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TL;DR: What does it mean when an OFF switch causes the White wire to become HOT as well as the Black wire?

I’m replacing and adding a light fixture in my kitchen. The previous owner(s) did some DIY remodels, and since I have everything pulled apart, I’m trying to figure out how to re-wire some things as “properly” as possible. Or, really, just understand what is going on (as much as possible with the wires I have access to).

simplified-kitchen-diagram

  • The switch (S1) operates all lights and fans in the kitchen, as well as the light above the cooktop
  • The light above the sink was removed before I bought the house, so I assume that it is also operated by the switch as well
  • As far as I can tell, the sink wires (C) feed the cooktop light (B)
    • B only has power if I connect C to D
  • Yellow (A) is HOT
  • With the switch (S1) ON, Black (D) is HOT
  • With S1 OFF, Black (D) and White (D) are BOTH hot

So, my main questions:

  1. Does Yellow (A) and Black (D) both being hot mean they’re tied together somewhere, or is something weirder potentially going on?
  2. Why does the White (D) go HOT when the switch is OFF?

Q: What could possibly make a wire go from 0.0V to ~120V when a switch is turned from ON to OFF?

Switch:

switch switch-wires

Ceiling/Ducting/Soffit Thing:

ducting

Sink: sink sink hole

Cooktop: cooktop

The wires are essentially dropped through a couple holes in the "ceiling". I say "ceiling" because it's a weird almost "ducting" space that runs the circumference of the kitchen (I don't think "soffit" is the right term?) Again... weird, old DIY renovations from the 70s probably played a part in all this.. I'm sure there's hard conduit running through it, and maybe that's where I would find all the tie-ins or junction boxes folks are asking about. But, as you can see from the pics all I can see from the kitchen are the trailing ends of the wire bundles I've diagramed, and no hint at to where they go. I'm not prepared to start pulling off the paneling, knocking holes, or going into the attic and digging down into the kitchen/weird-duct-space. At least, not for this one effort of replacing a bulb (and outside of just my quest for just general knowledge)... there is probably a larger kitchen remodel in my future.

So, ultimately, maybe there's just not enough info, and I'll just put things back the way they were (was probably going to anyway)... but I just won't have an answer, and move on with my life :)

