[Rav]

I'm currently reading Wiring Simplified to increase my understanding of things electrical and improve my DIY skills. I'm on the chapter about 3-wire circuits and MBWCs. There is an admonition against using the two terminal screws on receptacles and lighting outlets for neutrals, because, it says, "removing a receptacle, such as for replacement, would then result in a break in the neutral wire during the time there is no connection to the receptacle. This would in turn place all the receptacles connected to one leg (beyond the receptacle temporarily removed) in series with those connected to the other leg, all at 240 volts. Appliances would malfunction and be ruined or at least badly damaged, and lamps would burn out." It recommends using pigtails and wire nuts instead, so as to not have downstream receptacles depend on the device for continuity.

I don't understand this for a number of reasons. First, if I was replacing a receptacle, I would make sure to turn off the power to the circuit (or circuits) that feed that receptacle, and I wouldn't restore power until all wires were reconnected. Since the power is off, nothing downstream can be affected. Even if I had used wire nuts and a pigtail on the neutrals, I would never disconnect the pigtail from the terminal while it was live. Second, if I was permanently removing the receptacle, I would make sure to reconnect the wires so that they feed through before restoring power. Third, I don't see why properly connecting wires to the two terminals would be any less dependable than using wire nuts. If a device can fail, then so can a wire nut. Why bother having two terminals available if we're not supposed to use them? Plus to me it's easier to see what's going on when using the terminals rather than adding a pigtail and a wire nut, plus less wires to stuff in the box, etc.

Can people here help me understand this? Thanks.

 

[Daniel Holzman]

The point of the note was to emphasize that your wiring job should make sense to the next person who works on the circuit, not just you. If the next person decides to remove the receptacle, and is not aware that it is a MWBC, they could potentially nut off both neutrals without realizing the consequences. This could happen if they were temporarily removing the receptacle. So the point of the note is to be overly cautious, and look further down the line than just what you might do.

 

[IslandGuy]

You only have to read a few electrical posts here to see that the loss or disconnection of a feed-through neutral on a device is not an uncommon problem. On a typical 2-wire circuit this only results in the loss of continuity to downstream devices, but in a MWBC the result can be devastating for anything plugged in downstream.

 

[Rav]

The point of the note was to emphasize that your wiring job should make sense to the next person who works on the circuit, not just you. If the next person decides to remove the receptacle, and is not aware that it is a MWBC, they could potentially nut off both neutrals without realizing the consequences. This could happen if they were temporarily removing the receptacle. So the point of the note is to be overly cautious, and look further down the line than just what you might do.

Thanks -- it certainly makes sense that the next person might not know and that I should consider them. But why would they nut off both neutrals and restore power that way, even if just temporarily? Even in a two-wire circuit, that would be an open neutral for everything downstream, wouldn't it? Shouldn't they nut them together, just as they would the hots? Not trying to argue, I'll do whatever is recommended, just trying to understand. Thanks.

 

[Rav]

You only have to read a few electrical posts here to see that the loss or disconnection of a feed-through neutral on a device is not an uncommon problem. On a typical 2-wire circuit this only results in the loss of continuity to downstream devices, but in a MWBC the result can be devastating for anything plugged in downstream.

OK, I think I'm beginning to get it. No matter what I (or the next person) SHOULD do (such as make sure the neutrals feed through), the consequences of a mistake are so high in the case of a MWBC that it's just safer to do it safer.

 

[joed]

Here is a video demonstrating what happens with the circuit when the neutral opens.
Not only is it safer. Code requires you to do it this way.

 

[Rav]

Here is a video demonstrating what happens with the circuit when the neutral opens. Not only is it safer. Code requires you to do it this way. Video Link: https://www.youtube.com/watch?v=HsSlQnGCvdg

Thanks. I watched that (twice) plus a few other videos on open neutrals. I will definitely wire per code.

 

[mpoulton]

On a newly installed MWBC using a double pole breaker or handle ties, the risk is greatly reduced because both halves of the circuit will always be turned off at the same time and it's readily apparent from looking at the panel that it's a MWBC. It didn't used to be that way. Older MWBCs were often wired using non-adjacent breakers and without labeling. Turning off one breaker would kill power to the receptacle you're working on, but you'd have no way of knowing that there's another half to the circuit and current was still flowing in the neutral. Until you disconnect it...

 

[Rav]

On a newly installed MWBC using a double pole breaker or handle ties, the risk is greatly reduced because both halves of the circuit will always be turned off at the same time and it's readily apparent from looking at the panel that it's a MWBC. It didn't used to be that way. Older MWBCs were often wired using non-adjacent breakers and without labeling. Turning off one breaker would kill power to the receptacle you're working on, but you'd have no way of knowing that there's another half to the circuit and current was still flowing in the neutral. Until you disconnect it...

Thanks. Yes, in fact I suspect that my circuits 2 and 3 are (at least to a certain location) an MWBC. They are, of course, non-adjacent and not labeled as MWBC. I suspected that it's an MWBC because at one point near the panel, one NM cable alarms my non-contact voltage tester whether I have turned off breaker 2 or 3 separately. It's only when I turn off both breakers that the tester goes silent. So, with my slowly increasing but still newbie understanding of these things, that makes me think it's an MWBC since one cable contains both circuits. Since I want to have AFCI protection added to both circuits 2 and 3, and once it's verified that they are an MWBC, the plan is to move breaker 3 to slot 4, so that they are adjacent, and put in a double-pole AFCI breaker. Don't worry, I wont be doing that work myself. I'll have an electrician verify all this and do the appropriate work.

