Shared neutral wiring question

Are there proper electricians on here? I probably need to call one but may not be able to for a few weeks and wasn’t sure if something I noticed in a recently purchased house is urgent or not.

The house has a smart switch that requires a neutral. The switch is at the end of the run and controls an outlet (unnecessarily in my opinion). The outlet has a switch loop given the hot from the panel reaches it before the switch that controls it. I’ll call this circuit 1.

In the same box is a hot, neutral, and incredibly short bare ground from a different circuit. The hot wire is capped, and the ground is cut extremely short such that it seems unusable. I’ll call this circuit 2.

Both circuits are 15 amps.

The problem: the neutral wire from circuit 2 is used in the smart switch, with the hot and load from circuit 1.

I’m not an electrician, but this strikes me as a problem, correct?

To fix can’t I just use a wire nut to pigtail the hot (white wire given switch loop) and load together (both circuit 1) so the outlet is always on? Then connect the hot from circuit 2 to the switch, leaving the neutral form circuit 2 connected so the smart switch can still work without an actual load connected.

The other issue is the ground wire of circuit 2 is unusably short. Can I leave the ground from circuit 1 connected?

Attached a drawing if that’s more clear.

I don’t see very well, but your drawing is upside down, right? :thinking:

Also, you said the circuit is 15 A, I just wanted to verify that you’re in the USA.

There are a couple of electricians in the community, hopefully someone will have an answer for you. :sunglasses:

Thanks a uploaded the picture again. And yes I’m in USA.

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Paging @ritchierich

I’m not an electrician but I’ve been told that using a neutral and hot from two different circuits is A Very Bad Idea.

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This is correct.

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Using a neutral from another branch circuit is prohibited by the NEC. I’m pretty sure the Canadian code as well since I don’t know where you are. The reason is that you run the risk of running more current on the neutral conductor than it is rated for (15A, in your case).

You have a couple options.

1 - You said that you thought the switching of the outlet is unnecessary, so you can simply terminate at the receptacle. If the receptacle is split (half hot all the time and half switched), replace the receptacle. Of course, this leaves you with a switch that does nothing.

But since there is a hot and neutral in the switch box, you can add a smart switch or a multi-button smart switch to use as a scene controller. (Plus you can use a pocket socket at the now unswitched receptacle.

2 - If you really want the receptacle switched, move it over to the other branch circuit that’s in the switch box, presuming it’s not dedicated or you will overload it. At the receptacle, cap off the incoming hot and neutral. At the switch box, connect the smart switch to the other branch circuit in the box and use the 2-wire going to the receptacle as a switched hot/neutral. The only downside to this is that the entire receptacle will be switched.

NEC 300.3(B)(1) etc.

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Thanks. So option#1 is what I plan to do. Because I’m using the switch to control different smart plugs in the room.

But rather than open up two electrical boxes can I just pigtail together the wires labeled 1 and 2 in the picture (in the switch box).

Any thoughts on the ground wire issue I mentioned also?

I suppose technically you could pigtail those two together, but tbh, there is no need to send a hot to that switch box. If I were doing it, I’d terminate at the receptacle. It’s also less confusing for the next person looking at it.

I’d pigtail that short neutral giving you something to work with. If you can only get a minimal twist, use a copper crimp.

The reason I was thinking of pigtailing the ones in the switch box was down the road if I move, I can more easily put a regular switch there that doesn’t need a neutral. Plus I’d prefer messing with as few boxes as possible.

While I agree this should not be done, it would appear that the outlet is at least using both the hot and neutral from the same circuit which is the most critical as one could draw up to 15A from that outlet.

The other circuit (#2) is only providing a neutral to the switch (from the diagram it appears that hot (Wire #3) is disconnected) so the only undue load on Circuit 2 is whatever the switch itself draws (likely just a few mA).

This could have been way worse… say that hot (Circuit #1) were disconnected and the outlet were fed from hot (Circuit #2 / Wire #3), then you could definitely cause an issue by drawing significant current from the outlet as Circuit #2 would provide hot while Circuit #1 would provide Neutral.