isherwood
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Dura
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    If the breaker is somehow between the switch (S1) and the lights then something is seriously wrong with your wiring. How about clear, focused pics of the actual devices in question showing us how the wires are connected to the devices and where they go into the boxes. If you're comfortable, take the cover off of the panel as well and get a picture of the wiring on the breaker in question, too. [Edit] them into your post. – FreeMan Mar 03 '22 at 20:45
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    ...that you are using one of those non-contact testers and it's not very good at it's job, probably - most of them are not. The "similar common error" would be switching neutral, rather than hot, but it does not actually match up on all points to the described readings. – Ecnerwal Mar 03 '22 at 20:49
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    When you say switch off, are you going by the standard down is off or a label? Might be possible that the switch is turned upside down(done that). – crip659 Mar 03 '22 at 21:13
  • A switch leg is one place the white should be always hot and the black is the switched hot, prior to the 2017 code this was the standard . A switch leg is when there are only your 2 wires and possibly a ground at the switch. Power enters the circuit at the light fixture then white to the switch is connected to the black from the panel. Later code versions required the white to be marked as a hot (tape or sharpie) but this was overlooked a vast majority of the time. If it works don’t change it unless you know it is wrong. I have probably had over 100 calls where a diy turned into pro rework. – Ed Beal Mar 03 '22 at 21:31
  • Given how we've seen your NCV tester working in your house in previous questions; this switch's lack of grounding; the original problem being a flickering light... then you can either check the tightness of all the connections and put everything back... or turn this into "pro work". – Mazura Mar 03 '22 at 23:34
  • "take the cover off of the panel as well and get a picture of the wiring" - or if it's all exposed in the basement, at least just a picture. I'd like to know the house's scheme a little better. Mainly if it's EMT or a mixture of spaghetti, if "the previous owner(s) did some DIY remodels"... – Mazura Mar 04 '22 at 00:19
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    White (D) go HOT when the switch is OFF? was answered in the last question. That's an NCV being weird. - The diagram is wrong. Nothing goes from the panel to the switch (if it does it's just using the same EMT) .... D is the incoming and feeds the switch, which feeds back to where D is. - We have a picture of the switch : now do one of where D is. In that box is a bunch of neutrals nutted, two wires nutted that come in and go out, and the wire you're left with is the one that powers the light. – Mazura Mar 04 '22 at 00:26
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    @Mazura — I wasn’t testing with an NCV (thanks to the other discussion haha) Everything in this diagram is tested and confirmed with a multimeter. – Dura Mar 04 '22 at 02:46
  • @FreeMan -- Sorry, it's just a relative diagram, the breaker isn't "between" them, I was just trying to show that when the breaker is ON... both the wires I have access to Yellow (A) and Black (D) are hot, I just put the breaker in the middle for display. I have no clue "really" what goes on in the space between all the devices. – Dura Mar 04 '22 at 02:51
  • @Mazura — I don't think this question about (D) was answered, because I was asking about the switch in the other thread... you're saying "it's your NCV being weird", but I'm not testing the fixture (D) with an NCV, I've listed what my multimeter is telling me with the switch ON and the switch OFF, and I'm just trying to understand what's going on (even though I probably won't end up changing anything). Also, there's no "box" to take pics of (again, DIY crap) it's literally just those C and D bundles dropped through a hole in the ceiling :( – Dura Mar 04 '22 at 02:56
  • @crip659 -- I installed the switch, and I'm super anal about that so, when I say the switch is OFF, that means the label on the switch says OFF. (So, unless they messed it up at the factory, it should be OFF haha) – Dura Mar 04 '22 at 02:58
  • @Ecnerwal -- I'm not using an NCV (anymore) -- when I list them as HOT in this thread... that means my multimeter is showing ~120V when I connect to a known earth ground. – Dura Mar 04 '22 at 03:00
  • Are you really, really sure you have a yellow wire there? Either a) it is a white wire that has yellowed, or b) it is actually yellow and has writing on it, and, it's in the "conduit wiring method" meaning going down a pipe. There needs to be a junction box, unless the fixture provides this, check the installation instructions of the fixture. DVMs can be over-sensitive and read phantom voltage - but where are you putting the other probe? – Harper - Reinstate Monica Mar 04 '22 at 07:46
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    @Dura a good number of the questions for you here in the comments would be addressed by the pics requested in my very first comment. Clear communication can be very difficult - I run into it all the time at my job, and it's entirely possible for two experienced pros in the same industry to miscommunicate to say nothing of pros talking to amateurs. Pictures explain a lot without any words at all to confuse people. – FreeMan Mar 04 '22 at 12:30
  • @Harper-ReinstateMonica -- I'm sure it's a yellow wire with writing (illegible) on it. I've added pictures for clarity, there is conduit, but the box itself is apparently VERY ungrounded (from another thread). When I'm testing voltages I put the red probe of my DVM on the wire, and the black probe on a known, good ground on another circuit (eg, an outlet on the other wall or something). If this method has any chance of giving me false readings, then I really don't know what I'm doing here, and I'm kind of screwed anyway lol – Dura Mar 04 '22 at 18:53
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    It has a metal box. The wires being odd colors and having writing on them mean they are in conduit. Some conduit is metal, and carries the ground in the conduit shell. However, there are huge problems with the box installation or complete lack of boxes here. The switch would have grounded through the mounting screws, but unfortunately someone replaced them with drywall screws, ruining the threads. Boxes are not allowed to be inset when the surrounding material is flammable. Even in non-flammable drywall, only 1/4" is allowed. They make extension sleeves to fix this kind of problem. – Harper - Reinstate Monica Mar 04 '22 at 20:26
  • Describe how you are making your measurements. A volt meter with probes across a switch will measure 120 with the switch open or off and 0 with the switch closed – Ed Beal Mar 05 '22 at 15:35
  • @EdBeal -- My understanding is to put the red probe from my DVM on the wire I want to test, then the black probe on a known proper ground, preferably on another circuit. So, this is what I do. – Dura Mar 07 '22 at 17:10
  • Dura, if you are on the switched side of switch then you are measuring 0 because through the lamp you are at ground so no voltage. Turn the switch on and now there will be 120 with reference to ground. THIS IS BASIC ELECTRICITY. Ok it’s reversed from that again basic there will be 120 across an open switch and 0 across a closed switch there are only 2 real possibilities here in this case and both are simple, across the switch or across the load. – Ed Beal Mar 07 '22 at 20:22

2 Answers2

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A wire will go from 0 to 120V when a switch is turned from on to off:

If the switch is improperly in the neutral line between the load and the neutral, as opposed to being in the hot line between hot and the load.

When the switch is off, the fixture cannot draw any current, so the entire thing goes to 120V from hot, though the fixture, and on the neutral wire up to the point of the switch.

When the switch is on, the load is drawing current and the voltage is dropped across the load, and the neutral wire is connected through the switch to neutral.

Ecnerwal
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    Not entirely correct but no comment on a down vote gets a upvote from me. it all depends on where the measurements are being made. Sounds like a diy switch leg or wired backwards to code in AC there is no difference how the probes are connected as a note to the OP – Ed Beal Mar 07 '22 at 20:34
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All I see is a no-contact voltage sensor in use. Those devices are NOT a reliable way to determine if a particular wire is hot. Neutral wires can easily pick up enough voltage from stray inductance and capacitance from actually hot wires to light those things up. This is especially true when there are non-grounded circuits involved in any way.

kreemoweet
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