Legal/safe? Neutral in box goes to different panel than the one that provides the hot

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.