Change mwbc to add gfci

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  #1  
Old 01-09-18, 09:56 AM
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Change mwbc to add gfci

I have 2 single gang receptacles on my kitchen counter and they are mwbc.
i have a spare 2 pole 20 amp in my panel.
Can I upgrade these to 12/2 wire and run gfcis from a 2 pole breaker splitting the circuits again or must kitchen receptacles be on separate single pole breakers now?
 
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  #2  
Old 01-09-18, 10:20 AM
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Can you post a sketch of what you are proposing?
 
  #3  
Old 01-09-18, 10:24 AM
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Can I upgrade these to 12/2 wire and run gfcis from a 2 pole breaker splitting the circuits again
I am a bit confused about what you are trying to do.

Is existing circuit run in 14 and trying to replace it with 12?
How are you going to feed mwbc with 12/2 ? It has to be 12/3 to make mwbc.

You can have mwbc and have gfci receptacles.
 
  #4  
Old 01-09-18, 10:35 AM
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Black of wire #1 to first receptacle and into slot 1 of the 2 pole breaker.
black of wire o#2 to second receptacle and into slot 2 of the 2 pole breaker.
Shared neutral, no red wires.
Not possible?

Actually I guess I can't do that as it puts 240v on the neutral?
 
  #5  
Old 01-09-18, 10:55 AM
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Black of wire #1 to first receptacle and into slot 1 of the 2 pole breaker.
black of wire o#2 to second receptacle and into slot 2 of the 2 pole breaker.
Shared neutral, no red wires.
Not possible?

Actually I guess I can't do that as it puts 240v on the neutral?
What would be the purpose of sharing neutrals if you are running two 12/2 home runs?

Generally speaking, no, you cannot create a MWBC with two separate cables. The shared neutral has to be inside the same sheathing as the two feeders.

Why don't you just keep the MWBC as-is and put a GFCI receptacle in each box?
 
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Old 01-09-18, 11:05 AM
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There's a red and a black wire in each junction box, ie the upper and lower parts of the receptacle are on different circuits.
i can't connect both hot wires to the gfci as it doesn't have a tab to remove and split.
Or could I do a 14/3 to 1st receptacle and 14/2 to the second using GFCI
https://i.stack.imgur.com/nKHGX.png

I thought counters had to be 20a now...
 
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Old 01-09-18, 11:26 AM
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Black of wire #1 to first receptacle and into slot 1 of the 2 pole breaker.
black of wire o#2 to second receptacle and into slot 2 of the 2 pole breaker.
Shared neutral, no red wires.
Not possible?
If you will be pulling new cable, why are you sharing neutral? Cable #1 and Cable #2 will have their own neutral.


There's a red and a black wire in each junction box, ie the upper and lower parts of the receptacle are on different circuits.
i can't connect both hot wires to the gfci as it doesn't have a tab to remove and split.
Or could I do a 14/3 to 1st receptacle and 14/2 to the second using GFCI
https://i.stack.imgur.com/nKHGX.png
This is exactly how you should run it. But, no need to replace existing cable. Just cap unused wire.

Kitchen should be 20A, but if it is existing and don't have problem with 15A, you don't have to replace it.
If you need 20A, just run 12/2 to each receptacles and put 2 single pole 20A breakers. No need for MWBC.
If you really want to run MWBC, just run 12/3 to the first outlet and 12/2 to second outlet like the picture you linked.
 
  #8  
Old 01-09-18, 12:47 PM
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There were some unusual rules with Canadian code. A 15amp MWBC may be legal but not 15amp individuals. Need someone from up North to chime in.
 
  #9  
Old 01-09-18, 12:57 PM
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A two pole gfi breaker will provide the gfi protection.
 
  #10  
Old 01-09-18, 04:55 PM
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The easiest thing to install is the 2 pole ground fault circuit interrupter breaker unit. Then no changes to the receptacles upstairs are needed.

