I have the following wires in a box for a switch. I'm trying to replace the switch with a smart switch but cant figure out which is neutral. I thought it was the white one, but when I unbundle the white white, the light no longer turns on when I toggle the switch.

Also if I measure the voltage between the load and the box and it comes up as 120V, does that mean the missing ground wire isn't a problem?

US wiring.

enter image description here

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    If you really want to know which is hot and which neutral, you could buy a "live voltage sniffer" smile.amazon.com/gp/product/B01DFN42VG (circuit breakers have to be on for it to work). GL! – rogerdpack Aug 22 at 15:26
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    You really should have a better understanding of residential wiring before attempting work like this. There are plenty of books and online resources. Learn the basics before you start messing with your homes wiring or plumbing or anything else. – JPhi1618 Aug 22 at 15:59
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    If you do not understand why an open neutral disables the circuit then stop doing electrical work until you do understand. Apparently you had an open neutral just hanging out in a box while the hot was powered; that is very dangerous. You are putting yourself, other residents, and your structure at risk. – Eric Lippert Aug 22 at 20:48
  • Please try to avoid asking two questions in one question; usually only one gets answered. – Eric Lippert Aug 22 at 20:50
  • I can only underscore what Eric Lippert has said. If you don't understand electric circuitry, then messing around with this kind of thing can void your insurance policy and get you into legal trouble. It can also cause your house to burn down, and kill you or the people you care about. Please stop this at once. Electrical work is something that should be left to the professionals. – Dawood ibn Kareem Aug 22 at 22:32

You've got hot and switched hot on the existing switch. If the box is grounded properly then you will get 120V from hot (or switched hot when it is switched on) to the box. You will also get 120V from hot to neutral when the switch is off. If I understand it correctly (and Harper or one of the other real experts will correct me if I'm wrong), the circuit is actually:

hot -> switch -> switched hot -> light fixture -> neutral

The problem is often, particularly in older installations, that neutral is not present within the box because it is not needed by older switches. But neutral is needed by the light - just often that neutral will bypass the actual box since it wasn't (without a smart switch and prior to current code) required inside the box for any reason.

In your case, if I understand the pictures correctly, multiple black (hot) and white (neutral) wires nutted together. That indicates you have some additional circuits - e.g., outlets or more lights controlled by another switch - that are sharing the hot & neutral with this light switch & fixture. That is perfectly normal, but the result is that the neutral for the light fixture depends on those white wires being together. Removing the wire nut breaks the light circuit, and probably some other outlets or lights elsewhere in your house.

Bottom line: The white wires nutted together should be neutral, and you should be able to add another wire (pigtail) from the smart switch into that bundle/wire nut.

One more thing to be watch for: Hot vs. Switched Hot

In an ordinary single (i.e., one switch, as opposed to 2 switches - which is actually called 3-way) switch, it does not matter in any practical way which wire (hot vs. switched hot) is connected to which screw. However, with a smart switch you must make sure that hot and switched hot are connected according to the switch instructions. Hot is the wire that goes to the wire nutted bundle of blacks. Switched hot is the black wire (it doesn't have to be black, but it is in your particular case) going from the switch out of the box all by itself. If your switch instructions refer instead to line and load, line == hot and load == switched hot.

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    Thanks, I think this does make sense, I didn't realize the light, or anything else needed neutral, though that makes complete sense now that you say it. – ATG Aug 22 at 1:48
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    The confusion is because "regular switches don't need neutral but smart switches need neutral" masks the fact that all circuits need neutral because in the end any properly functioning circuit (except certain 240V circuits) uses a neutral somewhere - just not necessarily in the switch box. – manassehkatz Aug 22 at 1:53

Yes, there are also other cables heading off other directions to carry power onward to other loads.

The clump of all-blacks is always-hot.

The clump of all-whites is neutral.

You see where one wire on the switch is a short wire that simply goes to the always-hot clump. That style of wiring is called a "pigtail". The wire is also always-hot, and if the smart switch needs to know this, it is Line.

