two conductors powered by two breakers?


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Old 05-27-24, 07:36 AM
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two conductors powered by two breakers?

I'm new to electrical work and I need some help understanding this configuration. My home was wired in the 60s, and the panel and breakers were updated in the 90s.

I was trying to turn off the power for a 1/2" Armored Cable conduit which provides power to the garage door opener and the laundry room. The 1/2" AC cable has a red, black, and white conductor (12/3), but no green or bare ground, just three wires.

When I turned off the 20 amp Laundry breaker, the black wire measured no voltage, but the red wire still had voltage. I then turned off the 20 amp breaker directly below the first one, and the red conductor no longer had voltage.

In the junction box over my garage door opener, the black conduit passed right through the box, while the red and neutral had a pigtail that powered the garage door.

When Breaker 01 is shut off, the Garage Door opener and Laundry room both shut off, but there was still voltage in the junction box/receptacle, which required me to shut off breaker 03.

Do these breakers need to be tied together? Is this normal? Also, the armored steel Cable conduit coming into the box has a metal connector with just those three red, black, and white conductors, no ground wire. The similar 1/2" AC conduit leaving the junction box was also 12/3 but had a green conductor that had a wire nut around it, not the green pass through nuts, but it terminated it. Why would this green conductor not be grounded into the metal box and they just put a wire nut over it?

As I do things myself here, can anyone explain why the breakers are set up like this? And can I provide any further info? (FYI, no 240 volt receptacles, all 120V in laundry and home, all receptacles are 3 prong) also, I have not taken my panel face off, so I visually do not see two conductors feeding into two separate breakers, I believe that is what we'd see. I am learning electrical; this is all new to me. Please do not assume I can understand overly complex terminology but any help is appreciated.

Also, My assumption here is that in the 60s this AC conduit was the equipment ground. I have heard many different names for this stuff which I may have used incorrectly in the past? (BX, Greenfield, MC flex) and it seems like this is called (Steel) Armored Cabled Conduit, which I believe was used in the past a an EGC. So hopefully I am using the correct names.
 
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Old 05-27-24, 08:11 AM
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This is a multiwire circuit sharing the neutral so the breakers need a handle tie or replace the breakers with a double pole. Also verify the two breakers are on opposite poles/sides so not to overload the neutral.
 
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Old 05-27-24, 09:10 AM
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Multiwire circuits have been used pretty much when transformers and electricity was started. When you have a neutral that has the same potential as each ungrounded conductor you can "share" that neutral between the ungrounded conductors (2 in single phase, 3 in three phase) The current that the neutral carries is only the imbalance of the ungrounded conductors.

Example: You have 10 amps on phase/leg A and 7 amps on phase/leg B the current on the neutral is only 3 amps.

As Pattenp posted, if you are modifying anything on a multiwire circuit you are required to bring it up to the current code and install a handle tie. However, there is nothing wrong if you want to add handle ties to all your multiwire circuits for your own protection. Just make sure that you properly identify the multiwire circuits before installing the handle tie.

I have heard many different names for this stuff which I may have used incorrectly in the past? (BX, Greenfield, MC flex) and it seems like this is called (Steel) Armored Cabled Conduit
BX, MC, and AC are types of cables. Greenfield, Flex, and Flexible metal conduit (FMC) are types of conduits. Conduits are empty tubes where wires are installed after the conduit is installed, You can usually add wires to conduits at a later time. Cables come with wires already installed and you cannot add wires at a later time to cables. When you post "12/3" that typically refers to a cable and can be confusing to us when you then post "conduits".
 
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Old 05-27-24, 09:24 AM
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Here is a rough diagram of what I was planning. Moving the outlet 5 ' directly above the garage door opener now.
Adding Romex where the steel cable once was in a small section.
I am not sure how to verify that the breakers are on opposite poles/sides. I have two rows of breakers.
They are on top of each other on the left side row, I think it is called Circuit 01 position and Curcuit 03 position?
There are no 240v outlets in the house anywhere they are all 120v 3-prong outlets. If that means anything.
I have had two electricians look at the panel in the last 5 years. Not related to this, but they never said anything. Check out the diagram to see if anything looks wrong?

 

Last edited by PJmax; 05-27-24 at 05:48 PM. Reason: pic hosted/posted PJmax
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Old 05-27-24, 09:38 AM
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For the AC cable (with just the red, black, and white wires) you need to verify whether it has a bare ground wire or strip running the full length inside, outside the paper lining. It might be hard to see but could have been folded back at the end of the cable and visible under the cable clamp from the outside of the panel box or junction box.

If you don't see this strip, you must assume that the cable is not grounded and does not provide a ground path between the two boxes it is connected to. You can add a ground path (12 gauge wire for a 20 amp multiwire branch circuit) between these two boxes and this does not have to follow the AC cable into the wall.

To verify viable positions of the rwo breakers for this multiwire branch circuit, measure voltage between them when both are flipped on. Should get in the range or 240..

There could be reasons why the green wire in the other cable is not connected. For example it was damaged, like snipped off at the other end.
 
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Old 05-27-24, 02:22 PM
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Seems my jpg diagram is not posting. I can only see it in the "go advanced" So I do see it, it just will not show up with my above posting or in the forum. I will try to post it again here in this posting here, see if it works


 
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Old 05-27-24, 02:57 PM
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For the AC cable (with just the red, black, and white wires) you need to verify whether it has a bare ground wire or strip running the full length inside, outside the paper lining. It might be hard to see but could have been folded back at the end of the cable and visible under the cable clamp from the outside of the panel box or junction box.

