MWBC Troubleshooting....

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1793

1793

Senior Member
Location
Louisville, Kentucky
Occupation
Inspector
I could really use the help of others with a problem I'm having with a Multiwire Branch Circuit.

Situation:
2 story Pool House wired with all MWBC.
Outside 2 gang receptacles ( Red to GFI and Black to GFI )
Both receptacles quit working
I was called to investigate and I can not figure out this one.

Voltage, using DMM,
R-N=35
B-N=35
R-B=0
R and or B to ground 35.

The only way I can get 0.00 reading is to turn off breakers 5 & 7 and 8 & 10.

Breakers 5 & 7 are both Black and breakers 8 & 10 are R / B.

Turning off other combinations of breakers yields lower and different readings.

I'm stumped
 
Dennis Alwon said:
Are the neutrals from both MWBC tied together somewhere other than the panel.
Click to expand...

It appears or seems to be a Home Run but apparently not. I cannot follow the circuit, finished 2nd floor and lower level. I have looked in surrounding receptacles and switches in the lower level where the panel is located and the outdoor receptacles in question.

I could not get into the upper level today.
 
This is where you need to ring out some circuits, door bell on battery, or a pair of phones with a battery to read everything out on the circuits.

You could even test for cross bleed on second neutral.

Of course a meggar is ideal once you know the runs... :)

Besides all that why now, why did the "pool" area go nuts now... :)
 
Do you think that you have a pair of open ungrounded conductors and just picking up some induced 'ghost' voltage?

Try a load on the 35 vac.
 
1793 said:
I could really use the help of others with a problem I'm having with a Multiwire Branch Circuit.

Situation:
2 story Pool House wired with all MWBC.
Outside 2 gang receptacles ( Red to GFI and Black to GFI )
Both receptacles quit working
I was called to investigate and I can not figure out this one.

Voltage, using DMM,
R-N=35
B-N=35
R-B=0
R and or B to ground 35.

The only way I can get 0.00 reading is to turn off breakers 5 & 7 and 8 & 10.

Breakers 5 & 7 are both Black and breakers 8 & 10 are R / B.

Turning off other combinations of breakers yields lower and different readings.

I'm stumped
Click to expand...

I would lean towards a neutral connection lost somewhere. Probably a obvious question, but were you checking directly on the wires or through the receptacle openings? It wouldn't be totally out of question that for some reason you had 2 bad GFCIs. Just a thought.
 
Little Bill said:
I would lean towards a neutral connection lost somewhere. Probably a obvious question, but were you checking directly on the wires or through the receptacle openings? It wouldn't be totally out of question that for some reason you had 2 bad GFCIs. Just a thought.
Click to expand...

I was testing the wires. I did ring out and have continuity between the Neutral and EGC.
 
I would not get too hung up on the MWBC wiring. Regular trouble shooting techniques still apply.

An extension cord plugged into a known properly wired receptacle and a wiggy could be helpful. Check each of the troubled wires to the extension cord.

Don't discount the notion someone could have done something in that building and got in over their head. Then you get called to fix it but do not get the whole story.
 
Are these two Muti-curcuits. Do you have one cable with B and R or two cable with B and R. The reason I ask is because I recently had the same situation. In the panel they had put the red wires on one mini and the black on other mini. When I put the breakers out to check them, one breaker was so burnt that there wasn't any of the stab left.
 
You mentioned that there is continuity from neutral to ground. You can connect a circuit tracer from another circuit to this neutral and trace the neutral.

Sounds like you don't know which breakers supply this but the method mentioned will help find out. Also did you check for voltage at all breakers to start out, could be easy find if not all have voltage.
 
Went back today to look into this situation. I tested at the panel and all breakers tested out as they should.

As I was looking this over I tested with both a "Wiggy" and a DMM. When I put the DMM on the H-N I got 35V when I added the Wiggy the # dropped to 0.5.

After some head scratching I started looking at areas "outside of the box". I found a 3way switch at the front door of the upper level that controlled this exterior receptacle. I could not get into the main house where I think the other half of the 3way is located to confirm my thoughts.
 
I'm constantly amazed at how many journeyman electricians still get caught up by ghost voltage.

A guy in our shop was troubleshooting a long underground run the other day and kept reading ghost voltage with his DMM. I wanted to strangle him with his test leads. He should know better, you need to use the right tool for the job and that's a wiggy.
 
I use a Wiggy 99.999% of the time. I use a DMM to see the voltage on a circuit. For example I take readings at the device and I then check at the source.

Ghost Voltage does not get me "all fussed up".
 
1793 said:
I use a Wiggy 99.999% of the time. I use a DMM to see the voltage on a circuit. For example I take readings at the device and I then check at the source.

Ghost Voltage does not get me "all fussed up".
Click to expand...

I'm not trying to argue with you. All I said was it appeared to hang you up enough that you needed to start a thread about it before you realized your mistake. I thought it was standard trade practice to immediately get out a wiggy anytime you read voltages out of the ordinary, simply to take that variable(ghost voltage) out of the equation?

My apologies if I offended you.
 
Cow said:
I'm not trying to argue with you. All I said was it appeared to hang you up enough that you needed to start a thread about it before you realized your mistake. I thought it was standard trade practice to immediately get out a wiggy anytime you read voltages out of the ordinary, simply to take that variable(ghost voltage) out of the equation?

My apologies if I offended you.
Click to expand...

No need for apologies, and I'm not offended. By the way, what was my mistake. I guess I jumped into the description using the "Ghost Voltage" to help others help me with what could have been the problem of no Wiggy voltage on what appeared to be a Home Run. I received good information about shared neutrals on different circuits.
 
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