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TL;DR:

in summary: the load (light) is in the garage, it is fed by a hot that originates in the garage, and it sinks into a neutral in the garage. The odd thing is that a switch loop extends from garage to house -- and in the junction box at the house a neutral is present, but it's not the neutral from that garage light circuit. This neutral in the house j-box is not currently used.

My thanks for Greg Hill's comment, which I've copied verbatim above, and which is an excellent summary of my long question. Read on for additional details, if needed…


I have two "main" electrical panels at my house: one in the house itself, and one in the detached garage. The electrical service comes in at the garage, and then from there to the garage panel, and via a connection to the house to the house panel. (I don't know whether the house panel is acting as a sub-panel from the garage panel, or is served standalone from the main service; I suspect the latter, but can't confirm.)

At the garage are two switchable light circuits, floodlights and down-lights. These are both on three-way switches, with one switch each in the garage, and one switch each in house (i.e. four switches, two for each circuit). The lights themselves are all mounted on the exterior of the garage.

I had previously learned, for the down-light circuit, that the load wire is connected to the switch in the garage, and that the line (hot) wire is connected to the switch in the house.

What I discovered today was that the bundle of neutral wires in the box for the switch in the house (in the same building), all go to the house panel, even though the line (hot) wire in that same box comes from the garage panel (in the other building).

(I found this out, because I had previously tried to install a digital timer switch, locating it at the box in the house, but when I got it hooked up, then as soon as I tried to turn the breaker back on, it would trip. The electrician who did the wiring came to look, and explained the reason, i.e. that the neutral was going back to the wrong panel, so the breaker treated it as a ground-fault, since the current wasn't coming back to the breaker's panel.)

What is the legality of this configuration? Does it meet code? Any particular safety issues to be concerned with?

The issue is mitigated by the fact that, as near as I can tell, these neutrals aren't actually serving anything. The hot returns to the garage and then to the loads via the travelers hooked to the three-way switches. The actual loads are in the garage and are connected to neutrals that go back to the garage panel in the same building.

I found out about the issue when I tried to hook something up to them (the digital timer switch), and that immediately tripped the breaker. So it seems like the neutral wires in the house-side box are unused. Presumably, if there had been anything from the garage-side hot that was using the house-side neutrals, the same thing would've happened (i.e. breaker tripped immediately). So the issue seems fairly self-limiting, and I don't see any reason that would wind up changing.

That said, I have seen some other posts discussing hot/neutral issues. More of them involve multiple neutral wires going back to the same panel, and I didn't see anything exactly like my situation. But based on the discussion I saw in those questions, I have the impression that at least when the neutral is actually used, it's very important that it go back to the same panel (and even the same breaker?) where the hot used with that neutral comes from. And it seems like leaving the wiring like this today, could be laying a trap for someone tomorrow.

I gather that the problems include possible overheating of the wire, as well as overload and accidental contact risks. It seems that these problems only exist with the neutral is actually used, so technically I might not have any hazard now. But does code allow for that situation, even so?

Related posts I found:
Is it acceptable to have two circuits with a shared hot but different neutrals?
Why is this shared-neutral wiring bad?
How to determine which neutral to which power?

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    So, in summary: the load (light) is in the garage, it is fed by a hot that originates in the garage, and it sinks into a neutral in the garage. The odd thing is that a switch loop extends from garage to house -- and in the junction box at the house a neutral is present, but it's not the neutral from that garage light circuit. This neutral in the house j-box is not currently used. Did I get that right? – Greg Hill Jan 9 at 23:51
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    @Greg: yes, you have exactly summarized the situation (in a way that I'd wished I could've in my post...I'm going to copy/paste what you wrote, because it so clearly articulates what's going on.) – Peter Duniho Jan 9 at 23:53
  • Are the house-to-garage switch loops wired using cables, or individual wires in conduit? Can you post photos of the insides of both switch boxes? I think this is a solveable problem with some trickery, but I need more details to be sure. – ThreePhaseEel Jan 10 at 3:22
  • Also, what make and model is said digital timer switch? – ThreePhaseEel Jan 10 at 3:22
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    It is wise of you to consider whether you are building a trap -- or, in this case, worsening an existing trap -- for the next guy. I've left many comments and answers on this site about that pattern, which is in my experience extremely common. Someone wires up a circuit that is correct but weird; the next owner fails to understand it and modifies it into a working but unsafe configuration, and the owner after that gets a nasty surprise when attempting a second modification. Shared neutrals seem to be a particularly rich source of this antipattern. – Eric Lippert Jan 10 at 23:36
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That's indeed completely illegal, but is a common sight where people put smart switches on 3-ways. They get to a "remote" 3-way that's on a spur, and the only available wires are always-hot, traveler 1 and traveler 2, and they go "oh hey, I see a neutral over there on the other side of that divider!" (because it would be conceptually better if there were physical dividers, but there usually aren't).

