Home>Discussions>ELECTRICAL & LIGHTING>How to break up a circuit with a shared neutral?
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rickpantel

Hi again everyone. I posted the original question, and I've read over the answers from the experts on the forum. I still am not clear on what happens with a shared neutral circuit, and I need some further assistance:

JLMCDANIEL posted: ".....every thing is OK until the neutral break or bad connection, and you have one 60 watt light on in the bath and the vent fan is running. Now you are running 240 volts through the circuit, dropping 220+ volts on the fan and 18+ volts on the light and the circuit is drawing over 22 amps.....".

OK...I understand the part about balanced loads and how the neutral only carries the difference in load between the two hot sides. Attached is a diagram of how these these circuits are wired in my house. The ground bonding wire (or lack of it) is not shown for clarity...that is a separate question. Here are the points I'm stuck on:

1- What I can't figure out is how the loss of the neutral results in 240 volts going thru one side of the MWBC. Why would the loss of the neutral not result in everything just going dead? I see how you get 240 volts going to one of the end appliances, but it has nowhere to go. :confused:

2- if one of the end-run branch circuits loses a neutral, then that particular end run would just go dead, even though the HOT was still live...am I correct? :confused:

3- can you elaborate on why electronic appliances are particularly sensitive to being on a shared neutral circuit? If the loads aren't balanced is it possible to get greater than 120 volts thru the electronic devices? Would a surge protector help to protect against this? :confused: Our stereo keeps going on the
fritz, and I wonder if this is possibly the cause?? :(

Any help from the experts on the forum would be greatly appreciated!!

Attachment:
canuk

rick .... maybe this will help. I took your diagram and edited for illustration..

Quote:

2- if one of the end-run branch circuits loses a neutral, then that particular end run would just go dead, even though the HOT was still live...am I correct? :confused:

as you can see here one circuit is broken ... everything is fine except the one branch is broken and won't work. Whatever is still connected is running with 120 volt supplied because of the main neutral is connected to the mains supply.

with your example .... the loads are no longer balanced but there still is only 120volts applied to the loads.... it's only the current that is being carried by the main neutral wire that changes.

Quote:

1- What I can't figure out is how the loss of the neutral results in 240 volts going thru one side of the MWBC. Why would the loss of the neutral not result in everything just going dead? I see how you get 240 volts going to one of the end appliances, but it has nowhere to go. :confused:

3- can you elaborate on why electronic appliances are particularly sensitive to being on a shared neutral circuit? If the loads aren't balanced is it possible to get greater than 120 volts thru the electronic devices?

As you can see I've broken the main neutral .... so now 240 volts is flowing from the hot on the bottom right .... through the loads on the right .... continues onto the loads on the left ..... and returns through the hot on the bottom left.
In this example those loads are equal ( balanced ) they would have the same amount of voltage drop ( 120 volts ) and net current difference of zero .... not a big deal.

in this example .... the loads are different .... the same 240 volt pathway still exists as above except .... the load on the right is larger so more voltage ( and current ) will be dropped .... less voltage ( and current ) will be available for the load on the left .
Because there is a total of 240 volts the amount of voltage dropped at each load must add up to the orginal 240.

For example .... the larger load on the right might now have 200 volts which means the load on the left has to be only 40 volts ( 200 + 40 = 240 )

This would be one reason you might see lights being brighter and others being dimmer.

Hopefully this makes sense and helps.:)

rickpantel

Hey Canuck:

Thanks for the eloquent reply....

So, in other words with a shared branch-circuit neutral that is broken: the R side HOT can act as a return path for the L side HOT, and the L side HOT can act as a return path for the R side HOT at the same time. As long as the loads are balanced, everything will still work fine. However, if the loads are unbalanced, then a "voltage steal" situation would arise where one side develops a higher voltage than the opposite side, with the sum being 240 volts. I assume this would cause lights to burn brighter or dimmer, and electronics to either work poorly or fry themselves. Would I be correct? :confused:

I also assume that this same situation is replicated throughout the entire house if the main neutral from the power pole is broken. Would that be correct? :confused:

I would agree that it is much easier to avoid shared neutral branch circuits entirely, even though they are still allowed by the various electrical codes. I still plan to break up these circuits, as we discussed earlier in this thread. :)

canuk
Quote:

I also assume that this same situation is replicated throughout the entire house if the main neutral from the power pole is broken. Would that be correct? :confused:

Yep ... you can use the diagram to visualize if the neutral connection from the transformer to the house is broken could cause problems inside the whole house.

rickpantel

OK...mission accomplished!

