Electricians: mwbc yay or nay?

[NetWareHead]

Pretty much says it all. How do you guys feel about multi wire branch circuits? After some consideration, I decided not use them because of the following concerns:

If the neutral in an mwbc is opened, the resulting loads connected will see a spike in voltage. The two 2 120 volt circuits will change to a single 240 volt and will usually burn out an appliance. A neutral working itself loose or someone accidentally disconnecting it could result in a fire. Also you need to pigtail the neutral, since that pesky neutral can't be disconnected.

Secondly, failure to terminate the circuits to different phases in the panel will over load the neutral. Or overloading one leg will lead to an unbalanced condition; something I could see happening with receptacles. The average home owner just plugs stuff in with no understanding of a mwbc. It's arguable that heavier guage wire could be installed where even in an unbalanced condition, the wire could safely carry the extra current

The pros of mwbc? I understand the reasoning for it. Pulling 1 run of 12/3 is cheaper than 2 runs of 12/2. It might be easier to snake 1 cable vs 2, important consideration for old work. And one less conductor in the box could help to easier meet box fill requirements.

How does mwbc handle gfci/afci breakers? Are two pole breakers available like this? Is the resultant wiring compatible with afci/grci?

Regardless of the benefits, I think the cons outweigh and I try not to use them. I think the wiring, should it be disconnected, is fail-deadly vs a normal circuit where a loose wire fails safely. The connected loads and which leg they reside on are important in a mwbc. All for the reasoning of saving one cable run? Am I missing anything or are there any proponents of mwbc here? Is there a situation where an mwbc is the only solution?

 

[Red Squirrel]

I would only use one for a dedicated outlet. There is little to no chance of the neutral failing as it will be a straight run with only 2 terminations, one at the panel and one at the outlet. Make sure you use a double pole breaker as well. Don't use the back stab feature of the outlet either, use the screws. I personally like to use my plyers and make a small hook then hook it on the screw in the direction it has to be turned, then tighten it. Makes for a really solid connection. Don't forget to break the tab on the hot side! (been there done that. lol)

Though for non dedicated outlets (ex: strings of general purpose outlets) or for outlets that will be for a single use with no chance of growth (ex: a fridge) then I would just do a normal circuit.

Keep in mind that a neutral failure does not necessarily mean the loads see 240, what will happen is both sides will be in series with each other and have 240 together. Depending on their impedances they'll go out of balance so it's still a bad situation, but it's not always going to be bad. Ex: a load could see 140v and still work fine (but it will be out of spec and shorten it's life) and then the other might see like 100v. If one of them is off then the other won't work either. So say you have an alarm clock and a lamp plugged into a MWBC and the lamp is off, if the neutral fails the clock will just turn off.

 

[Micrornd]

Nay. For all the reasons you listed.

Plus 12/4 w/ground is readily available at both Lowes and HD, in addition to just about every electrical supply house (at least here in the US), so there is no excuse about having to pull 2 wire runs.
And right or wrong additionally, some local jurisdictions (here in FL, at least) have made it against code for some of the reasons you listed.

 

[Greenman]

Since most circuits require AFCI breakers it's a moot point, they don't work work with shared neutral.

 

[NetWareHead]

Since most circuits require AFCI breakers it's a moot point, they don't work work with shared neutral.

Thanks. One of the answers I was looking for. It seems that mwbc was borne of an era where the cost of each copper conductor was enough of a consideration to warrant it's use and permit a shared neutral between 2 circuits.

Nay. For all the reasons you listed.

Plus 12/4 w/ground is readily available at both Lowes and HD, in addition to just about every electrical supply house (at least here in the US), so there is no excuse about having to pull 2 wire runs.

Good point. Cable like this makes one cable pull possible and gives both circuits it's own dedicated neutral. I'd rather run 2 circuits over 12/4 than 2 mwbc circuits over 12/3.

 

[NetWareHead]

Keep in mind that a neutral failure does not necessarily mean the loads see 240, what will happen is both sides will be in series with each other and have 240 together. Depending on their impedances they'll go out of balance so it's still a bad situation, but it's not always going to be bad. Ex: a load could see 140v and still work fine (but it will be out of spec and shorten it's life) and then the other might see like 100v. If one of them is off then the other won't work either. So say you have an alarm clock and a lamp plugged into a MWBC and the lamp is off, if the neutral fails the clock will just turn off.

Good explanation. The series makes it not a conventional 240 v but had the potential to deliver as much. And one leg being off makes sense too, considering they are wired in series. The lamp seems to be in front of the alarm clock and if off, the clock dies too.

 

[stormkroe]

Believe it or not, homeline has some 2 pole afci breakers for use with mwbc. While I don't personally like them, there's no ultra-compelling reason not to use them (the entire house is fed off one, for instance). The most common point of neutral connection failure is outside the home on the transformer, in which case every 120v circuit in the home would be susceptible to possible 240v unless the panel was properly grounded.
OP, in the end of the paragraph of your 'reason #2' you mention using heavier wire due to circuit overloading. What is your reasoning for this? If a homeowner or anyone else plugs in too much load, the breaker will simply trip regardless of over sizing of the wire.

