Can two circuits' neutrals be tied together (not a single neutral wire, but two that have been connected)?

Question

Related to but different from this question, I have two three-way switches in my house, which are in multi-gang boxes at the top and bottom of the staircase and control lights at the top and bottom landings. I know for a fact, having fixed the "hot" side of the wiring to the three-way switches before, that the switches (along with others in both boxes) are fed by different circuits. However, all of the white neutrals in each box are twisted together into one bundle and wire-nutted; this predates any work I did on the circuit, but I don't know if this is the work of the original electrician or the "handyman" whose work I've been fixing since I moved in.

As these joined neutrals include the travelers of both three-way circuits, the neutrals from the nearest light to each box, and the neutral heading back to the panel from each box, these two circuits' neutrals have multiple junction points that tie the neutrals together.

Is this dangerous (from a fire hazard perspective)? My Google-fu brought up some similar questions, but the answers were all along the lines of answers to the related question in the link: "a shared neutral is bad". However, this isn't a "shared neutral" as in one white wire carrying the load from two circuits; this is two white wires carrying the load from two circuits, but the two white wires are both carrying the load from either circuit. So, to my way of thinking, there's plenty of copper to get the power back to the panel without overheating. And obviously there are no AFCI/GFCI breakers in the house (plenty of GFCI receptacles), because everything on both circuits works fine (they'd trip as soon as I turned a light on otherwise). The only concern AFAIK would be shock safety, as you'd basically have to make sure both of the breakers were off before working in either switch box (something I would do anyway; I wouldn't want ANYTHING hot in either box if I were messing with these switches, regardless of whether the neutrals are separate or not).

EDIT: picture == word*1000:

SteveR · Accepted Answer

EDITED FOR CLARIFICATION:

If this is wired as you have drawn it, then it will not be a safety issue. The additional neutral will only share the current in both circuits. Because both circuits are fused for 15A, each neutral will only see a maximum of 15A. This is providing that both neutrals are solidly connected! If one were to become loose or disconnected then the other can potential see the full load, 15A*2=30A.

If anything else is fed from either of those breakers, it becomes a whole other issue! 
I recommend you wire it the right way. However to answer your question, I don't see it as a safety issue if that is the only circuit on those breakers.

With either breaker off, that circuit will be isolated from the energized circuit. The only common path between the two is the shared neutral. If the energized circuit were to draw the breaker maximum of 15A, the de-energized neutral potential would be at maximum only a few hundred millivolts, not posing a safety risk.

Also it is not against code to have more than one feed in a jbox or switch box.

staticsan · Answer

So the two lights have different Active lines but the two circuits have their Neutrals tied together? This means both circuit's Neutral lines are sharing current return. This will be a problem if you ever put a safety-switch on either circuit because they measure the current imbalance between Active and Neutral to determine if there is a fault.

AFAIK, this is not legal wiring in Australia.

stevegt · Answer

It's not legal nor safe, and would not pass an inspection. There's the potential for some weird current-loop effects there as well; heating of metal components, Electromagnetic interference (EMI), etc.

Having said that, I've seen much worse.

Philip Ngai · Answer

This is a safety hazard. An electrical system must remain safe (not hurt a user or overheat and cause a fire) even if there is a failure.

In your situation it is true that the neutrals will not be overloaded under normal conditions. But if one of the neutrals were to get disconnected at or near the breaker panel (loose screw or accidental damage to the wire itself) then the other neutral could end up carrying the current for two circuits and overheat and cause a fire.

Karl Katzke · Answer

It isn't an immediate safety issue. Having a shared or bridged neutral is actually a very common occurrence in old work. As most other posters pointed out, there is a slight risk of overload if you're close to max draw on both circuits simultaneously, but it's incredibly unlikely.

Back on the other hand, it will rapidly become a nuisance if you place a GFCI device on one of these circuits or if you install a GFCI or AFCI breaker. AFCI breakers are now required by most electrical code even for retrofit installations. Any time you turn on a switch on the non-GFCI/AFCI circuit, the GFCI/AFCI device will detect an imbalance on the current that's passing via the neutral and hot, and it will trip. Finding this problem and fixing it later is a pain in the butt and will require not a little wall damage to rectify. (Ask me how I know.) As a result, wiring the circuit this way no longer meets code.

I would go through the work to wire it properly for future compliance.

ThreePhaseEel · Answer

You will need to separate the neutrals -- this is both unsafe as already explained and a 200.4(A) violation:

Installation. Neutral conductors shall not be used for more than one branch 
  circuit, for more than one multiwire branch circuit, or for more than one set of 
  ungrounded feeder conductors unless specifically permitted elsewhere in this 
  Code.

Thankfully, Tester101's already given us an illustration of how:

Also, to avoid this happening again, tag the neutral conductors as per 200.4(B):

Multiple Circuits. Where more than one neutral conductor associated with 
  different circuits is in an enclosure, grounded circuit conductors of each 
  circuit shall be identified or grouped to correspond with the ungrounded 
  circuit conductor(s) by wire markers, cable ties, or similar means in at least 
  one location within the enclosure.

Another argument against this is given by 300.3(B):

Conductors of the Same Circuit. All conductors of the same circuit and, where 
  used, the grounded conductor and all equipment grounding conductors and 
  bonding conductors shall be contained within the same raceway, auxiliary 
  gutter, cable tray, cablebus assembly, trench, cable, or cord, unless 
  otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

and 310.10(H)(1) (ignore the exceptions, they do not apply to residential wiring):

General. Aluminum, copper-clad aluminum, or copper conductors, for each 
  phase, polarity, neutral, or grounded circuit shall be permitted to be 
  connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG 
  and larger where installed in accordance with 310.10(H)(2) through (H)(6).

See this article for more details on why your current configuration is a bad idea in general.

Ben · Answer

Ever hear of a multi branch circuit? You can have two circuits share the same neutral wire even but they have to be on opposite phases. If they are on opposite phases then the current carried by the neutral will only be the differential between the two circuits and will never exceed the breakers limit.

Answered by Ben on June 9, 2021

Can two circuits' neutrals be tied together (not a single neutral wire, but two that have been connected)?

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