If you go with the two 12/2 separate branch circuit cables to replace the original 14/3 MWBC cable for added amperage then you need to double check the box sizes to be sure all of the wires can be accommodated.
 

Last edited by AllanJ; 01-09-18 at 05:27 PM.
  #11  
Old 01-09-18, 05:30 PM
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GFI breakers are 5-10 X the price of 2 GFI receptacles here.
also, must the breakers on a mwbc be tied together or can they be on different single pole breakers with a shared neutral?

If I went the 20A route then can the 2 receptacles can be on a 2 pole? I don't have 2 single pole 20A spare in the panel at the moment.

​​​​​​
 
  #12  
Old 01-09-18, 06:43 PM
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The mwbc must have the hots on opposite legs of the panel to prevent overloading the neutral. The NEC would require a handle tie or a two pole breaker .
 
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Old 01-09-18, 06:48 PM
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must the breakers on a mwbc be tied together or can they be on different single pole breakers with a shared neutral?
They can be 2 single pole with a handle tie. However, if you are going to put GFCI breakers, this cannot be done because each breaker has to have neutral for each circuits.

If I went the 20A route then can the 2 receptacles can be on a 2 pole? I don't have 2 single pole 20A spare in the panel at the moment.
I'm not sure if that is allowed or not, but single breakers are cheap enough to just replace the breakers with singles. Most 2 pole breakers are common trip, which means when one circuit trips, the other will trip as well.
 
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Old 01-09-18, 08:24 PM
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qwertyjjj, I can't help with the wiring, I just want to add some comments for the pros and see if it helps.
In the U.S. two 20A counter top circuits are required in the kitchen and they usually serve two opposite walls.
AFAIK in Canada, each individual duplex receptacle must be fed by 2 circuits (mwbc), not opposite walls.
This wiring is above me but I wanted to chime in. I don't think a GFCI can share a neutral.
 
  #15  
Old 01-09-18, 10:41 PM
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AFAIK in Canada, each individual duplex receptacle must be fed by 2 circuits (mwbc), not opposite walls.
Didn't know that. So, how is GFCI installed in Canada then? 2 pole GFCI breakers are the only way I can think of since GFCI receptacles cannot have top and bottom separated.


This wiring is above me but I wanted to chime in. I don't think a GFCI can share a neutral.
Shared neutral is not a problem as long as it is don't at the receptacle since that is before the load.
However, 2 single GFCI breakers cannot share a single neutral. 2 pole GFCI breaker can be used instead.
 
  #16  
Old 01-09-18, 11:37 PM
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In Canada the choice is 15a split or 20a circuit for a kitchen receptacle but you must have gfci near sinks.

Here is an mwbc but using 2 pole
http://ww2.justanswer.com/uploads/AssuredElectrical/2012-05-28_165807_mwbcgfci.jpg
Actually it could use 3 wire NM but the 2 gfci's would have to be close.
Does it satisfy the code if everything running in the same outer sheath? I'd have to run 12/3 from the first box to the second but remove the black cable making it 12/2?
 
  #17  
Old 01-10-18, 07:37 AM
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Multiwire branch circuit needs two adjacent breaker panel slots that have 240 volts between them regardless of the kind of breakers being used. If two single breakers are used then their handles must be tied causing both to go off if one trips..

MWBC can have GFCI receptacles provided that nothing is connected to the latters' load terminals other than a (non-MWBC) independent 2 wire subcircuit continuing to other outlets.
 
  #18  
Old 01-10-18, 10:57 AM
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Generally speaking, no, you cannot create a MWBC with two separate cables. The shared neutral has to be inside the same sheathing as the two feeders.
that's for the run from panel to the initial junction box. But if I have a 12/2 going to the second gfci, making the red (hot) connection in the 1st junction box then I can use a second cable to share that neutral?
I presume this code requirement is to ensure it's clear in the panel?
 
