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Should I combine this MWBC? General advice about running a new circuit...

Should I combine this MWBC? General advice about running a new circuit...

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Old 12-28-13, 10:13 PM
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Should I combine this MWBC? General advice about running a new circuit...

Hi!

I decided to run a new outlet to my home theater so it has it's own circuit and it turns out that the place I was going to splice into is a MWBC. For now the MWBC is only feeding a couple of lights and 1 outlet per leg. I've read that any failure of device, wiring or outlet could cause 220v to be sent into either end - in this case my entire home theater so here's what I'm thinking:

remove one of the 2 power wires in the MWBC and combine everything on these separate legs with my new outlet on a single circuit. Since both legs rarely see 2 amps would that be the best and safest option?

Thank you for any help. I'm experienced at wiring basic 2 wire stuff, saw the 3 wire today and was stumped for a bit and hoping this is a good solution.
 
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Old 12-28-13, 11:11 PM
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Welcome to the forums.

I decided to run a new outlet to my home theater so it has it's own circuit
If that was the case you'd run a new cable directly to the panel and not connect to an existing circuit.

When you set up a MWBC the critical neutral is where the three wire cable connects to the two circuits. In the diagram below..... I've illustrated the important neutral connection.

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Old 12-29-13, 12:04 AM
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Hi PJmax and thank you

Initially I was going to run a new circuit off the panel but after checking what was connected to each circuit I found that the 2 closest to where I want the new outlet are almost completely unused. (which I found out today make up the 3 wire MWBC)

To be clear, what I intend to do is remove one of the 2 power wires and connect both legs to the remaining power wire so that I have a standard 2 wire configuration. This will total 4 outlets and 2 60w style light fixtures all well under a 15 amp load. (including home theater)

The diagram you posted is indeed how it is currently wired. The 3 wire goes into a junction box and splits off. Is there any reason I shouldn't turn it from a 3 wire to a 2 wire circuit? (would it be unsafe or harmful to my electronics?) This is an older house and it was obviously wired that way to support devices that are no longer used. The way I am seeing it is that removing one of the 2 power wires and powering everything off the remaining wire turns it into a standard 2 wire circuit. (like the rest of the house)
 
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Old 12-29-13, 06:01 AM
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You can tie a (2 wire) 120 volt extension onto any multiwire branch circuit by tying onto neutral and one hot.

But evaluate and estimate the existing and future load on the existing circuit before tying on extensions.

While you are at it, should you encounter any receptacles on the MWBC with feed neutral and continuing neutral connected directly to the receptacle, it would be a good idea to improve upon that. Remove the neutral ends, snip off any nicked or badly squashed part, and tie them directly to each other (using a wire nut) with a third, short, wire (pigtail) going to one of the silver screws on the receptacle. (Also a good idea to re-treat the hot wires this way.)

The (rest of the) MWBC remains unchanged as an MWBC.

Keeping it as an MWBC, you can tie on twice as much load (some extensions tied to red and white, others tied to black and white) compared with converting the whole thing to a "regular" circuit and abandoning the entire length of the red wire. This might allow you to power a complex home theater without running a new circuit all the way to the panel.
 

Last edited by AllanJ; 12-29-13 at 06:33 AM.
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Old 12-29-13, 06:34 AM
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Im curious to why you think you need to go to the trouble to change this configuration? If it aint broke, don't fix it.....

The way I understand it is the concern in a MWBC is that there potentially could be voltage on the neutral while someone is servicing one of the two circuits with only one of them disabled. There is also the chance someone could disconnect the shared neutral while servicing the circuit live and cause the imbalanced 240 load across both circuits.

Neither are the case in your situation since 1. your aware its a MWBC and would turn both circuits off while servicing or add handleties for anyone else working in your panel 2. you or anyone else servicing your equipment shouldn't be working live and disconnecting neutral wires and 3. the shared neutral should be pigtailed accordingly at any devices.



I decided to run a new outlet to my home theater so it has it's own circuit .....
Since both legs rarely see 2 amps
Surely a home theatre is cranking away at more than 2 amps.... why do you need a dedicated circuit?



Good Luck
p.s.-
I probably shouldn't chime in, Im far from an electrician but I have a few MWBC in my house so I am interested in the topic. I hope a pro can shine more light on this for both you and I.
 

Last edited by rards; 12-29-13 at 06:58 AM.
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Old 12-29-13, 07:15 AM
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The most important thing as explained by PJ is that at the box where it splits the neutrals are pigtailed to the receptacle. The danger is if the neutrals are connected to the bridge on the receptacle. If the receptacle is removed the neutral can be lost and high voltages occur. However if the neutrals are pigtailed then they remain connected even if the receptacle is removed.
 
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Old 12-29-13, 07:55 AM
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what I intend to do is remove one of the 2 power wires and connect both legs to the remaining power wire so that I have a standard 2 wire configuration. This will total 4 outlets and 2 60w style light fixtures all well under a 15 amp load. (including home theater)
Since you are already planning on combining the loads from the two circuits, why not use the 2nd circuit that you have freed up for your home theater to provide the dedicated circuit you were originally wanting? There is nothing wrong with a MWBC, they are perfectly safe if installed correctly and yours appear to be installed correctly.
 
