Removing 1/2 of MWBC (one phase)

T

thomasg88

Member
Location
Detroit
Occupation
Electrician
Please please help.

Removed severely damaged and dangerous circuit in old home with original wiring in awful condition covering multiple receptacles seemingly all over the house. Following unrelated demo that gave more access found circuit was one phase of a MWBC. Now in process of fully disconnecting removed phase/circuit from remaining phase. Of course everything is out of whack.

Best research I’ve done (and understanding MWBC much better) is to disconnect removed circuit (red wire) from each receptacle. Any help is greatly appreciated. Sincerely thank you.
 
Welcome to the forum. :)

Maybe you can provide a little more detail as to how the existing circuits are wired and what you're trying to accomplish. Are you trying to get rid of the 1/2 of the MWBC which is connected to the red circuit?
 
Hello,

Yes. Circuits 19 & 23 15A w/ 14-3 comprise this (what I later determined was an) MWBC. The wiring was all original from 1929 and I initially identified circuit 19 as severely compromised, not treating it as 1/2 of an MWBC.

I began to remove circuit 19 and run new 12-2 from the effected receptacles to the panel. Renovation demo providing access, and trouble shooting revealed three MWBC’s including 19/23 (nothing had been done correctly at the panel, etc.).

Research indicates removing “old 19” (not connected to panel but still at 40v) from all receptacles with both 19 &/or 23, will convert circuit 23 into a single pole/phase 15A circuit (assuming neutral path is intact).

For the record, I am aware the MWBC for 19 should have been 21 to put it on a separate phase. This was one of many incorrect elements of the extant installation.

Any tips/affirmations of how to convert circuit 23 to a single pole/phase (and functional) circuit are greatly appreciated.

Thank you.
 
What does "severely compromised" mean? For example, if there was overheating, and it was in the same BX cable, maybe all the conductors of the MWBC are "severely compromised"??

When you see fewer neutrals in the panel that hots, there are MWBC or some hack job.
When you are not using conduit and see red wires, that is another good indication there may be MWBC present.
Back in 1929 they called them Edison circuits and I believe they were common.
 
Also...

Before about 10 years ago, the two breakers did not have to be next to each other and have a common disconnect. So, you cannot rely on the MWBC breakers being adjacent. Even now, someone could have installed it electrically properly, but not according to code as far as adjacent breakers and common disconnet is concerned.
 
thomasg88 said:
Any tips/affirmations of how to convert circuit 23 to a single pole/phase (and functional) circuit are greatly appreciated.
Click to expand...
Just swap the conductors on 21 & 23. MWBC's are fine and more efficient than two wire circuits, not only in material but in performance as well.
 
thomasg88 said:
Research indicates removing “old 19” (not connected to panel but still at 40v)
Click to expand...

Real quick question. When you say that the "old 19" circuit is severely damaged, do you mean physically damaged? Or is it because your read 40 volts with it disconnected at the #19 breaker?

If you are reading that 40 volts with a typical high impedance DVM, that is most likely a "ghost voltage" because the MWBC #19 red is still running in close proximity to it's energized pair on #23 in the 14/3 cable. This will create a capacitively coupled voltage that is easily read by a DVM, but "packs no punch". If you had a low impedance meter, or even a 25 watt light bulb, that ghost voltage would disappear.
 
infinity said:
Welcome to the forum. :)

Maybe you can provide a little more detail as to how the existing circuits are wired and what you're trying to accomplish. Are you trying to get rid of the 1/2 of the MWBC which is connected to the red circuit?
Click to expand...
hi infinity - thank you. Might you or others have tips/info on how to remove 1/2 of an MWBC? The replies refrain from any actual guidance. The info I am working off of is disconnecting the red wire (circuit 19) from all receptacles on the MWBC and then hoping the neutral path is still intact for circuit 23. Thank you.
 
ptonsparky said:
A 46-circuit panel in 1929, for a house?
Click to expand...
Second iteration of CB panels. Fuse box converted maybe early 1990s. Any guidance as to how to properly remove 1/2 of MWBC (red wire circuit 19) while keeping the other half (black wire circuit 23) functional would be greatly appreciated. Thank you.
 
MD Automation said:
Real quick question. When you say that the "old 19" circuit is severely damaged, do you mean physically damaged? Or is it because your read 40 volts with it disconnected at the #19 breaker?

If you are reading that 40 volts with a typical high impedance DVM, that is most likely a "ghost voltage" because the MWBC #19 red is still running in close proximity to it's energized pair on #23 in the 14/3 cable. This will create a capacitively coupled voltage that is easily read by a DVM, but "packs no punch". If you had a low impedance meter, or even a 25 watt light bulb, that ghost voltage would disappear.
Click to expand...
Hi MD - yes, physically damaged at receptacles and switches, ungrounded and never upgraded since 1929. Any guidance as to how to properly remove 1/2 of MWBC (red wire circuit 19) while keeping the other half (black wire circuit 23) functional would be greatly appreciated. Thank you.
 
tthh said:
Also...

Before about 10 years ago, the two breakers did not have to be next to each other and have a common disconnect. So, you cannot rely on the MWBC breakers being adjacent. Even now, someone could have installed it electrically properly, but not according to code as far as adjacent breakers and common disconnet is concerned.
Click to expand...
Hi TTJH - my understanding is that the two circuits of an MWBC must be out of phase so as to not overload the shared neutral. 19 & 23 are in-phase. Any guidance as to how to properly remove 1/2 of MWBC (red wire circuit 19) while keeping the other half (black wire circuit 23) functional would be greatly appreciated. Thank you.
 
thomasg88 said:
Hi TTJH - my understanding is that the two circuits of an MWBC must be out of phase so as to not overload the shared neutral. 19 & 23 are in-phase. Any guidance as to how to properly remove 1/2 of MWBC (red wire circuit 19) while keeping the other half (black wire circuit 23) functional would be greatly appreciated. Thank you.
Click to expand...
You keep saying MWBC, 19 & 23 would be on the same leg, and that is not a MWBC. That would just be overloading the neutral. I don't understand when you ask how to remove half of that? Just remove the red wire in the panel and see what doesn't work. You may have to open receptacle & switch boxes, maybe even junction boxes, to find where any splices may be.

For the record, I'm like ptonsparky, I've never seen a 46 space panel in the late 90's. With a #23, that would indicate at least a 46 space panel.
 
Little Bill said:
You keep saying MWBC, 19 & 23 would be on the same leg, and that is not a MWBC. That would just be overloading the neutral. I don't understand when you ask how to remove half of that? Just remove the red wire in the panel and see what doesn't work. You may have to open receptacle & switch boxes, maybe even junction boxes, to find where any splices may be.

For the record, I'm like ptonsparky, I've never seen a 46 space panel in the late 90's. With a #23, that would indicate at least a 46 space panel.
Click to expand...
I qwonder if they are twin breakers and the opp is counting it as 21 and 23 instead of 21 (a)/(B)
 
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