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I’m replacing a light switch in an old house with lots of uncolored wiring. I removed the switch plate to find two black wires (connected to the switch) and a capped up white wire (see photo below).

Is that white wire ground or neutral? What’s the difference? What happens if I think it’s a neutral wire but it’s a ground instead? (For what it’s worth, I’m installing a wifi switch that requires a neutral.)

switch

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    An electrical expert will weigh in with an informed answer. But I don't see anything in that photo that looks like ground. Only neutral. Neutral is the wire that current returns to the panel on when a circuit is being used. Ground is for shunting unwanted voltage. Both are needed, but the latter may not be present in older wiring. Adding it requires new wire; don't try to use the neutral as an alternative to ground. Your wifi switch probably wants both neutral and ground, and you should provide both. It's possible that metal box is grounded. – Peter Duniho Sep 26 '20 at 18:33
  • @PeterDuniho Helpful, thanks! Neutral is traditionally white, right? – Sam Sep 26 '20 at 18:35
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    Yes. I believe it's more than just tradition. There are rules governing wire colors, and neutral is required to be white or gray, if I recall correctly, and these colors are prohibited from being used in other applications unless some additional marking is used (i.e. colored electrical tape at the connection). I don't have the code memorized like some others here; I'm sure within the next 24-48 hours, you'll get an actual, informed answer to the question. – Peter Duniho Sep 26 '20 at 18:38
  • @PeterDuniho To address your comment about whether the Wi-Fi switch needs both neutral and ground: the instructions say that it requires a neutral but ground seems to be optional. Does that make sense to you? Honestly though, the instructions look like they were translated from Chinese maybe, so I don’t exactly know if the translation is trustworthy. – Sam Sep 26 '20 at 19:19
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    There are at least two considerations: operational and safety. It's entirely possible that the switch can operate successfully without a ground connection. But whether doing is safe, and especially whether doing so meets the electrical code requirement, I don't know. A regular (mechanical) switch typically won't need a connection to ground. But once you have current passing through the switch back to neutral, it's possible that a ground connection would be needed for safety. Products may or may not provide accurate information with respect to legal requirements (i.e. code). – Peter Duniho Sep 26 '20 at 19:45
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Green, yellow-green, or bare are supposed to be safety ground only.

In North America, neutral is required to be carried on white or gray wires.

If neutral is not needed in a cable, the white can be re-tasked to be a hot wire. Code now requires such white wires be marked with tape, but most installers do not.

Hots are every other color.

You have 2 white wires in that cluster. Even though they're not attached to the switch, they're equally important. Generally a cluster of all whites are neutral.

Rarely, you'll see 1 white in a cluster of hots; that's the "cable exception" I mentioned above.

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  • Got it. Thanks. So, it’s likely those capped white wires are neutral and just spit and capped in case they’re ever needed… right? – Sam Sep 26 '20 at 21:33
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    @sam no, they're being used right now! Current flows in loops. They are the other half of the loop. – Harper - Reinstate Monica Sep 26 '20 at 21:38
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    @Sam -- as long as all the existing neutrals are still connected together at the end of the day, yes, you can add the switch's neutral to the neutral bundle – ThreePhaseEel Sep 26 '20 at 23:26
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    I'd prefer you to claim "Ground is supposed to be green..." and "Neutral is supposed to be white". Making bold claims about what wire colors definitely will be what, on the internet, when you haven't inspected the particular installation to see if the previous resident, armed with even less knowledge than the current resident has, did to the wiring "to get X Y Z to work" could have fatal consequences – Caius Jard Sep 27 '20 at 8:18
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    @CaiusJard Conceded. However the earlier edit never said "(colors) are always ground", ha ha, right... it said "Ground is always (colors)". The latter was informed by my experience here; note my rep. – Harper - Reinstate Monica Sep 27 '20 at 15:45
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You have no ground visible. Hot is switched with Black in and out while Neutral is passed thru.

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  • Ok. So just to be super clear here, that white wire is neutral? Is that what you’re saying? Thanks! – Sam Sep 26 '20 at 19:10
  • Ok. Thanks. Can you explain the second half of your comment? “Couple of volts to ground…”? I understand electrical concepts, but I’m not good with the jargon! Ha! – Sam Sep 26 '20 at 19:14
  • "couple volts" means that neutral and ground are not always at exactly the same voltage potential. There could be some small difference, resulting in "a couple of volts" potential when you compare the two (i.e. put a volt meter with one probe on ground and one on neutral) – Peter Duniho Sep 26 '20 at 19:41
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    @J... not all homes are well earth bonded to neutral on site so the connection is at the DT, which can be that for away for a 200A service – Tony Stewart Sunnyskyguy EE75 Sep 28 '20 at 15:14
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    I measured zero point one Volts on range Neutral. Good. 0.0 in garage. No change in Neutral with full load on oven and stove, but that is 240 split phase for oven. Line dropped from 120.1 to 117.5 on range outlet on full load, so 2% on Line, 0% on N. confirmed your comment. TY. Which reminds me the previous house with 2 V on Neutral had ground and neutral reversed in the attic with 16 pot lights added by some rookie for previous owner, which I fixed. I also deleted my incorrect comment. That’s only when N-G are reversed under load. – Tony Stewart Sunnyskyguy EE75 Sep 30 '20 at 15:43
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To answer your original question, if ground and neutral are swapped, when the light is energized (on) you will have electrified your entire house ground as the return path to the main panel. This is bad and dangerous. The good news is, everything looks normal in the box as mentioned previously, the neutral feed is nutted to the lamps neutral and the hots (blacks) are switched at the switch. There is no ground unless it is achieved though emt metal conduit. If this is an old box pre 1970 it it possible there was no ground installed. Why do you think the neutal and grounds may be swaped?

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  • No, I don’t think they’ve been switched… just a bit new at this. So, can I uncap the white neutral cluster and connect a neutral from the new switch to this cluster? Thanks! – Sam Sep 26 '20 at 21:48
  • There does appear to be a ground but the OP would have to test it to be sure. The cable going into the box is flexible metal conduit which is probably grounded. you can see the red insulation barriers protecting the wires from the edge of the metal. That conduit connecting to the box would be an appropriate ground. – JACK Sep 26 '20 at 22:17
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    Older switches didnt need the neutral so it just passed on into the light through a wire nut pigtail. The new smart switches use the neutral for their own power (so there will be three wires under the pigtail) . This is why newer code requires the neutrals at the switch boxes even though it didnt used to be used there. – mark f Sep 26 '20 at 22:27
  • So, just to be clear, I can uncap the white neutral cluster and connect the neutral from the new switch to this cluster? Thanks for clarifying! – Sam Sep 26 '20 at 22:50
  • Yes. The three white wires will be: one from the panel circuit wiring, one to the fixture light, and the new one to the smart switch. – mark f Sep 26 '20 at 23:55
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The black wires are your "hot" or power wires. The white are your neutrals and your ground is green, bare copper or the box if ground is present.

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