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My house is from the 1950's and I've been replacing some light switches with smart ones that require a neutral wire. In my bathroom, the wiring appears to be a switch loop like the image below. There's no neutral in the switch box which is necessary. I'd rather avoid running a new cable from the light to change the configuration. Can I leverage a neutral wire from another device on the same circuit breaker?

On the other side of the bathroom wall, there's a switch for an hallway light. They're both on the same circuit breaker. It's a 3-way switch and is the secondary. In these houses, they ran dual 2-wire cables. So, there's an unused white wire in that switch box. A continuity test confirms the other end is in the hallway light fixture box.

My idea is to leverage that wire by connecting it to the neutral wire bunch at the hallway light fixture box. Then I can jump it over to the bathroom switch box with a short cable in the wall. Would this be sound and safe to get neutral to that switch?

image

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    If you're going to supply the switch and light from a new location (disconnecting/capping the current supply at the light), that's fine. If you're looking to just bring the neutral from one box to another by running a neutral wire, then no.
    – FreeMan
    Jan 8 at 16:40
  • Ok you've definitely answered my initial question. Following up on running the new supply to replace the existing one in my bathroom. Conceptually, the secondary switch should have the hot wire running to it, correct? (Need to verify whether that is actually the case). So as a replacement feed, my cable to the bathroom switch (from hallway siwtch) will connect off the hot black wire and the unused white wire (which will be get connected to neutral at the hallway light)? And the exiting feed at the bathroom light D/Cd. Jan 8 at 17:51
  • The answer to this question is "no". If you'd like to ask a new question, then please do so. That will give plenty of room to discuss possibilities and give you full answers as opposed to doing things here in comments. Please take the tour to familiarize yourself with how things work around here.
    – FreeMan
    Jan 8 at 18:02

2 Answers 2

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To provide neutral at the switch box you either:

  • replace the cable to the switch box
  • or you use a special version of "smart switching" equpment where you make the white to the switch box neutral again, connect it to the smart switch, and the smart switch tells a relay at the light location to actually turn the light on.

Borrowing a neutral (even from the same circuit) violates keeping neutral and hot/live in the same cable, which is required. If your source switch can provide unswitched hot/live and neutral then you could supply the switch that way, with both. Just neutral, no.

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  • If OP has a hot also at the second switch box, Would they be able to run a hot plus neutral cable from there to the first switch box?
    – crip659
    Jan 8 at 16:45
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    Yes. See @FreeMan's comment on the question and the last paragraph of my answer.
    – Ecnerwal
    Jan 8 at 16:46
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Ecnerwal's answer is (as usual) 100% correct. But two additional things to consider:

  • Currents must be balanced in every cable.

This is why you can't use a neutral from a different part of the same circuit. While the entire circuit will still be balanced (so if you have a GFCI/breaker it will be fine), the neutral wire you use will no longer be balanced with its partner hot/switched hot/traveler wires.

In theory you could grab hot and neutral from the hallway box, if actually present (I don't think they are in your case, since you said "secondary" which to me implies panel->switch 1->switch 2 (=secondary)->hall light), which would typically have switched hot and neutral in switch 2 box, but not "always hot") and rewire your bathroom light to not use a switch loop. However:

  • Your 3-Way is Likely ILLEGAL

A 3-way switch must have all 3 wires in the same conduit or cable. But you said:

they ran dual 2-wire cables

and that's just totally wrong. Each cable must be balanced but you have:

  • Cable 1 = common + traveler 1
  • Cable 2 = traveler 2 + spare "unused white wire"

Common may be hot, switched hot or neutral, depending on configuration. Which means that if your light uses 1A then when it is on you either have:

  • Cable 1 = +1A/-1A
  • Cable 2 = nothing

which is perfectly safe. Or:

  • Cable 2 = +1A/nothing
  • Cable 2 = -1A/nothing

which is unsafe. (+ and - flip back and forth 120 times a second, consider this as current at peak of each cycle.)