An alternative solution - but I am not an electrician - might be to use wires #1 and #2 to bring hot and neutral from circuit #2 to the outlet. You would then just need to disconnect both hot/neutral from Circuit #1 and cap them off. You would have to connect Wire 3 to the Switch on the Hot terminal rather than it being disconnected. In summary: Wire 3 to Switch Hot terminal. Neutral from Circuit 2 to Neutral of smart switch. Wire #2 also connected to Neutral terminal of the Switch and on the other end connected to the outlet’s neutral terminal. Wire 1 connected to the Load terminal of the switch and on the other end to the hot terminal of the outlet.

While I believe this eliminates the sharing of neutral/hot between circuits, it introduces another potential issue. You now have an outlet that will potentially (if switch is on) remain live when circuit 1 is switched off at the breaker and one might not expect that unless exception is noted at the breaker panel.

I’ll try to draw something up to make it more clear.

Again - I am not an electrician.

Edit: Diagram of the “alternative solution”:

Thanks, all helpful feedback.

I think I’m going to terminate circuit 1 at the receptacle as @Bry suggested and connect the hot from circuit 2 to the switch. Not comfortable moving a receptacle to a different circuit.

I just don’t know what to do about the ground wire. It’s entirely sheethed in Romex from circuit 2. I don’t think I can even cut it loose from the Romex let alone strip it and attach some jumper. Assuming the circuits are both 15A, is it against code/otherwise problematic to keep the circuit 1 ground wire attached to the switch?

Can you post a pic of the problematic Romex?

If you are in the US AND your locality has adopted has adopted NEC 2014 (or presumably later), then you can use a ground from another branch circuit, as long as both circuits originate in the same panel. (NEC 250.130(C)). If you are still on NEC 2011, then that practice is prohibited. Like everything with the NEC, you need to rely upon local regulations as well. But from a practical standpoint, you might infer the practice is safe if the NEC was changed to allow it.

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I’ll try to send a pic later. But on second thought wouldn’t it be simplest to just undo the switch loop at the receptacle and pass regular hot, neutral and ground all from circuit 1 to the switch. Then cap the circuit 2 hot and neutral separately in the switch box. That way the outlet will always be on. The switch will work for scene control without a load and have all wires from same circuit.

@William_Knecht - If you want to power the switch to use it as “scene controller” without a load, then just use Circuit 2’s hot and neutral to power the switch, and use Circuit 1’s hot and neutral to power the outlet. Any wires between the two boxes would have to be disconnected. The two can be, and likely should stay, on separate circuits. I am guessing NEC would advise not to bring power from multiple different circuits into the same box due to the chance one might bind them all together (or some other reason). . The two circuits might be on different 120V legs in your panel so it might spell big trouble if you were to accidentally tie them together. Maybe @Bry can confirm this would be an issue and what happens if it were to happen.

The outlet itself could be replaced with one that can be switched, if that is still needed. You can find both zwave and zigbee decora style outlets that switch the bottom outlet. If you plan on doing that, I would look into using an “Association” (therefore I believe both devices need to be Zwave) to control the outlet so that you are not dependent on the whims of ST and the cloud. That would likely get you as close as possible to a wired setup (no lag?).

Disclaimer: I am not an electrician.

Good point! Yep, you can do that too. That eliminates the short ground problem. If the tab connecting the two outlets is broken to split it, just replace the receptacle.

I’m not aware of any prohibition against having conductors from two or more branch circuits in the same box. Just don’t bundle the neutrals and grounds together, and be mindful of the box’s capacity.

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I would assume that the same applies to the hot wire too. I imagine that if a live from a circuit on one leg were to be connected to a live from another circuit on another leg of the subpanel, something bad would happen. In all of the switch & outlet boxes I’ve worked on, I have never found wires from different circuits, but good to know it might happen.

Yes, it depends upon the phases of the conductors, but as a general rule I would say you won’t be happy with the loud noise, the melting conductors, potential fire and whatever else comes before the breaker trips.

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It’s actually a pretty common use case for an entryway. You have a double gang switch and one switch controls the porch light which is on circuit A and the other switch controls the interior hallway light which is on circuit B.

There’s a maximum voltage limitation, but below that it’s not an issue.


Very good. Thank you so much! Don’t like touching these things but seems safer not having the shared neutral.

I’m going to cap separately those two unused and wrap wire nuts in electrical tape for good measure.

If I get around to it I’ll find the box that feeds the not needed wires to the switch box and terminate that circuit there.