Multiwire Branch Circuit question

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Old 08-28-08, 06:35 AM
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Multiwire Branch Circuit question

I have been hearing that there is a right way and a wrong way to wire circuit breakers to a MWBC. Is this true? Someone told me that the breakers cannot be on the same phase as each other? I am not sure what this means and what that would look like in the main panel if that was the case. Currently, I have 2 MWBC circuits in my panel. They are wired with 2 15 single breakers one underneath the other..not side by side see diagram:

Buss Bar 1 Buss Bar 2


*********** **********
*********** **********
*********** **********
first MWBC breaker **********
second MWBC breaker **********
*********** **********
*********** **********
*********** *********
First MWBC breaker **********
Second MWBC breaker **********


In the above diagram, the stars stand for conventional breakers either 20 amp or 15 ect.
LIke I stated above, these MWBC are on top of each other and each one has a diff wire (black for one, red for the other) and then the WHITE wire for BOTH sets are wire nutted together with a single piece of white which goes to the neutral......is the a correct setup???

Thanks!
 
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Old 08-28-08, 06:54 AM
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What you describe is not a shared neutral. You said both white wires are wire nutted together to neutral. In a shared neutral you only have one neutral and the cable would be 3 wire with ground, black, red, and white.

A shared neutral connection must have the red and black wires on different poles. A double pole breaker is used to accomplish this. It would occupy two slots in the panel vertically.

It sounds like (maybe) someone used 4 wire cable with ground to run two circuits????

On a shared neutral if both the black and red wire were on the same pole at the main the neutral could be subjected to the combined currents of both legs and that would exceed the wire capacity. If they were on different poles, like they should be, the current would never exceed the maximum current of either pole. They also cancel, so that if the exact same current flowed on both poles there would be no current in the neutral.
 
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Old 08-28-08, 07:38 AM
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The fact of having the 2 whites serving 4 different circuits all connected together does not sound correct at all.

What do these cicuits serve? Are there available holes in the neutral buss?
 
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Old 08-28-08, 07:57 AM
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OK
This work was done by a qualified electrician well over 15 years ago. The house was built in 57. How I discovered this mess is because when I went to install a double pole AFCI on one MWBC setup, I noticed that the white wirefor that circuit, which is supposed to connect to the neutral on the AFCI, was wirenutted to another white from another MWBC. To these 2 white wires, a white jumper was used to go to the neutral buss. Yes, there are OPEN spaces and I even thought about splitting the whites on each MWBC to a seperate hole on each neutral bar but I dont want to go messing around because I am not sure what this setup is or if its even safe! Again, an electrician did this when the original panel was updated 15+ years ago and I am now just discovering it because I am installing AFCI's.
The MWBC 15amp single breakers are placed vertical so I assume that they are out of phase to negate overloading the neutral?
When I Installed that double pole AFCI, it immediately tripped so I just took it back out and put the original standard breakers in it and it is working OK now......
What these circuits power:
2 bathrooms
3 bedrooms
utility room lights
4------15 amp circuits in all set up in 2 MWBC's
****I wanted to protect them both with double pole AFCIs (I have them but I think that there is something wrong...)

I know CODE requires seperate 20 Amp bathroom circuits GFCI protected but I was saving that huge rewiring project for a later date due to tight budget).......
what am I dealing with here?
job for an electrician??
Thanks everyone
 
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Old 08-28-08, 10:52 AM
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Here's a simple rule of thumb. If you have a "cable" with red, black and white (and hopefully green) terminating at a double pole or two singles, there must be 240 volts between the red and black. In certain cases the two singles (even if they read 240v between them are required to be joined so that they trip together.
 
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Old 08-28-08, 04:27 PM
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if im getting what your saying right which i think i am your telling us that you have 2 three wire cables basically out of both you have a black,red,white,and green in each cable the red and the black out of those two cables go to your breakers and the white from one cable and the white from the other cable go to a splice pigtail off and go to the neutral bar. correct? if thats the case there is no doubt your afci's are not working because of the simple face that afci's will not work on 2 circuits that share a neutral it requires (like a gfci) that the hot wire off the breaker have its own white wire so it can monitor through a loop. for lack of better words being that they share a neutral you are getting extra sensitivity or mixed signals being that you have two afci's monitoring back on the same neutral. there is a saying "if its not broken don't fix it." that type of circuit you have is long since been revised and is now illegal for future work. Each circuit must have its own neutral except for the obvious devices/appliances that are two pole that require it. what i suggest is that you skip messing with whats in the panel, just put in afci receptacles in the rooms that you want them in cause it sounds like your circuits go to multiple rooms anyway so if you can find the receptacle in each room that feeds the rest put an afci there and that will provide you with the protection you need and i think that will bypass the hassle of it shutting off the circuit instantly because your upstream of the neutral splice and the afci does not monitor back to the panel only where you put it and receptacles downstream so you should be fine putting in afci receptacles. Just in case your unclear though, work that was already done is grandfathered in you dont have to go back and fix its fine. but if it was your choice to do it then what i said above would be your best bet.
 
