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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:

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A wiring diagram might help explain things a little better. –  Tester101 Mar 13 '12 at 2:56
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I'll see what I can bodge up in Paint in less than 10 mins –  KeithS Mar 13 '12 at 3:00
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That's why there is a code, to keep it the same from place to place. Just because you found it does not mean an electrician would find it if they where rushing things. What that code is I could not tell you. But you asked if it was dangerous, so that would be a yes. i203.photobucket.com/albums/aa80/chris_kennedy/DSCN2315.jpg I think an inspector would still call it under MWBC 210.4B –  lqlarry Mar 13 '12 at 3:35
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This is a simple fix, just separate the neutrals Like This –  Tester101 Mar 13 '12 at 13:39
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Wired the way you show it, it is not a safety issue. The fact that one breaker is off, and there still will be power into the switch box from the other circuit is not a code violation. More than one circuit is allowed in a common box. One circuit can not energize the other the way it is wired. The neutral is bonded to ground, and will be at the ground potential. Why can't you just separate the neutrals? –  SteveR Mar 13 '12 at 19:39

5 Answers 5

up vote 2 down vote accepted

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.

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There is also the chance that one line is longer then the other which would result in the resistance being larger on one leg over the other. This would result in current being directed more towards one leg over the other. So in this sense, it can be a safety issue. –  Kellenjb Mar 13 '12 at 20:01
    
@Kellenjb- 14AWG wire has a resistance of .025 ohms/10'. If the length difference were even say, 10' long and the current in the circuit were 15A, the vdrop accross the wire will be 375 milli volts, 0.375V. I don't see that as a safety issue. If that were the case, you better not touch your sink faucets:) In reality its probably a 100W bulb, Round that off to a current of 1A * .025 (ohms) = .025 volts... –  SteveR Mar 13 '12 at 20:41
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I didn't say it would be a safety issue, just that it could be one. Plus in this circumstance its not as much about the voltage drop, rather the extra current that one line would be taking over the other. It is also possible to get some nasty things going on when you get loops like that going on. –  Kellenjb Mar 13 '12 at 21:16
    
Both neutrals are correctly sized for the circuit current. If anything they will share the load and neither will see the full load. I'm not saying that shared neutrals should be ignored, I just don't see any safety issue with this scheme. If either of those breakers feed other loads, then yes the shared neutrals can become a real problem, but he does not show that being the case. –  SteveR Mar 13 '12 at 22:24
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Multiwire circuits (two hots sharing a neutral) have to be on a single double breaker, or the two breakers have to be tied such that turning one off turns the other off. This IS a safety issue. –  Jeremy W. Sherman Oct 7 '12 at 2:18

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.

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Also if you turn of a circuit to work on, then it's neutral can still give you a shock from the other circuit. –  Walker Nov 6 '12 at 12:43

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.

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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.

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It's not legal in the USA either. –  Brad Gilbert Mar 13 '12 at 17:49

It isn't a 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.

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