Help me trace a fault?

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wwhitney

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So - should a neutral wire "never" be shared between circuits?
Let me try again from a different point of view:

Yes, neutral wires should never be shared between circuits, in the sense that if you were to go to a panel, and lift any two branch circuit neutral wires [smaller than 1/0] off the neutral bar, there should not be any connection between the two free ends you get. Those neutral wires should not be connected together in any downstream box.

Cheers, Wayne
 

wwhitney

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OK, that last statement wasn't strong enough. One more try:

Ungrounded (hot) conductors can't share neutrals in the sense that each ungrounded conductor has only one associated neutral. You have to use that neutral for loads supplied by that ungrounded conductor, you can't just grab a different neutral. [The converse is not always true; a neutral may have more than one associated ungrounded conductor, as in MWBCs.]

Cheers, Wayne
 

ChuckGM

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Okay - This is interesting stuff!! You've both been VERY helpful on this thing! Now, to take some of your "professional" discussion and break it down a little bit. :) I follow that a 2-pole breaker is essentially automatically on 2 legs of the service and the neutral should not be shared anywhere. So.... do I also hear you saying that when, for example, a 12/3 wire is used as a convenience to get two circuits wired at the other end of the house etc... that those two circuits should be placed on opposite legs in the service? --- (also, I've tried a couple of threads in different forums over the past couple months trying to grasp "why" I seem to have some "ghost current" on a couple of circuits. I'm going to take a wild guess and figure that shared neutrals may account for some of that?) I'm going to be opening a lot of boxes in the next couple of weeks - good it's a small house! - but back to original question: If there "IS' a shared neutral - separate legs? - or, even in those situations another neutral should be run??
 

Reach4

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I follow that a 2-pole breaker is essentially automatically on 2 legs of the service and the neutral should not be shared anywhere.
Did not understand that one. "Anywhere"?
do I also hear you saying that when, for example, a 12/3 wire is used as a convenience to get two circuits wired at the other end of the house etc... that those two circuits should be placed on opposite legs in the service? ---
Yes, but MWBC also has the advantage of being more power efficient -- losing less power in the wires. With non-adjacent single pole breakers, in theory that could work if the two are on different legs. However there are now code rules that say the two breakers in that MWBC would need to be tied together to be shut off to work on the circuits. The two breakers can trip independently. There is discussion about what would constitute proper tying.

So what works technically is somewhat independent of what the rules allow.

When using GFCI MWBC (shared neutral) then you need a 2-pole breaker because the shared neutral must go through that breaker. I guess the same thing would apply for 3-phase 208. I have not tried to look at what breakers would be available.
 
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ChuckGM

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You guys are great - and thank you again for carrying on this conversation with me! When you're attempting to answer my questions remember I'm a retired air-traffic controller not one of you! ;) So, to follow up on the last. For me, for practical and safety purposes, in an old house (my last), where I am attempting to clean up old wiring while remodeling 1/2 the basement to include an efficiency apt. we can rent out.... I like to see if I can "make things right" - make it safer - more efficient etc....
1.- For me, a 2-pole breaker is just understood as how you wire a 220 circuit. Are you saying that one can also be used to wire "separate" circuits sharing a neutral but that if one trips they both trip?
2. - Are you also saying... If I have a 12/3 wire running down to the far end of the house, powering completely separate circuits but sharing a neutral that the "best" thing for me to do would be to ensure that the breakers for those circuits are on separate legs? (understanding there may be some debate amongst professionals re. how new rules are applied)?
3. - So, to clarify in my head, - #1 and #2 are both viable options but #2 is generally easier and more practical?
4. - but/and/?? if one (or both) of those circuits sharing a neutral has a gfci receptacle or is a gfci breaker that it MUST either have it's own neutral OR it is a case for #1 above?
--- that make sense?
 

wwhitney

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1) Yes. Except I wouldn't call them "separate circuits sharing a neutral" I would call them an MWBC (multi-wire branch circuit). Mostly because it's a sufficiently special case that the notions of separate and share are not so clear, but MWBC describes it exactly.