 

[NetWareHead]

Believe it or not, homeline has some 2 pole afci breakers for use with mwbc. While I don't personally like them, there's no ultra-compelling reason not to use them (the entire house is fed off one, for instance). The most common point of neutral connection failure is outside the home on the transformer, in which case every 120v circuit in the home would be susceptible to possible 240v unless the panel was properly grounded.

Interesting, never considered this. Makes me glad I ran a dedicated #4 copper wire to a rod outside.

OP, in the end of the paragraph of your 'reason #2' you mention using heavier wire due to circuit overloading. What is your reasoning for this? If a homeowner or anyone else plugs in too much load, the breaker will simply trip regardless of over sizing of the wire.

I was discussing with my electrician about mwbc and he worked a job where he had to correct numerous code violations including a homeowner running an mwbc but used 2 separate breakers (without a handle tie) and on the same phase. I understand that an mwbc that draws from 2 separate phases and when using separate phases, the return current on the neutral is the difference. On a miswired job that draws from the same phase, the return neutral current is the sum and can overload the neutral wire's capacity. I was speculating that perhaps using heavier gauge wire for just the neutral would be a way to prevent this neutral overload...or just install it correctly in the first place.

 

[Red Squirrel]

Speaking of grounding, wonder how many people get plumbing done where they replace copper with pex and it ends up breaking the ground and they don't realize it. It happened to me recently when I got my main valve changed. I'm aware of it and need to fix it, but I bet most people don't even think of it.

 

[Greenman]

Believe it or not, homeline has some 2 pole afci breakers for use with mwbc. While I don't personally like them, there's no ultra-compelling reason not to use them (the entire house is fed off one, for instance). The most common point of neutral connection failure is outside the home on the transformer, in which case every 120v circuit in the home would be susceptible to possible 240v unless the panel was properly grounded.
OP, in the end of the paragraph of your 'reason #2' you mention using heavier wire due to circuit overloading. What is your reasoning for this? If a homeowner or anyone else plugs in too much load, the breaker will simply trip regardless of over sizing of the wire.

Never heard of such a thing. Though when you think about it, there isn't any reason why it shouldn't work.

I've been getting call backs because of fussy AFIC's. Some of them won't take an electric motor, a blow dryer or even a curling iron. I have one customer that wants all of them replaced with standard breakers.

 

[stormkroe]

Never heard of such a thing. Though when you think about it, there isn't any reason why it shouldn't work.

I've been getting call backs because of fussy AFIC's. Some of them won't take an electric motor, a blow dryer or even a curling iron. I have one customer that wants all of them replaced with standard breakers.

They're a nightmare. I understand the sleeping quarters code, where people cram their dressers back and kink the clock/lamp/charger cord behind it, but the blanket usage is really nothing more than manufacturers controlling new codes.
In canada, refrigerators are exempt from afci requirements, but your deep freeze isn't (you know, the one that has hundreds of dollars of meat in it usually). Microwave magnetrons trip them, vaccum cleaners, some tvs do, etc. And lets not forget the increase in price that the customer has to take. If I have 15 afci's in a house (they're almost $50 more than a standard each) that's a TON of money for a homeowner.
Frustrating.

 

[Greenman]

I also hate that they take up a full space in the panel. Most of the remodels I do we have to put in a sub panel because there isn't room in the main for all the fat AFCI breakers. I always end up having to get a 125 sub and down grade it to 60 amps just fit them all in.

I just had an inspector bitch because I bundled the wires in the sub to keep it neat. On another job 6 blocks away in another city the inspector bitched because I didn't bundle the sub panel wiring, he said it looked sloppy. Sometimes you really can't win.

 

[NetWareHead]

I also hate that they take up a full space in the panel. Most of the remodels I do we have to put in a sub panel because there isn't room in the main for all the fat AFCI breakers. I always end up having to get a 125 sub and down grade it to 60 amps just fit them all in.

Yep... https://goo.gl/photos/fEnmGeuDz9SS5dGJ7

An old square D QO panel from the 80s I thought didn't need upgrading. I mean afcis fit right and they work with the panel?? Now Im stuck with it and Im never going to open it again unless absolutely necessary. Barely room as is and the afcis cover the grounding bar on the side. Barely room to run wires up and out the top. If I need to change/add a breaker on the left side, minimum 3 breakers have to temporarily come out. Plus I hate the stupid top neutral bar. I should have changed it when I had the chance but too late now...

I just had an inspector bitch because I bundled the wires in the sub to keep it neat. On another job 6 blocks away in another city the inspector bitched because I didn't bundle the sub panel wiring, he said it looked sloppy. Sometimes you really can't win.

A fickle bunch those inspectors are. I came from the low voltage cabling world and neatness was godliness. Loved creating cool huge cable waterfalls with cat5/6 etc... When I started doing electric, I carried that habit forward and got slapped on the wrist because my tendency to aggregate the cables together into neat progressively larger bundles was a no-no. Its gotta be neat enough so the inspector can see what you did but if you bundle all your cables together too neatly, the heat buildup can cause a fire. The most I ever bundle together is 3-4 at a time now.

 

[Greenman]

OCD is a great thing to have if you're an low voltage guy.