  #19  
Old 01-10-18, 12:00 PM
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Here is something I learned about Canadian code for kitchen receptacles.
Kitchen receptacles can have 15A split or 20A non split receptacles.
What you have currently is 15A split, where top and bottom receptacles of duplex receptacle is on different circuits.
Therefore, if you do what you were suggesting, it will be in violation of the code.

If you replace cable with 20A (12/3) you can do what you are suggesting.

I'd have to run 12/3 from the first box to the second but remove the black cable making it 12/2?
Do not remove wire form 12/3 to make 12/2. Just cap unused wire instead.
 
  #20  
Old 01-10-18, 12:21 PM
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And I'm not allowed to tie wrap 2 12/2 cables together making a 12/4 and just use 3 of the wires putting red tape on one of the blacks?
i have spare 12/2 that's why.
 
  #21  
Old 01-10-18, 12:32 PM
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E3701.5 Multiwire branch circuits.

All conductors for multiwire branch circuits shall originate from the same panelboard or similar distribution equipment.


Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

The ungrounded and grounded circuit conductors of each multiwire branch circuit shall be grouped by cable ties or similar means in at least one location within the panelboard or other point of origination.

Not positive, but I don't see anywhere that explicitly says you can't.
 
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Old 01-10-18, 02:08 PM
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And I'm not allowed to tie wrap 2 12/2 cables together
If Canada allows you to do this, you will have to keep them together at all times, including where they enter the box. You can't have them enter the box via two different NM clamps / knockouts. The code disallowing this or restricting it wouldn't be in the section on MWBCs.

It seems kludgy to do this just to use up some spare cable, IMO.
 
  #23  
Old 01-10-18, 02:13 PM
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And I'm not allowed to tie wrap 2 12/2 cables together making a 12/4 and just use 3 of the wires putting red tape on one of the blacks?
i have spare 12/2 that's why.
You can pull both wires together, but that doesn't make it 12/4. It just makes 2 cables in a junction box. Don't even need MWBC in that case. Just put them on 2 single breakers.
Also, why not just pull straight to each junction boxes if you will be pulling 2 cables anyway?
 
  #24  
Old 01-10-18, 02:46 PM
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You can pull both wires together, but that doesn't make it 12/4. It just makes 2 cables in a junction box. Don't even need MWBC in that case. Just put them on 2 single breakers.
Also, why not just pull straight to each junction boxes if you will be pulling 2 cables anyway?
I have an empty 20a 2pole breaker on the panel. Didn't want to go out any buy 2 single pole breakers if I could use the existing one.
 
  #25  
Old 01-10-18, 03:38 PM
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When you have a 2 conductor cable (hot and neutral) continuing from the (first) outlet box to the (second) box, it is treated as a non-MWBC branch even if it came from an MWBC in the (first) box. This 2 conductor cable may not join another cable in the (second) box to continue on as a MWBC.

But the 2 conductor cable can continue and be treated like a non-MWBC from that point on.

A MWBC can split into or spin off non-MWBC branches at any time and two of the branches continuing on may enter the same outlet box further downstream (but not have their neutrals wire nutted together).
 
  #26  
Old 01-10-18, 04:29 PM
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I have an empty 20a 2pole breaker on the panel. Didn't want to go out any buy 2 single pole breakers if I could use the existing one.
2 separate circuits will work fine with 2 pole breaker, but I just am not sure if there is any code that doesn't allow it.

Single pole breakers are very cheap. I'd just replace them.
 
  #27  
Old 01-10-18, 04:46 PM
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All conductors of the circuit need to be in the same raceway or cable.
 
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Old 01-10-18, 05:05 PM
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All conductors of the circuit need to be in the same raceway or cable.
always seemed an odd code requirement since if I carry on the circuit, it's a separate cable with a receptacle in the middle.
I can join a 12/3 with a 12/2 or vice versa for 3 way switches and they are different cables aren't they?