2) Yes, not just "best" but "mandatory". Failure to have the two "hots" on opposite legs is dangerous because the neutral can be overloaded.

3) In #2, the individual breakers are required to be next to each other and connected with a handle tie. Otherwise, yes.

4) The restriction applies to wiring that is downstream of and protected by a GFCI, any load has to be supplied either only by conductors going through the GFCI, or without using any conductors going through the GFCI.

So for GFCI breakers, in (1) you'd need a 2-pole GFCI breaker, and (2) is not an option. But you could still have GFCI receptacles on an MWBC. You can either just not use the load side terminals (so if you had 5 receptacles you wanted GFCI protected on an MWBC, you'd use 5 separate GFCIs), or you can split the neutral to comply with the above restriction (so with 5 receptacles, at the first GFCI, you'd split the 3-wire MWBC into (2) 2-wire circuits, and you could just use (2) GFCIs, with the other receptacles supplied from the load side of one of the GFCIs).

Cheers, Wayne
 

Reach4

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1.- For me, a 2-pole breaker is just understood as how you wire a 220 circuit. Are you saying that one can also be used to wire "separate" circuits sharing a neutral but that if one trips they both trip?
Yes. A GFCI fault would turn off both. Now do they have a GFCI 2-pole breaker which will only trip out one side in the case of an overcurrent trip? They could, and maybe they do, but I did not find that in my brief search. It should exist.

2. - Are you also saying... If I have a 12/3 wire running down to the far end of the house, powering completely separate circuits but sharing a neutral that the "best" thing for me to do would be to ensure that the breakers for those circuits are on separate legs?
Sharing a neutral is called MWBC (multiwire branch circuit) and it is important that they be on different legs. This is where rules changes have come in to let an electrician be able to readily turn off both hots of the MWBC for the safety of the person working on the wiring. This is an area of discussion as to how this can be done.

3. - So, to clarify in my head, - #1 and #2 are both viable options but #2 is generally easier and more practical?
I have an incomplete comment. Separate breakers have the advantage that overcurrent on one does not take out the other. If you have independent trip on a 2-pole breaker, then overcurrent one one does not shut off the other side. That feature should IMO be available with GFCI, but I am not sure it is. Easier for whom? The designer, the electrician running one more neutral wire through a conduit, and keeping track of which neutral is which? More practical for the occupant?

MWBC allows mixes of 240 and 120 volt outlets. That can be advantageous at times.

4. - but/and/?? if one (or both) of those circuits sharing a neutral has a gfci receptacle or is a gfci breaker that it MUST either have it's own neutral OR it is a case for #1 above?
--- that make sense?
No and no. If the outlet has its own gfci, then it doesn't care what neutrals have done before the input ("line") terminals. Any loads on the load terminals must use the neutral on the "load" terminals of the gfci.
 
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wwhitney

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Now do they have a GFCI 2-pole breaker which will only trip out one side in the case of an overcurrent trip? They could, and maybe they do, but I did not find that in my brief search. It should exist.
No, I don't think they exist. 2 pole breakers are always common trip to my understanding. If you want non-common trip, use two single pole breakers. Which is obviously not a possibility for GFCI breakers.

[Quad breakers (a two position double tandem) are another story and may or may not be common trip on the outside two connections. But I'm not aware of any GFCI tandem or quads, I believe GFCI breakers are always full size.]

Cheers, Wayne
 

Reach4

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No, I don't think they exist. 2 pole breakers are always common trip to my understanding. If you want non-common trip, use two single pole breakers. Which is obviously not a possibility for GFCI breakers.
Here is a search for you: "independent trip" breaker It is possible that a breaker have independent trip for overcurrent, and common trip for GFCI. That is the combination that may not exist in practice-- yet.