If you don't see this strip, you must assume that the cable is not grounded and does not provide a ground path between the two boxes it is connected to. You can add a ground path (12 gauge wire for a 20 amp multiwire branch circuit) between these two boxes and this does not have to follow the AC cable into the wall.


I have cut this conduit open and there is only red black and white.
No bare ground or green wire.
Some folks including my electrician have indicated that the Armored flex conduit is the equipment ground.
I guess that is what they did in the 60?


 
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Old 05-27-24, 05:49 PM
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Last edited by PJmax; 05-27-24 at 08:20 PM.
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Old 05-27-24, 08:13 PM
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When I connected the CGCI plug in the configuration above. The breaker flipped.
Bummer. Not sure what went wrong.
Red is 120v
Black is 120v
Together they are 240v
The new GFCI receptacle is pigtailed to the Red ( Brass screw) and white ( silver screw) and ground ( green)
I left the tape on the receptacle that identifies the load screws. I only used the two top Line terminals. Brass and Silver. Red and neutral.
The black is not connected to the new receptacle it just passes through the box providing power in the Laundry room. ( Just like in the diagram above.)
Why would the GFCI receptacle trip the breaker when it was installed. I flipped the breaker on in about 5-7 sec the breaker flipped off.




 
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Old 05-27-24, 08:21 PM
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Assuming nothing is plugged in.
If you disconnect the GFI receptacle.... the breaker doesn't trip ?
If that is correct.... the GFI receptacle has a problem.
 
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Old 05-27-24, 09:14 PM
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PJ Max
When I flipped on the breakers I had the garage door opener plugged in to the new GFCI Receptacle.
The breakers tripped.
When I disconnected the receptacle from the configuration, the power all stayed on.
It seems like the odds of this being a "me problem" is greater than this being the receptacle's fault?
Gotta be something I am doing wrong
 
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Old 05-27-24, 09:52 PM
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As I laid out in the diagram above, I have a shared circuit correct? Seems that is the case.
So I have a black line coming out of breaker 01 and a red line coming out of Breaker 03.
2 rows of breakers, these are vertically stacked and both breakers share a neutral.
All three conductors run through one flex metal conduit.

I just read this on Quora about GFCI receptacles. Can anyone confirm this as I do have a shared Neutral as pointed out initially. Is this true?
"GFI’s need a dedicated neutral to function properly. If your circuit is sharing a neutral with another circuit (which is not unusual) then you have a bigger problem than you know."
 
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Old 05-28-24, 04:41 AM
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The neutral needs to be pigtailed before the gfi.
 
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Old 05-28-24, 11:41 AM
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The neutral is shown as pigtailed in the picture.
GFI receptacles are connected to MWBCs all the time. Extremely typical in a kitchen.

The neutral MUST always be complete.
 
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Old 05-28-24, 01:55 PM
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Thanks for the diagram
I am pretty sure that is how I have that wired in there.
I put pigtails on the the receptacle before hand. ( Photo included)
Any ideas as to what I can troubleshoot now?
Both breakers are on and working. 120 v (117-118v) on each line measure after my joints.
240 when measured together.
This Multi wire Branch Circuit is currently on and working.
It was strange when I introduced the brand new 15 amp receptacle, flipped both breakers on at the same time. 5-7 sec later the breaker at the panel tripped.
I do not think there were many devices on downstream. So not a ton of amps being drawn when i turned on the breakers.
I am embarrassed to say I am not a 100% sure which breaker tripped of the two. I just immediately ensured they were both off. I got a bit spooked and felt terrible that all my hard work did not pay off.
So I am now trying to take a step back and trouble-shoot this project and figure out why one of the breakers would trip when this receptacle was installed?
Let me know if you have any ideas to start with while I wait for an electrician and manually open and close my garage door now. I will be in my room crying until then. LOL



 
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Old 05-28-24, 09:51 PM
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Mystery solved.
My electrician came out, he said that the existing green wire in the conduit was being used as a leg and said it was not a ground wire. ( See initial diagram I posted)
So when I saw the green conductor coming from inside the conduit I simply grounded the green.
Apparently this tripped the panel breaker.
So initially when I powered up the breaker, power was sent down stream to the devices, my GFCI receptacle was not the issue, the wall switch in the laundry room downstrem was turned on, and this sent power to the ceiling light but also back upstream via the green leg wire which was now grounded. This Leg wire was what I had grounded in the junction box and it shorted and tripped the breaker.

no matter how much work I did to prepare for this DIY first electrical project, I never thought to consider a green wire a hot LEG wire. So this was a good lesson learned. You can see in my diagram I posted early on, there is a green wire in my diagram. So that was being used a a LEG hot wire from inside the laundry room. I think at some point, in the 60s, if you flipped on the laundry room light it must have also flipped on the garage light?
Maybe I should have known? If the Metal flex conduit raceway was the Equipment ground, maybe I should have questioned why would this conduit have a green ground wire in it. And maybe I could have somehow figured out the green was not ground and it was a hot leg going know where. And it would have only been hot, when a light switch in the laundry room was flipped on, so testing it with a volt meter, I am not sure what that would have shown?
So my Electrician said to just terminate both ends of the green wire so it is not in use by anything now.
Everything is back and now I am replacing my light switch he blew out while finding this short circuit.
I could have never done this without that last bit of help.
So this was my fault. Glad no one got hurt and I learned so much.






 
 

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