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All of the electrical boxes at my house have every neutral tied together and all grounds separately tied together.

I understand this is standard practice because you can control 2 light switches on the same circuit in parallel rather than in series.

But what about 2 circuits and 2 switches?

My example is 2 circuits, not 1.

I have 2 circuits in a single-gang box where all the neutrals are connected and all the grounds are separately connected. This is the case in every box I've seen so far with 2 circuits. They also all have travelers (usually only white and black).

  1. The load comes in, goes into the switch.
  2. That goes to the lights.
  3. Instead of going back to the panel it came from, it goes to another circuit's neutral wire.

Like so:

LEGEND:

  • B1 & B2: Two different breakers.
  • L: Power going out and return power for lights.
  • T: "Traveler" line wire pairs.
  • N: Neutral power return for what I assume is B2 at the breaker.

two lights switches, 2 circuits, 2 travelers in one box

My house is from 1992.

I'm trying to figure out if this is code or problematic. I know code changes, but to me, this seems like it would've never been allowed.

But I don't know anything about electrical wiring, so it's possible I'm completely wrong. All the houses in my neighborhood should have this same wiring job because they're all from the same builder. None of them have burned down in 30+ years.

None of the 6 breakers that share neutrals in various boxes have the breakers tied together. Also, all 6 of those breakers share a neutral with each other:

  • In one box, B1 and B2 share neutral.
  • In another box, B2 and B3 share neutral.
  • ...
  • In the 6th box, B6 and B1 share neutral.

From everything I've seen online, shared neutrals are only talked in code where you have 2 breakers tied together with a black and red wire in the same box, not two separate black and white wires from different boxes. This is where I wonder if I'm not understanding something.

From my guesswork, I believe the issue with shared neutrals is if one circuit should trip the breaker, half of that load will be distributed to another breaker; therefore, it won't trip and could cause a fire because it should've tripped.

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    Two separate circuits cannot connect their neutrals together. Two breakers on a MWBC can share the neutral. Older MWBC circuits did not need their breakers together and have handle ties. Neutral must be white. A white wire does not need to be neutral, as in switch loops or three way switches. You are lucky if they are marked as not neutral. Just a bit of information to further confuse you/us. Always check for no power before trying with your fingers. Commented 2 days ago
  • I don't think this is MWBC. There aren't 2 circuits in one box. It's 1 circuit + travelers from another circuit. Looking into it more, it's actually crazier than I thought. Where B1 comes in, it goes out B2's neutral, B2 goes out B3's neutral, and so on. Not consistently though. Some places do have 2 circuits going to the same neutral wire. Commented yesterday
  • Code is different in different countries. Nobody can answer a question about what "code" is, unless you say what country you are in. Commented 13 hours ago

3 Answers 3

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If what you are saying is true - that the neutrals are from circuits protected by different breakers, and those breakers are not tied together, then you are correct. The neutrals should not be joined together. Otherwise, you can have current running through the neutral, even though you turned off the circuit's breaker.

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  • I think this answer gets to the point, briefly and correctly. Commented yesterday
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    No, this answer is just plain wrong. Separate neutrals must NEVER be joined together even if on handle-tied breakers (outside of specialized paralleling situations). Multi-wire branch circuits use ONE and ONLY ONE neutral. Commented yesterday
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    @nobody Saying WHY you can't join neutrals would add a lot more usefulness rather than just saying you can't do it without explanation. Commented yesterday
  • @nobody When I say "neutrals" in this answer, I just mean the non-hot white wires in the boxes. I'm showing deference to the fact that the OP is a novice. Commented yesterday
  • @nobody I thought the answer was clear that "neutrals should not be joined together" and you say the same, and say the answer is wrong. I thought that the purpose of a panel is to create circuits, and a circuit is a hot wire going out and a neutral to return the current to the same breaker on the panel when the circuit is in use. Having the neutral return the current to another breaker defeats the purpose of having a panel to organize the circuits. If I'm wrong about that, then I've worked too hard to make sure my circuits aren't connected to one another. Commented 22 hours ago
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I deduce USA NEC as your code, from the coloring of the wires in the picture. Below explains the problem and how to fix it.

NEC requires the going and returning current to be in the same wire bundle (cable or conduit), so the magnetic fields thrown off by the current in the wires cancel each other, thus preventing magnetically inducing current flow in adjacent metal objects, which could cause heating in the adjacent objects and possibly fires.

In the left side of your picture, you appear to have one end of a standard 3-way light switch arrangement:

good 3-way with balanced wires

Power comes in on one or the other of the T travelers, goes through the 3-way switch to the common screw C then to the black L to the light, then returns on the white L from the light to the white in the traveler bundle. Magnetic fields are balanced in the LL cable and in the TNT cable. The left side is good, you don't need to adjust it, except to take out the right side's neutral.

The right side is problematic. It looks to me like someone wanted a light that only had one switch controlling it to have two switches in a 3-way arrangement, and wired the light illegally to achieve this goal.

What probably used to be there was this:

normal one-way switch

Power came in on black T, through the switch to black L to the light, then returned on white L to white T. Magnetics were balanced in both the TT and LL cables.

Here's what you've got now: bad 3-way with no neutral coming in

To get the one-way-switched light to be a three-way-switched light, they illegally re-purposed the black and white TT cable to be travelers. But there's no wire for the return current in the TT cable. So they bootlegged off the other neutral in the box.