I split up the old shared neutral circuits, as discussed, and took the redundant HOT wire and turned it into a ground bonding wire. Everything works fine and the circuits are now properly grounded.

Thanks for everyone's input on this question! :)

peacher

I want to respond mostly to the originator but this might help others on this as well. I found this thread because I ran into a problem last night trying to add an AFCI to a replacement subpanel I was re-doing in my home. First off, I am not a licensed electrician (although my brother is), but I not sure all of them even understand (as evidenced by some of the replies). What they are not telling you, and why I see you can still be confused, is that the situation totally depends on the the source of the two hots. You have to consider phases. If they are opposite, you can get the 240v if you lose the neutral... [And, by the way, that's why house feeds share a neutral of the same size with no problem - because it never sees any of the 240 loads, or any balance of the 120v loads, either for that matter.] BUT if your shared neutral hots are the same phase (connected to the same side of the box feed) they will be IN PHASE. Losing neutral will simply mean rise and fall phases across both sets of loads and they see NO voltage. That's the good news... no 240 risk. But here's the bad... since they are in the same phase, the neutral sees the sum of both loads, and you potentially get 30 amps on a single wire rated to carry 15.

When I found a single shared neutral circuit last night in my box, I cursed the [probably licensed] electrician who did it, because now I can't bring my bedroom in line with 2008 NEC because I can't add the AFCI without major rewiring.

But, all in all, I think your original and final solution was excellent. Yes, there is some risk of a misrepresented [red?] being used as a substitute ground somewhere in a wall, but minimal if you re-label everywhere it shows up in a box.

In my situation, I did forego the AFCI [at least for now], but debated as to the phase question. I chose the non-240 option. When I re-looked at how I had done it last night, I was worried that since breakers alternate sides in a typical box that I would have to skip a breaker to maintain same phase. Turns out I had connected both red and black to a single tandem 15 so they were both being fed automatically by the same rail. When I looked again at the original box setup, they WERE adjacent CBs, but the design of the [old!] box was such that they were on the same rail. Luck was with me, but I would have switched one of them if I had found opposite phases.

Hope this helps. If I'm wrong, just remember, I'm not a licensed electrician.

JLMCDANIEL

.

Preacher this is one reason I don't like MWBC wiring is a residential installations. If on 2 legs and a lost neutral you risk 240 volts through appliances, if one wire is moved to a breaker on the same leg you risk overloading the neutral. To me the cost savings of using 3 conductor cable over two 2 conductor cables is not worth the risk.

Jack

P.S. To increase your knowledge only, residential electric service is generally single phase made up of 2 legs and a center tap commonly called a neutral.

ZZZ

It is both unsafe and a violation of the NEC to feed both hot wires on a MWBC from the same phase. While each hot may be protected at 20 amps on a #12 conductor, it is very possible you will have 32-40 amps on the neutral. This far exceeds its safe current carrying capacity and will cause overheating. It is also required to have both circuits open simultaneously (NEC 210.4B) which can only be done through the use of a double pole breaker. A tandem breaker will not suffice. MWBC should also be grouped together in at least one place inside the panel where they originate by a wire tie or similar. (NEC 210.4D).

We never use them in residential buildings, but they are an absolute necessity in commercial buildings, especially the lighting where one three pole breaker feeds three lighting circuits and only requires one neutral. In a panel with 36 lighting circuits, you only need 12 neutrals saving considerable expense in wire conduit and labor. In buildings with hundreds of lighting circuits this can amount to tens of thousands of dollars.

JLMCDANIEL

Being against code may not stop a home owner who needs another circuit from changing the MWBC from two breakers on separate legs to a mini double breaker with both on the same leg. Not all work is done by licensed or knowledgeable people.

Jack

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