What should you do before cutting a Multiwire Branch Circuit Neutral?

What should you do before cutting a Multiwire Branch Circuit Neutral?

  • Turn the building main off

    Votes: 1 3.6%

  • Turn off all the panels containing conductors in the MWBC

    Votes: 0 0.0%

  • Turn off all the branch-circuit breakers for the MWBC

    Votes: 22 78.6%

  • Turn off the breaker feeding the load in the outlet box you are working in

    Votes: 5 17.9%
  • Total voters
    28
  • Poll closed .
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F

fmtjfw

Senior Member
Location
Fairmont, WV, USA
A MWBC has more than 1 hot wire (with equal voltage between the hot wires) and a single neutral.

They generally come in two forms:

120/240 single phase [2 hots]

120/208 or 277/480, (or 347/600 more rarely) [2 or 3 hots]

If you need to cut the shared neutral for any reason, what do you do first?
 
Post the question here on what to do?
1. Identify for sure which hot wires share the neutral, looking at handle ties, wire groupings, and other clues.
2. Open the breakers or disconnects for all of those hot wires.
2b. LOTO if appropriate.
3. If I am feeling particularly cautious, put a clamp-on ammeter on the neutral before I cut it.
 
First you have to know how to find out if the circuit is a MWBC. If you don't know how to find that out, then you should shut down the panel feed.:)
 
don_resqcapt19 said:
First you have to know how to find out if the circuit is a MWBC. If you don't know how to find that out, then you should shut down the panel feed.:)
Click to expand...

Well, if the MWBC is grouped per 210.4(D) you are in luck. If not, it would be way beyond a qualified electrician's ability to determine and would be best to run away for safety reasons as iwire indicated.:)
 
don_resqcapt19 said:
That has only been a code requirement for two code cycles so the vast majority of the existing are probably not grouped.
Click to expand...

I hadn't realized that. As you might guess, we don't do a lot of work with MWBC's. I don't recall any being used for fire alarm power, but I don't remember them being forbidden for fire alarm either.
 
don_resqcapt19 said:
That has only been a code requirement for two code cycles so the vast majority of the existing are probably not grouped.
Click to expand...
If it predates the grouping requirement, well, then you are screwed. This simply can't be worked on or repaired. The only solution is to tear the building down and start over with current code.
 
ptonsparky said:
The real answer wasn't listed.

Explain to your new help why you should shut off both circuit breakers, then promptly open the neutral and burn up all the smoke detectors in the house because you shut one of the wrong breakers off.
Click to expand...

That was a very clever instructional demonstration. They will remember the lesson.
 
Is what I do pretty regularly is identify the circuit I'm working on, find the conduit the ungrounded conductor goes in to and turn off all circuits in that conduit. LOTO all affected circuits. If they aren't ID'd it's the surest way I know of not burning stuff up.
 
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