There are many Code sections covering it, but the most straightforward one is

Currents must be equal in each cable or conduit.

So if you have a switch cable, if you split the wires by which way the currents are going, the westbound currents must equal the eastbound currents (or whatever). If you give the currents a polarity, they should add up to zero. That is exactly what a clamp meter does if you put it around the cable.

Now there's a corollary to that rule: Cable/conduit must be in a tree topology, with no loops allowed (except for equipment safety ground wires). If you think about ants running up and down a tree all day, never leaping from one branch to the other since ants don't leap, then it's plain that no matter where on the tree you put an "ant turnstile", you'll count exactly the same number of ants going up the branch as come back down. If your cable/conduit topology is a tree, it's hard to screw that up.

Likewise, if you're drawing a circuit diagram in Paint, and you use the Paint Can to fill all adjacent same-color areas, the entire background should fill.

It's OK for different cables to come into the same junction box, effectively making a loop; but then, they need a Chinese Wall between groups of cabling to prevent the loop.


So, if you rearrange the wiring to enforce that partition, that will bring to an end your GFCI trips.

Of course, there are some things now possible that won't be possible if you do this. That's why you don't want to do this. Unfortunately, it is a Code requirement. You will need to "find a new way to do the thing" - smart switches are actually a good option here, but you may need to use them in a more sophisticated way that allows you to re-task some of your wires to other purposes.


The other rule is that, indeed, if one neutral returns current for 2 hots, it is at serious risk for overload. Neutrals don't have fuses; their protection comes ONLY from the principle that neutrals only return current for their hot(s). Plural due to multi-wire branch circuits, which provide an engineered way for hots to safely share a neutral.


It is rather unlikely that a bunch of neutrals are unused. This is a common myth; people don't see them connected to any switches, and conclude they are idle or spares. Actually, they are returning the current for the lights or onward loads. Current flows in loops.

  • I think you may have misunderstood. I'm not looking to get the timer switch working (it works at the other end, which is where it is now). I'm just trying to know what's legal and safe with respect to the wiring as it is. "It is rather unlikely that a bunch of neutrals are unused" -- you are right, it's possible these same wires, at some point elsewhere in the wall, connect up with neutrals serving some other fixtures/outlets/etc. But, I can say for sure, they are not being used for either of the switched circuits that the box they are in holds. – Peter Duniho Jan 10 at 2:26
  • @PeterDuniho Then where are those circuits returning neutral? Try separating them and capping each one off. I bet a bunch of stuff breaks. If they're truly unused, due to the equal-current rule that means their hots aren't in use either, which means the cables are entirely unused and should be capped off. – Harper - Reinstate Monica Jan 10 at 2:39
  • "Then where are those circuits returning neutral?" -- sorry, I'm not sure which of the various circuits you're asking about when you write "those". Are you asking about the switched circuits? As I mentioned, the neutrals for those circuits are in the garage, and return to the garage panel. ... – Peter Duniho Jan 10 at 16:20
  • ... If you're asking about the unused neutrals in the house, I can't find any hot wires that correspond to those neutrals. I have no idea why they are in the box. The electrician miswired a number of things initially (failing to hook up switches, or putting them in the wrong place) so some of these wires might be "left-over". – Peter Duniho Jan 10 at 16:20
  • @PeterDuniho yes, the clump of "spare neutrals" you mentioned. The hot wires are the partner hot wires that are in the cable. Is it just a loosy goosy hot wire with no cable? . Or is your house wired in conduit? – Harper - Reinstate Monica Jan 10 at 17:57
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It's a routine and appropriate thing to find conductors from multiple circuits cohabitating in a junction box. As the linked questions make clear, it's important that a hot conductor and its corresponding neutral belong together in cables, raceways, and junction boxes.