A MWBC can split into or spin off non-MWBC branches at any time and two of the branches continuing on may enter the same outlet box further downstream (but not have their neutrals wire nutted together)
Not sure I follow. 12/3 to first box. 12/2 to second using the red hot. These 2 boxes share the neutral (using 2 legs)
 
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Old 01-10-18, 05:15 PM
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Of the hot is not in the same cable as the neutral you will have heating issues. I don't think you understand this concept at all. You do have all the conductors in the same cable.
 
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Old 01-10-18, 05:16 PM
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All conductors of the circuit need to be in the same raceway or cable.

always seemed an odd code requirement since if I carry on the circuit, it's a separate cable with a receptacle in the middle.
I can join a 12/3 with a 12/2 or vice versa for 3 way switches and they are different cables aren't they?
There is nothing odd about it. A conductor is individual wire. What the code says is all conductor for a single circuit has to be in the same cable or raceway (conduit). And the circuit here means is circuit between junction boxes. Joining 12/3 and 12/2 in a junction box is not a problem. The problem occurs when you are trying to run 2 12/2 and share neutral from one of the cable because that is two different circuits now.

Pulling 12/3 to one junction box and separating into 2 circuits from that point is not a problem.

Pulling 2 12/2 to one junction box is 2 different circuits as well. Also there is no reason to wire it as MWBC when you are running 2 cables from the breaker to the junction box. In that case, you don't have MWBC to begin with.
 
  #31  
Old 01-10-18, 05:22 PM
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always seemed an odd code requirement
You won't think it's such an odd code requirement when you discover that something metallic is glowing red hot in your walls. Of course, that metallic something is usually out of view so you won't notice it, and won't know what happened until the fire investigator is going through the charred remains of your house.

You're just confusing the issue with the 3-way discussion. It comes down to this, it's very simple: In a given cable, the outgoing current should exactly equal the returning current. In that cable or raceway. This is always satisfied with conventional 3-ways, MWBCs, splitting MWBCs after a recep, you name it.
 
  #32  
Old 01-10-18, 06:23 PM
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Of the hot is not in the same cable as the neutral you will have heating issues. I don't think you understand this concept at all.
no I don't but interested to learn why.
Neutral doesn't reduce heat does it?
In a 240v circuit, there's 2 hots no neutral.
I would have thought a 12/3 would be warmer than the 12/2 ?
 
  #33  
Old 01-10-18, 06:47 PM
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When a current flows in a single conductor, a magnetic field is created. With AC, this current changes direction 120 times per second. This is what creates the heat in metallic objects near that wire. If you carefully split apart a lamp cord and wrapped something metallic around one of the insulated wires, you'd potentially have issues (but with a lamp there isn't enough current to do much harm).

In a plain 2-wire cable, the two conductors are both creating these magnetic fields. But they create the fields in opposite directions which cancel the other out. The hot current exactly equals the neutral current but is in the opposite direction at any given moment. Thus anything outside of the cable never sees a net magnetic field. With 12/3 it's the same thing, but the current of one conductor is balanced by the other _two_ in that case.

It's all about the magnetic fields. It has nothing to do with the heat created by the current going through the wire.
 
  #34  
Old 01-10-18, 07:13 PM
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In a split phase system, each line is out of phase to each other.


Above graph is what you will get in a split phase system.
Let's assume 100% load. Current will have same graph if there is 100% load. The current take path of least resistance and current from L1 will flow to L2 through N since L2 is -120V when L1 is +120V or visa versa.
Therefore, neutral will not see any current when load between L1-N and L2-N is same.

In the real world, current on L1 and L2 will never be the same, and that is where neutral comes with play. Current on the neutral will be the the difference between the current on L1 and L2.
Current on neutral is not current on L1 + current L2, instead it is current on L1 - current on L2.
 
  #35  
Old 01-10-18, 08:07 PM
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Change --> A MWBC can split into or spin off non-MWBC branches at any time and two of the branches continuing on may enter the same outlet box further downstream (but not have their neutrals wire nutted together at that downstream location).

