NEC 210.4 opinions welcome!

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jclint07

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210.4- MWBC shall have means to SIMULTANEOUSLY disconnect all ungrounded conductors...so here we go....

Contractor installs a 12/3 MWBC, with BOTH ungrounded conductors terminated on a single 20A circuit breaker, which is actually listed for his two conductors (QO-120 breaker).

#1- His MWBC ungrounded conductors DO SIMULTANEOUSLY disconnect, with this setup.

#2- Obviously, neutral current can no longer cancel out, since both hots are now on Line 1, instead of Line 1 & Line 2. So 40 amps COULD possibly flow on his neutral conductor.
BUT, the 20 Amp circuit breaker limits the neutral conductor to only 20 Amps.

#3- Technically, by putting both ungrounded conductors on same breaker (which is listed for such install), he no longer has a MWBC.

This type of install will never fly in my jurisdiction and it didn't. Appropriate double pole circuit breaker was finally installed.

Opinions??
 
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jclint07 said:
210.4- MWBC shall have means to SIMULTANEOUSLY disconnect all ungrounded conductors...so here we go....

Contractor installs a 12/3 MWBC, with BOTH ungrounded conductors terminated on a single 20A circuit breaker, which is actually listed for his two conductors (QO-120 breaker).
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All he has is a single circuit now (unless you meant this was a tandem breaker but the "QO" seems to say a single pole) with two conductors from one breaker

jclint07 said:
#2- Obviously, neutral current can no longer cancel out, since both hots are now on Line 1, instead of Line 1 & Line 2. So 40 amps COULD possibly flow on his neutral conductor.
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only if it is a tandem breaker.


jclint07 said:
BUT, wouldn't the 20 Amp circuit breaker limit the neutral conductor to only 20 Amps?
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Yes if it is a single pole breaker

jclint07 said:
#3- Technically, by putting both ungrounded conductors on same breaker (which is listed for such install), he no longer has a MWBC.
Click to expand...
Correct

jclint07 said:
This type of install will never fly in my jurisdiction and it didn't. Appropriate double pole circuit breaker was finally installed.

Opinions??
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Once again, if this is a single pole breaker there is no problem and the only violation might be wasted materials. ;)

Roger
 
My issue was that the shop owner will eventually get tired of the 20 amp breaker constantly tripping, due to the fact his equipment should have had 20 amp dedicated circuits. At some point, 30 amp breakers could be installed, do to this "nuisance tripping", and then that's where problems would start.
 
jclint07 said:
My issue was that the shop owner will eventually get tired of the 20 amp breaker constantly tripping, due to the fact his equipment should have had 20 amp dedicated circuits. At some point, 30 amp breakers could be installed, do to this "nuisance tripping", and then that's where problems would start.
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How can you inspect for "what ifs"? How do you handle say 40 amp fused circuits with fusible disconnects, the disconnect would be 60 amp rated and would allow some one to fuse the conductors at 50 or 60 amps at a later date?

If the equipment calls for dedicated circuits that's a different story but, there is no violation with what is installed.

Roger
 
I suppose once being an electrical contractor myself, I've seen the "what ifs".

Roger, as far as the fuse question, if equipment is rated up to 60A, then as long as fuses never went over 60 amps, there would never be an issue.

Never seen MWBC such as this or what was supposed to be MWBC. Appreciate the opinions.
 
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jclint07 said:
Roger, as far as the fuse question, if equipment is rated up to 60A, then as long as fuses never went over 60 amps, there would never be an issue.
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In my scenario the conductors and equipment are rated for 40 amps but it takes a 60 amp rated disconnect for 40 amp fuses so, there will always be the possibillity that someone will install larger fuses and we can't do much to control that.

Roger
 
Install was for dedicated 120V machine shop equipment.

Roger, you're absolutely correct about the "too large a fuse" scenario. We can not control that.
In my profession, I just try to help limit it. :D
 
I think the point was a larger OCPD could always be installed. That is when the violation occurs. Not accounting for "what if"s and "may be"s does not constitute a code violation.

Having said that if 40 amps worth was supposed to be installed then that's what should have been done.
 
jclint07 said:
Install was for dedicated 120V machine shop equipment.
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Then the installer definitely did not do the job he was supposed to do and I would cite that as the reason for red tagging the installation.

Roger
 
roger said:
All he has is a single circuit now (unless you meant this was a tandem breaker but the "QO" seems to say a single pole) with two conductors from one breaker

only if it is a tandem breaker.


Yes if it is a single pole breaker

Correct

Once again, if this is a single pole breaker there is no problem and the only violation might be wasted materials. ;)

Roger
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roger said:
Then that just leaves the possible wasted material as the only problem.

Roger
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Twoskinsoneman said:
I don't see any bases for condemning the install. There is no MWBC and no violation.
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Having the benefit of OP'er stating for dedicated equipment it may be a violation. On a single pole, you'd have parallel conductors smaller than permitted if connected to the same load. On a tandem you may have parallel ocpd, which is not permitted, and-or an ungrounded conductor protected beyond its ampacity.
 
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Smart $ said:
Having the benefit of OP'er stating for dedicated equipment it may be a violation. On a single pole, you'd have parallel conductors smaller than permitted if connected to the same load. On a tandem you may have parallel ocpd, which is not permitted, and-or an ungrounded conductor protected beyond its ampacity.
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Agreed if it in fact landed on a single piece of equipment but then there would have been no reason to consider this an MWBC in the first place but, I think the OP has cleared this up and there was more than one piece of equipment that was supposed to have a dedicated circuit.

Roger
 
roger said:
Agreed if it in fact landed on a single piece of equipment but then there would have been no reason to consider this an MWBC in the first place but, I think the OP has cleared this up and there was more than one piece of equipment that was supposed to have a dedicated circuit.

Roger
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Agreed. I just felt potential violations needed stated, which no one had covered.
 
I believe the installer couldn't get passed where I mentioned "210.4, MWBC must have ungrounded conductors disconnected SIMULTANEOUSLY. In his version, placing both hot conductors on the same breaker, rated for his 2 conductors, corrected the violation. In my version, it did not.
 
As an inspector one cannot fail an install because one doesn't like the way things were done. I think others have covered all the possibilities. If the load is too great for the circuit that is one thing but this is no different then running a 12/2 to a jb and going to the two different machines. Now the breaker may not be listed for 2 conductors but I don't understand why the installer went thru the trouble of running a mwbc and cheaped out on a breaker tie and a sp breaker.
 
Dennis Alwon said:
As an inspector one cannot fail an install because one doesn't like the way things were done. I think others have covered all the possibilities. If the load is too great for the circuit that is one thing but this is no different then running a 12/2 to a jb and going to the two different machines. Now the breaker may not be listed for 2 conductors .
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OP said the breaker was listed for two conductors.
Contractor installs a 12/3 MWBC, with BOTH ungrounded conductors terminated on a single 20A circuit breaker, which is actually listed for his two conductors (QO-120 breaker).
Click to expand...

but I don't understand why the installer went thru the trouble of running a mwbc and cheaped out on a breaker tie and a sp breaker
Click to expand...

Maybe he originally had a MWBC on two breakers and needed the room in the panel, so he replaced one of the breakers with one that was rated for two conductors. I'm sure there may be other reasons as well. Just MHO.
 
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