So power is now coming in on the black and white TT "travelers", through the 3-way switch to the black light L, returning on the white light L, then going off to another circuit's neutral. While the LL cable here is balanced, the TT "travelers" don't have a balancing neutral in them, so they're throwing off magnetic heating inside the wall. The return neutral power is going on another circuit's neutral, so that current is extra in the other circuit, unbalancing its magnetics, also heating the wall.

Note that even if this is a multi-wire branch circuit, the neutral return current is not balancing the power current within the same cable. This is a problem.

To fix it, you'll have to convert the right side back to a one-way.

Turn off the power to both circuits and check for voltage. Then remove the right side white L from the left side bundle. Remove the white T from the switch traveler screw. (You can keep the existing 3-way switch in place: a 3-way can be used in a normal one-switch circuit simply by not using one of the traveler screws.) Connect the white T and the white L together under a tight wire nut or wago. Your goal is to get back to the "What probably used to be there" picture above.

Then you'll have to fix the other end of this bad 3-way circuit. Open the other end box, and you might find something like this:

bad 3-way other end

Power and neutral come in on the PP cable. White P dangles, connected to nothing. The travelers TT take the power onward, but the neutral return current doesn't come through these cables, so the magnetics heat the wall.

Remove the switch: Connect black P and black T under a wire nut or wago. Connect white P and white T under another wire nut or wago.

Now the light is back to a one-switch and legal wiring.

"But it was convenient to have the light 3-way!"

Devices are available that can convert a one-way light circuit to a 3-way circuit safely and legally, that don't require changing the cables in the wall. Swap one of these devices in for the switch, and possibly for the removed switch, depending on the device instructions, and you'd be back to 3-way safely and legally.

If you don't want to rely on a special device, you'll have to replace the cable now being used as illegal TT travelers with a three-wire cable like the left side has.

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  • Since the light switch for these wires come from the living room (and this switch is in the entryway), and since all my 3-ways are like this in the house, I actually think this is how the original wiring was. All of these looked completely untouched when I opened them. The plastic on some switches also broke off on the back when sticking a flathead in the hole to pull out the pushed-in wires. But I agree, it's strange they're using that existing neutral wire for the other circuit. Commented yesterday
  • Ah yeah, the faceplates on all my switches are also from the same timeframe. Quite a few broke when messing with them which is also why I've been switching them out. Commented yesterday
  • Another method I came up with to deal with this shared neutral situation is to use 2 smart switches. The first switch would control the light circuit (which I'd have it set to anyway), the second switch (in this box) would be virtual-only. It'd be connected by pigtail line + neutral, but it wouldn't control the lights. Doing this eliminates the missing traveler setup and needs software for light control (as I already have everywhere anyway). Commented yesterday
  • I actually have this exact box with 2 AUX switches now. Still the same issue as shown because I still need 3 wires: power, neutral, traveler (signaling); and I only have 2 (traveler and power). 2 smart switches eliminates the need for the 3rd traveler. Commented yesterday
  • True, @Sawtaytoes , smart switches would cover this. Also switches with a remote radio transmitter, not "smart" as in the usual add-on bloat with Wi-fi and all the rest, just a radio remote control that looks like or at least mounts in a switch box. Commented 18 hours ago
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Another problem with one neutral serving two different breakers is that the neutral could be overloaded. For example, if two 15 A breakers are on the same leg of the panel, then a shared neutral would carry the sum of the currents in the two hot wires. So, for example, if 12 A was passing through one of the two breakers and 10 A through the other breaker, then the shared neutral would be carring 22 A! This would not trip either breaker and this neutral wire would get very hot and perhaps start a fire.

If two breakers are on different legs of the panel (and so the voltages and currents are 180 deg out of phase), then a shared neutral would carry the difference in the currents through the two hots and the current in the neutral would always be less than the current in one of the hot wires. Following the example above where one hot is carrying 12 A and the other 10 A, the neutral would carry only 2 A.

You should determine the voltage between the two hots in each box with a shared neutral. 240 V difference between two hots means they are on different legs. 0 V difference means they are on the same leg.

If any of your neutrals are shared between two hots on the same leg, you have a potentially dangerous situation. But even if all of your shared neutrals are serving two hots on different legs, then this is an unsafe way to wire a house.

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  • Based on the wiring diagram the OP shows it may be that whoever wired the house was saving money on cable for lighting circuits with 3-way switches. Perhaps the shared neutrals are limited to the lighting parts of circuits and maybe even to pairs of 3-way switches. If so, then the immediate risk is lower than if receptacles were wired with shared neutrals. Modern LED lighting much less powere than old incandescent. This possibly fixed by doing away with one 3-way switch in each case. This would reduce functionality, but perhaps this could be restored with a few modern 3-way remote switches. Commented yesterday
  • That's a good call-out. If there is only three wires in the panel and the two hots are connected to different legs, then you can have too much current through the one neutral. From the OPs description, there are two neutrals in the panel, but they are connected together in the switch boxes. Commented yesterday
  • @JimStewart, I believe you are correct, but I don't know for sure. As far as I understand, it only affects the lighting circuits, but only my dad has been in the outlet boxes. Some light circuits control outlets. I also remember, my dad was replacing an outlet in the office, and while we turned off the power, the living room lights started flashing when he was disconnecting a wire. That one outlet looks like it was put after the house was built, so I dunno. While only the low-amperage light circuits might be like this, it's possible other circuits were done in this "save on cabling" way. Commented yesterday

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