Based on additional comments, it appears to be one /2 cable, so this cable alternates between:

  • 1A/nothing
  • nothing/1A

which is definitely not good. And that means the feed to one switch is always hot/nothing and the feed to the other switch is switched hot/nothing.

As I understand it, wires near each other need to be balanced to avoid problems. Wires that are far from each other (like old knob & tube) are OK in that respect. But in any case, this violates modern (many decades) code.

The proper wiring with cables is either a 3-wire cable (most common) or a 4-wire cable (rarer, but becoming more common as it allows for always hot + 2 travelers + neutral, which is needed in some situations).

So stop worrying about the bathroom smart switch and instead work on (you can ask a new question for help, with pictures please) the possibly problematic 3-way circuits.


FIX THE 3-WAY!

The good news is that thanks to smart switches, it is actually possible to fix the 3-way switch problem and get hot/neutral over to the bathroom.

Based on OP's description, it looks like the 3-way switches are currently configured as follows:

  • Hot and neutral from panel to hall light fixture box (shorthand: hall light)
  • Hot (by itself - second wire not used) from hall light to switch 1
  • Two travelers (black/white in one cable) from switch 1 to switch 2
  • Switched hot (from switch to hall light) and neutral (from hall light to switch box but not used) from switch 2 to hall light
  • Switch 2 box is physically close to bathroom switch
  • Bathroom is a standard old-style switch loop - hot/neutral to bathroom light fixture box, hot/switched hot to switch

Based on this:

  • At hall light, connect incoming hot to black wire going to switch 1 (already done) and incoming neutral to white wire going to switch 1 (new). Disconnect incoming neutral from light fixture and from the white wire going to switch 2
  • Remove switch 1. Wire black to black and white to white. Install a wireless remote (e.g., Lutron Claro Smart Accessory Switch) that is compatible with the new switch 2.
  • Replace switch 2 with a smart switch (e.g., Lutron Claro Smart Switch). Connect hot and neutral to the wires (formerly travelers) from the switch 1 box (but not actually connected to switch 1). Neutral also connects to the white wire going to the hall light. Switched hot connects to the black wire going to the hall light.
  • At hall light, light fixture connects to switched hot and neutral from switch 2.

To extend to the bathroom:

  • Disconnect bathroom light fixture from incoming white neutral. Cap the incoming black hot and white neutral wires as they will no longer be used.
  • Connect the bathroom light fixture to the cable that is going to the switch box, both black switched hot and white neutral.
  • Use a short cable (with appropriate clamps, etc.) to connect hot and neutral from the hall switch 2 box to the new smart switch for the bathroom light.
  • Connect the bathroom light black to the new bathroom switch black switched hot.
  • Connect the bathroom light white to the new bathroom switch white neutral and white from hall switch 2.
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  • Appreciate your detailed thoughts on this. I'm going to post a new question, but first wanted to pull the thread on what you mentioned with traveler arrangement. I went through the kitchen 3way circuit in depth previously, and both the travelers were in a single /2 cable. Is that also unacceptable? Jan 8 at 19:52
  • Travelers in a /2...where is the hot or always hot or neutral - in a separate /2? Or is the /2 with the travelers the only cable between the 3-way switches? Or did they use the bare ground in place of something else? Jan 8 at 19:53
  • Sorry if that wasn't clear. Travelers in a single /2 and then each switch has another /2 that heads to the lightfixture. Notably, the unused wire going at the secondary switch (white) was actually connected to the neutral group at the lightfixture. Jan 8 at 20:14
  • That's definitely not good. But if that's the case then you should have "always hot" in to one switch (with other wire unused), travelers to second switch, "switched hot" to the fixture, with the other wire unused (though as you say, connected to neutral). Is that correct? Jan 8 at 20:21
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    Too late to edit my last message, but it finally clicked in my head that balanced wires prevent induced currents from the alternating fields. Jan 8 at 20:39

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