Examples of independent trip 2-pole breakers would be BR215AFIT and CH215AFIT Also find "Independent Trip" 7 times in https://www.platt.com/CutSheets/Eaton/eaton-ch115afcs-catalog page.pdf
For me, for practical and safety purposes, in an old house (my last), where I am attempting to clean up old wiring while remodeling 1/2 the basement to include an efficiency apt. we can rent out.... I like to see if I can "make things right" - make it safer - more efficient etc....
For that, replace all outlets with combo gfci afci outlets rather than breakers. Where you know there are no neutrals shared beyond a point, you could use the load side of the gfci outlet. I think this would work: to test if the neutral beyond a point is shared, you could open the neutral but leave the hot connected. Make sure there is no power on the downstream outlets with the breakers all back on.

If the boxes are not big enough for gfci/afci outlets, put in 2-pole common trip gfci afci breakers.
 
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wwhitney

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Here is a search for you: "independent trip" breaker It is possible that a breaker have independent trip for overcurrent, and common trip for GFCI. That is the combination that may not exist in practice-- yet.

Examples of independent trip 2-pole breakers would be BR215AFIT and CH215AFIT Also find "Independent Trip" 7 times in https://www.platt.com/CutSheets/Eaton/eaton-ch115afcs-catalog page.pdf
So, for regular breakers, not AFCI or GFCI, there's no reason to make independent trip multi-pole full size breakers, as the functionality is the same as using multiple single pole breakers with handle ties.

The example you found of independent trip double pole AFCIs is from 2003, so it refers to the now obsolete first generation AFCIs, (Branch/Feeder), not the second generation now in use (Combination). So obviously it is possible to make such a breaker, but I've not seen any current model AFCI or GFCI multipole independent trip breakers. Maybe they are out there but I've missed them.

Cheers, Wayne
 

ChuckGM

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Okay - I continue to learn - this is awesome!
couple more questions.
A. - I don't remember that it was answered specifically (?) so, I'm still thinking that shared neutrals might very well be my cause of "ghost current" - right?
B. - What is the downside with using a single pole afci/gfci breaker with old wiring circuits? i.e., why new receptacles rather than breakers?
C. - remember my level of "expertise" here ;-) I have to keep kicking myself to remember that two breakers next to each other on the same side ARE on separate legs!
---- other than that, it could take me a couple of weeks to find the time to run through everything I've got to do. Don't take any long vacations or anything! I'm sure I may have questions along the way!
Thanks!
 

ChuckGM

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Oh shoot...
"D." --- My 200A main panel (square D) has a neutral bar on each side (grounds and neutrals mixed) - Is there one on each side simply for "convenience"? or are they tied in differently somehow?
 

jadnashua

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The 240vac power coming into the house is coming in from a transformer. Each of those hot leads is coming from opposite ends of the secondary of the transformer coil. 120vac is sort of a phantom, as the power goes through the transformer coil, what they call neutral is tapped off of the middle of the coil so that measured from there to either end, you get 120vac. But, a property of a coil like that is that when one end is high, the other end is low on the sine wave, which makes the middle half-way. In a MWBC, each hot lead is connected to one end of that transformer, so, when one line is at the high end of the sinewave, the other end is at the low end. If you're running power entirely on one branch, the power goes out on one hot and returns on the neutral, so each wire has the same amount of current on it. But, if you now start to pull power off of the other branch, what is returning on the neutral from that side is out of phase from the power on the former branch, and they cancel out, so essentially, the neutral is never carrying the combined power...if it added, the neutral conductor would now be trying to carry up to twice the current the wire gauge in the cable was designed for. Because the neutral is half-way across the coil, it functionally makes it like two transformers, and remember that the phase on each end is opposite of the other, so when one hot lead was positive, the middle (neutral would be negative)...the other side of the coil would be negative, but RELATIVE to it, the neutral would now be positive...the positive and negative values (when the current is the same) totally cancel each other out, but as the power used in each branch normally isn't identical, it could go from 0-100% the same, the neutral won't ever try to carry more than the hot from its associated circuit.

A downside of a MWBC is, if you lose the neutral connection, you might end up seeing 240vac where you were only expecting 120vac.

Because the neutral can be carrying power from either branch, when you trip a breaker, you expect you've cut off all paths for the power...but, if both branches don't shut off at the same time, the shared neutral could easily still be having power running through it in the box where you have the other branch shut off...