In years past it was routine (and allowed under code) to make a switch loop for a light which didn't carry the neutral along with it. Code no longer allows this. We don't know which code was applicable at the time your house was built so we can't easily say whether the absence of neutral in your situation is a violation of the code that applied when the work was done. This style of wiring isn't inherently hazardous, but it does induce people to make subsequent changes that would create a nuisance (GFCI trips) or a hazard (neutral overload) by stealing a neutral corresponding to another circuit.

You specifically mention that the junction box in the house does have a neutral but you didn't mention whether that junction box also contains the hot that belongs with this neutral. If it does, then maybe there's an alternate method of achieving your true goal (something about controlling the garage lights with a timer?). You could ask another question if you'd like to read other ideas for cracking that nut.

  • "We don't know which code was applicable at the time your house" -- house was just built, started in 2016. I'll double-check the box, but I'm pretty sure the only hots in the box are from the garage. The timer switch works fine if mounted in the garage where there's garage-panel neutral wire, so I'm not having any trouble with the practical side of things. I just want to know if the current situation is legal and safe. – Peter Duniho Jan 10 at 1:17
  • On this point: "In years past it was routine (and allowed under code) to make a switch loop for a light which didn't carry the neutral along with it" -- given that in this case, the neutral would be unused in that loop (the current flows out on hot, flows back on one or the other traveler...I didn't see the wiring in the wall, but presumably the hot and travelers follow the same path between garage and house), does code carry an exception to allow there to be no neutral in that run? – Peter Duniho Jan 10 at 1:21
  • Speaking of residential wiring in particular, it used to be common that the hot and neutral would be brought to the light box in a room. A single piece of 2-conductor NM cable (aka "Romex") would bring hot down to the wall switch and return the switched hot for the light. In the case of a 3-way switch, a single piece of 3-conductor NM would would carry the hot out and bring back the two travelers. Code allowed the neutral to be omitted from those switch loops. I've seen two pieces of 2-conductor used in lieu of the 3-conductor, but I don't know if code ever really allowed that.. – Greg Hill Jan 10 at 3:24
  • So if I understand you correctly, code allowed (and possibly still allows? the electrical inspection was some time in late 2017, if I recall correctly) for the wires in use to be as they are (i.e. no neutral between the garage panel and the house switch-box). That would leave as the only remaining question the legality of unpaired neutrals in the same box, going to a different panel. – Peter Duniho Jan 10 at 16:23
  • It seems that the 2011 NEC was the first to require bringing neutral to switch locations even if it won't be used. It's up to each AHJ to decide which version of NEC they'll require. They often don't adopt new versions of NEC immediately, but one might reasonably expect your 2016 construction would have used at least 2011 code. – Greg Hill Jan 11 at 18:17
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The big danger of borrowed neutrals comes when someone tries to do work on the system. They turn off the circuits they are working on and go through "safe isolation" procedures and confirm that it is dead.

Then they start to disconnect stuff and suddenly get an electric shock! The neutral wire had no significant voltage on it during testing, but once disconnected there was nowhere for the current to go and so the neutral rose up to mains voltage.

  • Thanks for the note. However, I do understand the general risks involved. Unfortunately, your post doesn't really address the question I asked. These aren't "borrowed neutrals" (as in, in use for some other circuit). They appear to be "spare" (though as Harper points out, they must've arrived at the box with a hot in tow, not being single-wire runs...I need to investigate that). – Peter Duniho Jan 10 at 19:38

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