 

[Oso954]

I suspected that it's an MWBC because at one point near the panel, one NM cable alarms my non-contact voltage tester whether I have turned off breaker 2 or 3 separately.

You really need to invest in a multi-meter or wiggy for testing. Non-contact voltage testers are only a quick safety check. They often give false readings, indicating that something is hot when it is not. And occasionally they can fail to ID a hot.

All the alarm should tell you is, don't touch me until you shut me off or put a real voltage tester on me.

While it might be a MWBC, it could also just be that the wires/cables for those two circuits parallel each other at some point.

If you have two XX/2 NM cables involved, that's a good indication that is not a MWBC. If you have an XX/3 NM, it is a good indication that it may be.

 

[Rav]

You really need to invest in a multi-meter or wiggy for testing. Non-contact voltage testers are only a quick safety check. They often give false readings, indicating that something is hot when it is not. And occasionally they can fail to ID a hot. All the alarm should tell you is, don't touch me until you shut me off or put a real voltage tester on me. While it might be a MWBC, it could also just be that the wires/cables for those two circuits parallel each other at some point. If you have two XX/2 NM cables involved, that's a good indication that is not a MWBC. If you have an XX/3 NM, it is a good indication that it may be.

Thanks. It's just a single NM cable, not two cables (at the point that I used the tester). I'm not home now but when I am I can open the junction box it goes into and let you know what I see. I have thought about getting a multi-meter, and looked at a few, but got overwhelmed with the choices. Can you recommend a decent one for home DIY use? I don't want to spend more than necessary or have it be more complicated than my needs require, but want it to have the capabilities that basic wiring work would need (replacing receptacles, lights, stuff like that). From what I've read, auto-ranging would seem like a good thing to have.

 

[Know A Little]

I don't understand this for a number of reasons. First, if I was replacing a receptacle, I would make sure to turn off the power to the circuit (or circuits) that feed that receptacle,
Can people here help me understand this? Thanks.

It is not just the circuit for that receptacle you are working on, it can be other circuits connected to the multiwire circuit that that receptacle neutral shares. You may not ever be aware this is a multiwire circuit if you do not remove the panel cover. Even then the circuits may not be landed adjacent to each other.

 

[Oso954]

I have thought about getting a multi-meter, and looked at a few, but got overwhelmed with the choices.

You are over analyzing it. Any multimeter is better than no multimeter.
Buy yourself an inexpensive multimeter. For occasional homeowner use, it is all you need, particularly starting out.
I know people that are happy with a harbor freight freebie.

Personally, I think beginners are better off starting on an analog, but if you want digital, go for it.

 

[Luke M]

If you kill only one of the circuits and disconnect the neutral, then 120V will appear on the downstream neutral. If you touch it you'll be in series with the load, and likely be shocked.

Incidentally, the same happens if the neutrals of two circuits are cross connected.

If you disconnect the neutral without killing either circuit, then really strange things happen. Each load sees between 0 and 240V, depending on the relative load of the two circuits. This is why some people don't like shared neutrals.

 

[Know A Little]

I.

Incidentally, the same happens if the neutrals of two circuits are cross connected.

Explain please

 

[Luke M]

Explain please

Imagine two long circuits eventually meeting in a dual gang box. One outlet is wired with circuit 1's hot and circuit 2's neutral, the other with circuit 2's hot and circuit 1's neutral.

Upstream of this box, you kill circuit 1 and disconnect the neutral to change a socket. Put a volt meter on the downstream neutral, it reads 120V. Unless you kill circuit 2 also.

With the use of AFCI breakers nowadays, neutral wiring errors are getting caught. But in the past they weren't.

 

[Know A Little]

Imagine two long circuits eventually meeting in a dual gang box. One outlet is wired with circuit 1's hot and circuit 2's neutral, the other with circuit 2's hot and circuit 1's neutral.

Upstream of this box, you kill circuit 1 and disconnect the neutral to change a socket. Put a volt meter on the downstream neutral, it reads 120V. Unless you kill circuit 2 also.

With the use of AFCI breakers nowadays, neutral wiring errors are getting caught. But in the past they weren't.

Unless I missed it I did not see yo saying you were opening a neutral in this example. Kind of vague to me.

Incidentally, the same happens if the neutrals of two circuits are cross connected.

 

[TimPa]

nice job n the video joed!

for some time manufacturers were installing a 120 mov ($0.10) on the input of sensitive appliances. haven't opened anything up in a while to see if they still do.

 

[Jim Port]

There is only one brand of two pole afci breaker available AFAIK. You may not be able to add afci protection. You could switch to a standard two pole breaker.

 

[Rav]

There is only one brand of two pole afci breaker available AFAIK. You may not be able to add afci protection. You could switch to a standard two pole breaker.

In another thread I started, Jon Ciccarone thought he found an AFCI breaker that would work in my panel. He said:

Yes, if possible, the simplest solution would be to swap the circuit breaker (and hot wire) in slot 4 with the circuit breaker (and hot wire) in slot 3, so that the MWBC is stacked. Then replace the now stacked circuit breakers 2 and 4(formerly 3) with a double-pole AFCI breaker that fits your load center, like this one: http://m.homedepot.com/p/Square-D-Q...H=REC-_-irg_aic-3-_-202353327-_-204844653-_-N

Is that the one you mean, and do you think it would in my panel? Thanks.

Panel: QO Load Center Cover, Cat. No. QOC3OU, series G1, type 1 enclosure, Square D Company, stamped with number “F0605031."