# Can neutrals be tied together on different circuits? [duplicate]

I have a 2-gang switch panel, controlling 2 sets of lights. One switch is a normal switch, the other is a 3-way switch. Each switch is controlled by a different breaker.

I found that the neutrals are tied together in this box, is this allowed?

Can grounds on these different circuits be tied together? I'm going to be replacing my breaker panel soon and would like to try and install AFCI breakers if possible. Would the tied grounds cause any interference?

I'd like to get some smart switches inside this 2-gang box, but it's pretty tight in there.

Is there anything I should know before I put a 3gang box in there?

Thanks!

• Neutrals should not be tied together from different circuits. Think the main reason is it could be dangerous. Think grounds can be tied/connected together, but my knowledge is not for AFCI breakers. Think the only problem with three gang box is finding a three box cover with a blank section. Aug 16, 2021 at 18:20
• Depending on what you are in need of, a different approach to more space in the box is a 4x4 or 4-11/16x4-11/16 box with a 2-gang cover plate, rather than a 3-gang box, if you only need two devices, but more working space. Aug 16, 2021 at 19:03
• Aug 17, 2021 at 4:46
• Can they be? Sure, your situation shows they can. Should they? Is it safe? Is it legal? Most likely no, to all three. It makes me wonder who did that and if they ever mixed out of phase neutrals to learn an important lesson. Aug 17, 2021 at 18:13
• Have we rules out it being an MWBC? MWBC could explain two breakers separately controlling two lights, on the same neutral. Aug 18, 2021 at 2:33

I found that the neutrals are tied together in this box, is this allowed?

Absolutely never. If a box contains 2 different groups of hot wires that do not interact with each other, their neutrals must not interact either.

Neutral is the normal return current path, it flows the same amount of current as the hot. You notice we don't put fuses on neutrals. Why should we have to? Neutral flows the exact same amount of current as hot, so the hot breaker will protect it... right?

Well, as soon as you cross neutrals between circuits, you wreck that protection. Now 1 neutral could return 2 hots' worth of power. Now we need fuses or circuit breakers on neutrals. No thanks.

Also, AC power throws quite a powerful magnetic field proportional to current. As long as currents are equal and opposite among wires in the same cable, those magnetic fields cancel each other out. However if current flows up 1 cable and back another cable, it creates all sorts of mischief.

Also, we do not like paralleling - providing 2 pathways for the same current. It causes many problems, including the way it fails silently and destructively if 1 of the 2 wires has a loose connection, forcing 2 wires worth of current onto the other wire.

Can grounds on these different circuits be tied together?

Yes, that is fine. Because grounds only carry current during a fault condition, which we hope is ended in seconds when the breaker trips.

I'm going to be replacing my breaker panel soon and would like to try and install AFCI breakers if possible. Would the tied grounds cause any interference?

xFCI breakers have zero tolerance for crossed neutrals. The xFCI will incessantly trip until you correct all of them.

However, they don't care about grounds. If you look very closely at a breaker and how it attaches to the panel, you see the breaker doesn't even have access to ground.

However, xFCI devices will seem to care about ground. That's because ground enhances their ability to detect ground faults. Diagnostic procedure will reveal that removing the ground stops the incessant trips, and so the novice will blame ground. In fact the appliance is faulty, and removing ground only defeats the detection. It's like pulling the battery out of a carbon monoxide detector because it won't shut up.

I'd like to get some smart switches inside this 2-gang box, but it's pretty tight in there.

Is there anything I should know before I put a 3gang box in there?

You don't need to. You can use a box extension.

One of the most elegant is the Legrand Wiremold Surface Conduit Starter Box, which gives about a 1" lift. It's meant for you to then attach surface conduit to the sides of it... but if you don't do that, well, I won't tell :)

• The box extender is an interesting idea, but it's right next to a hand railing on my stairs, so I don't think it will look too good.
– KZ.
Aug 17, 2021 at 4:53
• Funny. After raising trouble about some AFCI breaker problems I received a replacement AFCI breaker from a certain manufacturer that doesn't expect the neutral back. Aug 17, 2021 at 19:04
• @Joshua Yeah, it's possible for an AFCI to rely only on the hot wire, but it requires exceptional digital signal processing. GE has one, but it's only listed/safe for GE panels. They specifically market it for MWBCs, since you can handle-tie two singles. Aug 17, 2021 at 19:25
• "we do not like paralleling - providing 2 pathways for the same current. " Purely an aside, but the UK sure does! And it comes with all the caveats you've outlined above.
– J...
Aug 17, 2021 at 22:58

Generally neutrals for separate hots should be isolated, grounds should always be jointed.

The exception to the neutral is a Multi-Wire Branch Circuit. You could have a 3 conductor cable leaving the panel, black/white/red with the black and red sharing a neutral. In a modern installation the circuits would be adjacent with handle ties or a multipole breaker. At some point the black and red may part ways and both circuits connected to the common neutral.

But normally if multiple neutrals enter the box they are required to remain separate and AFCI breakers will not properly function if tied together.

• Ha! Ask me how I learned about MWBCs :p. I've got these in my home, but there are no handle ties at my breaker box (built in 1980). The two hot wires I've found here are not on the same MWBC either.
– KZ.
Aug 17, 2021 at 4:47
• I should've gone into more detail. Will edit. In the big picture handle tied breakers are a recent requirement. Imagine the headache when working in a 3 phase panel and we would find two neutrals going into a conduit with 5 or 6 random hots. Aug 17, 2021 at 11:59

Grounds can be tied together. In a conduit rather than cable situation, the grounding wire can be shared.

Neutrals can only be tied together at the breaker (or fuse) panel originating the circuit. In the case of GFCI/AFCI, they are tied together beyond the GFCI/AFCI breaker (from a circuit-side view. From a distribution-side view, the supply neutral is split to the breakers and then to the circuits, individually.)

And improperly tied-together neutrals will cause problems as more and more circuits are required to be GFCI and/or AFCI.

• Could you clarify what you mean by: "In a conduit rather than cable situation, the grounding wire can be shared."? Thanks!
– KZ.
Aug 16, 2021 at 19:01
• In a conduit, one grounding wire of the largest size needed is adequate for all circuits sharing the conduit. In metallic (non-flexible) conduit properly joined, the conduit itself can be the grounding conductor and no grounding wire is needed. Since cables have grounding wires one per cable, you'll have multiple grounding wires when you have multiple cables. Aug 16, 2021 at 19:06
• @KZ For that matter, if you are using /2/2 cable (black, white-black, red, white-red, bare) you have 1 cable supporting 2 independent circuits that uses 1 ground wire. Aug 16, 2021 at 20:05