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Old 12-29-13, 08:00 AM
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Neutrals are required to be pigtailed as shown in PJs diagram. The breakers should have a handle tie or be a two pole.

Your plan to move the one wire to the other breaker cuts your capacity in half and also creates a code violation.
 
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Old 12-29-13, 10:59 AM
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[HR][/HR]. So as long as the critical neutral has a good connection and the wire from that connection to the panel is good there is no chance of sending 220v to either side? Can someone explain how exactly 220v could be sent down one side and how to prevent it?

There are a few reasons I am considering combining the two legs:

The combined load for everything ( with new receptacle connected) will be well below 15 amps so I don't see the need for multiple circuits, especially if a fault on one side can fry devices on the other side.

I am splicing the new receptacle wiring onto the same spot where the mwbc starts so making it a 2 wire circuit is as simple as removing one wire and connecting that leg to the other power wire in that same box.

My biggest problem with keeping the mwbc is I cannot check every connection in the entire run. House and wiring are old and it just seems safer to make this change.

Rards: 2 amps was referring to the entire load that is currently on the mwbc=main reason I am considering this.

Pcboss: I am not moving the wire to another breaker, I am disconnecting one of the 2 power wires in the 14/3 and combining the 2 circuits into one. From what I understand this makes the mwbc into a standard 2 wire circuit.
 
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Old 12-29-13, 11:09 AM
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The service coming into your house is one large MWBC. As long as the neutral spliced you are good. You are reducing you capacity for no benefit with your plan.
 
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Old 12-29-13, 11:13 AM
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Casualjoe: I think that's exactly what I would do if I were 100% sure a fault on that leg couldn't fry my home theater or start a fire. My thinking behind this is that everything is easily powered by a single 120v/15 amp circuit so if that presents less chance of my electronics being damaged, that's preferable.
 
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Old 12-29-13, 11:22 AM
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Pcboss: i am sorry for my ignorance in this. I am trying to understand how the critical neutral would affect either circuit when that is the 2 legs only connection? I think what I'm understanding is the importance is that the critical neutral is at the beginning of each circuit? So that is the only place where the 2 120v lines share neutral?
 
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Old 12-29-13, 11:31 AM
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I think what I'm understanding is the importance is that the critical neutral is at the beginning of each circuit? So that is the only place where the 2 120v lines share neutral?
That is 100% correct........
 
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Old 12-29-13, 11:42 AM
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Ok now I get it. Allow me to ask this= say there is a short or faulty device that shorts out on one leg. Does this then make the neutral hot on the other leg or does it only pass back into the panel?

Also, does a mwbc share the same characteristics as the main panel, seeing as all neutrals connect to the same post at the panel? I guess what I'm asking is, does a mwbc pose any more or less risk of fire/overload than the individual circuits on the main panel? (As long as critical neutral is respected)
 
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Old 12-29-13, 12:12 PM
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As long as the hot are on opposite legs of the panel there is no risk of overloading the neutral. Using a two pole or two handle tied breakers assures a proper wiring.

A fault or short on one leg will trip the breaker.
 
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Old 12-29-13, 02:14 PM
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I think that's exactly what I would do if I were 100% sure a fault on that leg couldn't fry my home theater or start a fire.
Don't be paranoid, it's just where the circuits split that you have that critical splice. Neutral connection problems anywhere else after the circuits split will not cause you to have 240 volts across your theater equipment. That being said, that splice where the circuits split is no more likely to cause you problems than where the neutrals connect in the panel.

pcboss

The service coming into your house is one large MWBC. As long as the neutral spliced you are good. You are reducing you capacity for no benefit with your plan.
Pc boss is exactly right.
 
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Old 12-29-13, 05:08 PM
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I really appreciate you guys helping me. Thank you.

I admit I am paranoid and even though I've worked with wiring a lot in my life I have never been comfortable with it, especially in a circuit panel. But I am careful with my work and good at the actual wiring.

What you said about the neutral being no more likely to cause problems than the neutral bar at the panel - this is something I was wondering. So each leg would never send power back down the other leg's neutral, only back to the panel. The one thing I can't figure out is what happens if the critical neutral running to the panel is damaged or becomes loose? Do both circuits cease function and/or would this be a situation where 220v on one leg is possible?

Also, the panel in this house is an old style main and range, no breakers but fuses and there was no indication in the panel that this MWBC was there. I figured it out when looking over the wiring in the attic. Does having fuses change anything in the way I should do this?
 
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Old 12-29-13, 05:33 PM
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Also, the panel in this house is an old style main and range, no breakers but fuses and there was no indication in the panel that this MWBC was there.
There would have to be. The red would go to one fuse and the black to another.

If you don't see any red wires in the panel then maybe what you found was not a MWBC but one circuit with a switched line running on red.
 