When you have 12-3 cable as a MWBC bringing power to the first outlet box with black and white conductors powering a receptacle unit there and a 12-2 connected to just the red and white and continuing to the second outlet box, then semantically the continuation to the second box is not regarded as having a shared neutral.
 
  #36  
Old 01-11-18, 06:05 AM
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It's all about the magnetic fields. It has nothing to do with the heat created by the current going through the wire.
This is not true. Heat is caused by current flowing through a wire and the resistance it encounters (electron collisions). Google "Joule Effect". If it were due to the magnetic field, then why would a 30 gauge wire get much hotter than a 12 gauge wire for a given current? The magnetic field would essentially be the same because the current is the same, yet the smaller wire would get hotter. The difference is the cross section of the wire and increased resistance, which causes the smaller wire to heat up more. Hence the reason we use larger gauge wire for higher current. If magnetic fields created heat, then you wouldn't be able to hold a magnet in your hand.

We've gotten off topic here. The OP's question has been answered.
 
  #37  
Old 01-11-18, 06:12 AM
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This is not true.
Mossman, if you look back at what I quoted and what started all that, it was in response to the OP saying that the requirement was strange that the conductors for a given run need* to be in the same cable or raceway. Yes certainly current in wire also creates heat but that's not what we were talking about. It wasn't what he was questioning.

And I don't think it's off topic since it's precisely what the guy was asking about. I don't think his question has been answered to his satisfaction because I get the feeling he'e still wanting to cobble something together with his junk roll of 2-conductor cable.

* = This is of course not truly required in the USA for all cases, but that's a minor point.
 
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Old 01-11-18, 06:32 AM
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Mossman, if you look back at what I quoted and what started all that, it was in response to the OP saying that the requirement was strange that the conductors for a given run need* to be in the same cable or raceway.
I see. You're referring to the magnetic fields interacting with the conduit (assuming it is conductive). That makes sense. The potential hazard is the magnetic fields inducing current in the conduit which would also cause it to heat up The current would be a non-issue for plastic conduit (obviously), and the heat generated within the wire would be greater than the heat generated in a metallic conduit due to an induced magnetic field. I'm thinking there's probably another reason conductors of a circuit need to be in the same raceway/conduit and apparently cable (does NEC explicitly state "cable"?)
 

Last edited by mossman; 01-11-18 at 08:06 AM.
  #39  
Old 01-11-18, 09:51 AM
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Thanks!
so, any other shared neutrals have to come from the white in the 1st box.

Lastly, I can understand the heating issue now in the different cables. The code allows it in conduit though. In a cable, the conductors are a fixed distance apart. In a conduit there could be a 1-3 cm gap between the conductors depending on how they place. Doesn't this create hotspots with the slight magnetic field differences?
 
  #40  
Old 01-11-18, 04:10 PM
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I can understand the heating issue now in the different cables. The code allows it in conduit though. In a cable, the conductors are a fixed distance apart. In a conduit there could be a 1-3 cm gap between the conductors depending on how they place. Doesn't this create hotspots with the slight magnetic field differences?
Wires may be a little cooler in conduit than in a cable, but that has nothing to do with how MWBC works.
Wire or cable's maximum load is determined by size and material of the conductor and thermal rating of insulation. NM-b, THHN/THWN, and UF-b 14AWG is rated for 15A and 12AWG is rated for 20A. (There are more types but listed are most common in household uses).

Magnetic field is not a big factor in how the wiring is rated. Most heat is generated from electrical resistance of the conductor. Higher the resistance and higher the current (load), more heat. Thicker conductor of same material has lower resistance, thus can carry higher current.

Magnetic field is a concern when a conductor is coiled multiple times or run parallel to the other wire. A long length of wire running in paralleled will have induction to wire next to it. When hot and neutral is run in the same raceway (conduit), magnetic field of hot and neutral is reverse of each other because the current flows in opposite direction. Therefore the magnetic field cancels out. and no inductive heating.


May be following link will help as well.
https://www.mikeholt.com/mojonewsarc...s~20031029.htm
 
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