What complicates this a bit more, is that they bond neutral to ground, so you may not be able to measure a voltage on it, but that doesn't mean that there's not still the possibility of current running through the connection. The ground is there to offer an alternate path for power when there is a fault, to trip the breaker or blow a fuse...it should never be used as a current carrying conductor in normal operations.

Did that help, or confuse things more?!
 

Reach4

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For me, for practical and safety purposes, in an old house (my last), where I am attempting to clean up old wiring while remodeling 1/2 the basement to include an efficiency apt. we can rent out.... I like to see if I can "make things right" - make it safer - more efficient etc....
B. - What is the downside with using a single pole afci/gfci breaker with old wiring circuits? i.e., why new receptacles rather than breakers?
Old circuits may have shared neutrals. Sharing of neutrals before the GFCI does not cause a problem. So it saves having to analyze the wiring.
 

wwhitney

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Old circuits may have shared neutrals. Sharing of neutrals before the GFCI does not cause a problem.
Depends on your definition of problem. It's an NEC violation, and it will generate EMF. But the loads will still work.

Cheers, Wayne
 

Reach4

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Depends on your definition of problem. It's an NEC violation, and it will generate EMF. But the loads will still work.
Let's define sharing too... MWBCs is sharing, and that is not a violation per se. Right? Generating EMF? uhhh...
 

wwhitney

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A You'll have to define what you mean by "ghost current." If you shut off one leg of an MWBC, you'll see current in the neutral from the other leg; that's normal and expected. And you should be able to tell in the panel it's an MWBC because the neutral and the two "hots" should be leaving the panel in the same cable, or otherwise be associated with each other (e.g. zip tied).

However, if you have two 120V 2-wire circuits, and their neutrals are intermingled/interconnected/mixed up somewhere, that's a problem worth fixing in my book. You could again detect current at the panel in the neutral associated with a breaker that's off. You can figure out what circuit is the source of the current by turning off the other breakers, one at a time, until the neutral current goes to 0. Then you can try to figure out where those two circuits are likely to have gotten mixed up, search for the problem, and fix it. Just like the problem that started this thread.

B Reach4 covered this

C Not a question

D Yes, convenience. The main panel is the only place where neutral and ground may be mixed. Most likely the two bars are neutral bars, and you have a (possibly green) grounding screw as your main bonding jumper connecting those bars to the case, and the EGCs (grounds) are just landed on the neutral bar out of convenience. If your panel were not the main panel, there would be separate bars attached to the case for the EGCs, and the neutral bars would not be connected to the case.

Cheers, Wayne
 
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wwhitney

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Let's define sharing too... MWBCs is sharing, and that is not a violation per se. Right? Generating EMF? uhhh...
Well, I've been consistently calling an MWBC one circuit in this thread, and so when I refer to sharing I mean two 2 wire circuits with the neutral getting mixed up. If you want to call an MWBC two circuits with a shared neutral (as is also common and sometimes a useful way to think about it), you'll need to distinguish between those two kinds of "sharing".

As to EMF, current generates a magnetic field. The "supply" and "return" conductors are supposed to be in the same cable/conduit/etc, and then the magnetic field is confined to within the cable/conduit. When they aren't (because the neutral of one 2 wire circuit is connected to either a neutral of another 2 wire circuit, or a load supplied by a different circuit), then the magnetic field spans the area/volume within the entire current loop. Which isn't necessarily a problem but is certainly bad form, and might conceivably interfere with sensitive electronics (probably not at household power levels). Certainly it can be detected with a sensitive meter (which you can make with a big coil of wire). I'm not endorsing any of the kooky "EMF is bad for you" stuff, nor do I know if their "meters" are actually measuring what they claim.

Cheers, Wayne

P.S. One of the reasons for the "all circuit conductors in the same cable/conduit" rule is that when not, if there's any ferromagnetic material in the current loop, it will heat up from the induced current. So the NEC prohibits that (and has an exception when there's no ferrous materials involved). The Canadian Electrical Code recognizes that it's not a practical problem below current levels on the order of hundreds of amps, so their prohibition only kicks in at 200A.
 
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