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Old 08-28-08, 04:57 PM
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Chewlu
You hit the nail right on the head. That is EXACTLY how things are with respect to the wiring of the 2 MWBC circuits. Now, was this a common wiring schematic back in the 50's? Why would they wire it like that? To me, that makes no sense. It seems like a very complex circuit and after seeing how all this is set up, I doubt that I will mess around with it anymore.
Does this setup mean that those 2 MWBC's share neutrals with all those circuits? I am way confused as to why this was engineered the way it was..care to enlighten me some more?
 
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Old 08-28-08, 05:28 PM
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usually when an electrician does a service change/upgrade the wires have the chance of being to short because of the different panel configurations and have to be spliced to make it where they need to go. the electrician that did yours took the hack way out and spliced the neutrals from both cables together at the panel. As far as the two cables are concerned you should break the splice after shutting off the power and make a separate pigtail from each white wire from each cable and bring them back to the neutral bar separately, that will make it better, as far as why they did the multiwire branch circuits, it was just acceptable back then they saw no harm in it, but years down the line having troubles with it and such they made amendments to the code to change it, i mean not saying yours is gonna burn your house down tomorrow look at it this way it has been working all this long dont worry about it, if you want the afci protection just put it in the rooms like i said you'll be fine but bear minimum if you dont do that just do what i said above with the white wires. if you dont feel comfortable doing it dont its not something you should be doing anyway but thats what should be done
 

Last edited by chewylu103; 08-28-08 at 06:25 PM.
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Old 08-28-08, 09:20 PM
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AFCI receptacles have not yet been invented. So I understand. The neutral wire of a multi-wire circuit cannot be spliced with the neutral of another multi-wire circuit and then terminated at the neutral bus-bar. Each neutral wire must independently be terminated under a seperate neutral bus-bar screw
 

Last edited by sidecutter; 08-28-08 at 09:26 PM. Reason: add-on
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Old 08-29-08, 04:49 AM
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OK points taken.
If that is the case that MWBC's have to have the neutral wires seperately terminated at the neutral bar, then why would a licensed electrician use a pigtail on both whites coming off the MWBC's to a single neutral connection on the panel neutral bar?

If I take apart that pigtail and use seperate whites the the neutral, will that be OK? Everything is working fine as is NOW; I would hate to seperate them and get a fire or worse.......because I find it really strange that it was a 'casual mistake' on the part of an electrician.....In other words, does anyone think that there is a plausible explanation as to why it was done like this??

If this matters any, The breakers on the second MWBC were not on top of each other (I guess that would be out of phase?)
Instead, there was one breaker (the one with a black wire) then underneath that one was a completely different breaker for another circuit, then under that one, the "sister breaker" of that MWBC circuit (this one had the red wire)...(I would assume that this MWBC was "in-phase")

Mind you, the FIRST MWBC breaker were on top of each other....again, I dont know if this was done deliberately or just a mistake...BUT I took it upon myself to place the breakers on the second MWBC on top of each other as in the first MWBC....because it is MY understanding that this kind of circuit has to have each "leg" 180 degrees out of phase so as to avoid overloading the neutral...am I correct in doing this or am I playing with fire??
 
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Old 08-29-08, 02:30 PM
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you are correct in moving the breaker basically on your panel it goes starting from the top the top two breakers are on say leg A and the second two are on leg B right below it the next set are leg A and so on and so forth, they are every other one. As far as the AFCI receptacles are concerned i saw a bogus site i guess but i mean if you want to look at it there it is: http://jyjz.en.alibaba.com/product/5...eceptacle.html
no that will not make it a fire hazard thats the way the neutrals should be wired the reason that the electrician did it that way was because like i said the panel was changed and sometime the wires are too short so or there is not enough space on the neutral bar and they pigtail the wires and why he did this is not easily understood i mean just because someone has there license doesnt mean they are the most competant person in the world.
 
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