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Old 12-29-13, 06:47 PM
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yes, it is wired in the panel like that. But there's no indication with the cover on like you would see on a modern breaker panel.
 
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Old 12-30-13, 12:35 PM
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I think the original poster is thinking his MWBC as a deficiency in his electrical system. He's referring to it as a fire hazard in some posts.

Is the original poster justified in thinking of this as something he should change?

I have at least 4 MWBC in my old house, im hoping that would be a normal thing. If not I have a major issue on my hands. I would like to read what the pros think.
 
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Old 12-30-13, 01:06 PM
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edit* I don't feel I need to change this because it is a MWBC

Hi again, rards. I've read enough to know a MWBC is perfectly safe if wired correctly and labeled correctly in the panel. My hesitation to use it for my home theater is I did not install the wiring in this house and cannot check the full run from panel to critical neutral. I already don't trust the wiring in this old house (currently half the house is on one fuse/breaker and at some point all fuses in the panel were changed to 30amp) I am correcting these issues now.

If I installed the MWBC or could check it from panel to critical neutral I would be confident in using it. But I can't and the wiring is very old so I don't like the extra risk for my most expensive devices. That and the 2 circuits are completely unnecessary in that area of my house.

I've been doing more reading on this wiring method and it seems the biggest risk is if the critical neutral becomes broken/open. In a normal circuit it is not possible to get 220+v on a 120v line but in that case a MWBC can. I also believe this poses a greater risk of fire than a normal 120v line. I'm still reading and have postponed my work until after the new year but I am pretty sure I am just going to pull one circuit and make the MWBC into a standard 2 wire. That said, what I should be doing here is just adding a brand new 15 or 20 amp circuit, just for simplicity but working around that old panel with so many old wires is just too much stress when I have that underutilized MWBC right there.
 
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Old 12-30-13, 01:42 PM
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Poptart- Makes good sense to me. Just so you don't feel too overwhelmed: In my house I have at least the following circuits sharing neutrals:

Furnace/Living Room
Dishwasher, RangeHood/Sump Pump Water Softener
Den with TV and computer/Bedroom/Bathroom

Paranoia indeed.....

Im not sure if I should call in electricians or treat it like how 99.9 percent of other homeowners would (ignore it).
 
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Old 12-30-13, 04:48 PM
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You would be better off with a new grounded circuit so there is no need to rely on the old questionable wiring.
 
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Old 12-30-13, 06:29 PM
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Paranoia indeed.....

Im not sure if I should call in electricians or treat it like how 99.9 percent of other homeowners would (ignore it).
If you are really concerned, locate the box where each MWBC splits and assure yourself that the neutral connection is proper. After that, I wouldn't worry about it, it's a perfectly safe wiring method. This is one of very many reasons why electricians must demonstrate a degree of competency in order to be licensed in most areas and why most municipalities require permits and inspections.
 
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Old 12-30-13, 06:36 PM
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In a normal circuit it is not possible to get 220+v on a 120v line but in that case a MWBC can. I also believe this poses a greater risk of fire than a normal 120v line.
I would strongly disagree with that generalization. I don't know how old the wiring is in your home nor do I know who installed it, how it was installed, the condition of it now or the condition of your home. To say a MWBC poses a greater risk of fire is simply not true. If it were true, MWBCs would not be allowed by the NEC.
 
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Old 12-30-13, 07:17 PM
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Joe, is it not a bigger problem to lose neutral connection to the panel in a MWBC vs a 2 wire?

pcboss, I planned on doing just that for simplicity but looking over the panel I just don't see a way to get wire into it comfortably. If these unused circuits weren't right there I would be forced to run the new one. I have lived here over ten years and we must have a solid wiring system to most the house as we've never had electronics die early or any big issues.
 
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Old 12-31-13, 08:00 AM
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Joe, is it not a bigger problem to lose neutral connection to the panel in a MWBC vs a 2 wire?
It may be a bigger problem if that were to happen, but it's still not likely to ever happen if the MWBC were installed properly in the first place. It would be much more likely to lose the utility neutral at the servicehead or transformer and then you risk possibly having 240 volts ON ALL 120 VOLT CIRCUITS!
 
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Old 12-31-13, 06:34 PM
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I know this isn't an answer to any of the problems posed here but to what extent would surge protection safeguard against the over voltage situation we are freaked out about?
 
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Old 12-31-13, 07:34 PM
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I know this isn't an answer to any of the problems posed here but to what extent would surge protection safeguard against the over voltage situation we are freaked out about?
Probably wouldn't help at all.
 
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Old 12-31-13, 09:21 PM
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Joe is correct..... it won't help at all.
 
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Old 01-01-14, 07:03 AM
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I assumed a surge strip would fry and turn off before harming the electronics in an overvoltage situation.... why is my reasoning wrong?

DISREGARD: Answeed my own question by reading the clamping voltage rating of 330 on the back of my surge strip.
 

Last edited by rards; 01-01-14 at 09:14 AM.
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