations, jurisdiction dependent).

Switch Loops and Smart Switches

Many houses (mine included) have many switches using cables with black, white and bare ground wires going from a light fixture to a switch. This is called a switch loop. It is a great idea because if your cables are all running in the ceiling, you just drop one cable down to the switch instead of using two cables (one for incoming power, one to the fixture). However, that does not provide neutral at the switch. No problem for simple switches, but smart switches need constant power and the preferred way to do that is with neutral. Without neutral the old standard was to leak some current through the light when "off", but that doesn't work well with LEDs (flashing or dim glow instead of appearing "off").

The alternative, when permitted by UL (safety of the switch) and NEC (maximum current allowed on ground), is to use ground like a neutral. Based on your description, the new switch works that way, except it doesn't work because ground is not properly connected to neutral (either at the panel or at a junction someplace else.)

In order to avoid exactly this problem, newer versions of the NEC require neutral at most switch boxes. Which typically means that a switch loop needs a cable with 3 wires + ground (black hot/white neutral/red switched hot/bare ground), eliminating much of the advantage of a switch loop (saving on wire). But existing circuits do not need to be changed, so many people are experiencing this problem as they add smart switches.

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    Thanks for the fantastic answer. The outlets I checked all had good voltages. The switch in the "other" bedroom (on the same circuit) has the same bad voltages, but the hallway switch on the same circuit presumably before the two bad bedroom switches has good voltage (and the smart switch works normally there). So I guess the problem is either between the hallway and bedroom wiring or that both bedrooms are miswired in the same way. Either way it sounds like something I'll need professional help on, but at least it makes a lot more sense now. Thanks again!
    – cbmanica
    Dec 19, 2022 at 1:01
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    @cbmanica great work on the process of elimination. You're saving yourself dollars by reducing the time an electrician needs to spend finding the cause. Next, try and see if the good hallway point goes to one or other bedroom, or somewhere else. Then clean and re-terminate the connections. You may surprise yourself, just take your time and think it through. There's no rush when its your own time.
    – Criggie
    Dec 19, 2022 at 2:38

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