Disregarding green/bare ground, the 1 remaining wire on the switch is switched-hot or Load if the switch cares about that.

If the switch has more than 2 screws for (non-ground) wires, then it's a multi-way switch and life gets a lot more complicated, and the above advice would not apply.

Sometimes you see a switch with only 2 screws, but more than 1 wire going to one of the screws. This is using the switch as a splice block. In that case it's better to convert it to a pigtail, such as what you see in this box.

I believe the white wires are indeed neutral. The home run hits this box, where black is switched, then continues to the light. (The other wires are other branches, maybe to other switches / lights, maybe to receptacles, etc.)

When you disconnect the neutrals in the box, you disconnect the neutral going to the light, so it's expected that it would not light up.

This appears to be an armored cable that uses the armor for grounding, such as type AC or MCAP, that would explain why there are no grounding conductors entering the box.

type AC cable

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    @AmandeepGrewal Yes, all of this is Alternating Current (not just US, worldwide, though with differences in voltage, frequency and wiring schemes). AC here refers to Armored Cable as opposed to NM Non Metallic, or other types. – manassehkatz Aug 22 at 2:11
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    BTW Armored Cable is not the norm in US households. – sleblanc Aug 22 at 12:27
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    Have to give + I saw the insulator and metal box also and agree.+ – Ed Beal Sep 3 at 23:09

You've asked two questions in this question; try to avoid that. If you have two questions, ask two questions.

Your second question was:

if I measure the voltage between the load and the box and it comes up as 120V. Does that mean the missing ground wire isn't a problem?

Define "problem".

First of all: this is evidence that the box is grounded, which is good; it should be grounded. The fact that the box is grounded does not imply that all is well with the grounds. Here are some things that could be wrong in your scenario:

The box could be grounded via an armored cable. That is not compliant with code in some areas. Why is this potentially a problem?

  • A future inspection might note that it is not to code and you might have to fix it.
  • People doing electrical work sometimes assume incorrectly that working circuits are to code, and make modifications which would be correct in a to-code circuit, and incorrect in a not-to-code circuit.

The box could be grounded completely by accident. Perhaps it is touching a pipe that is in the same wall, and the pipe is grounded. Why is this a problem?

  • As in my previous point, you do not want to be in a situation where a future modification to the plumbing changes the grounding of the box.
  • Suppose we are in this scenario and the plumbing was never explicitly grounded to the electrical system anywhere but here. Now suppose there is a fault somewhere else that causes the plumbing to have a different ground voltage than the electrical ground. You can now be in a situation where there are two objects, both allegedly grounded, that have potential between them, which could shock a person. (This situation is common in older houses; the first modification I made to my 100 year old house was bonding the plumbing and electrical systems together so they had a common ground.)
  • Do we know that the plumbing, or whatever it is that is grounding the box by accident, actually can safely carry a fault current to ground such that the current is strong enough to trip the breaker? Maybe there is resistance; remember, conductors heat up and metal becomes less conductive when it is hot. Maybe there is a gap somewhere in the system that will spark, and ignite a mouse nest in the walls? Who knows? The emergency safety system only works by accident in this scenario.

Is everything probably fine? Yes, it's probably fine. But that wasn't your question; your question was do we know that there are no problems with a safety system? No, we do not know that; we're guessing that there are no problems with a safety system. Absence of evidence of a problem is not evidence of absence of a problem!

  • Thanks, I've now called an electrician to check the ground and do the work to install the new switch. Thanks for taking he time for the detailed answer! – ATG Aug 23 at 6:57

The white wires most likely are neutrals. They're nutted together because the circuit continues on to other outlets - you need to disconnect the wire nut, add a new wire, and then nut the new wire and the old wires together. Then connect that new white to the receptacle.

Note that disconnecting the neutrals on a powered circuit that you know nothing about is a bad idea - if that was a multi-wire branch circuit, you could have sent 240v to everything on it. Be careful.

  • Hello, and welcome to Stack Exchange. Thanks for the answer; hope to see more from you. – Daniel Griscom Aug 23 at 2:11

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