Snap switches for Multiwire Branch Circuits

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markstg

Senior Member
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Big Easy
No, I specifically asked about neutral conductors.

But the reality is it hasn't, this is a highly over exaggerated problem

And I agree that the biggest problem (IMO) with additive harmonics is in transformers.

Roger
Now that I think about it, standard design is MWBC, and plenty of PC's are connected to these circuits and yes it only has shown up in the transformer neutral.

Why, do you not care about efficiency in wiring?

I just find it ammusing that people are so scared of MWBC's that they will grasp for any argument to defend their stance.:smile:

Roger
I'm not afraid of MWBC and have been specifying them all my career. The cost savings in copper and labor is real, and customers should benefit from this reality. But 1pole breakers have always been acceptable.

With the new NEC requirement that multipole CB's be used for MWBC, I'm not for common tripping of 2 additional circuits when only 1 is the problem. Nor am I for turning off 3 circuits when only 1 is troublesome.

BTW, the breaker does not have to a common trip.

Roger
Installations needing to meet the NEC 2008, MWBC are required to be connected to CB with a common trip. Isn't that what all the hub bub is all about.
 
Now that I think about it, standard design is MWBC, and plenty of PC's are connected to these circuits and yes it only has shown up in the transformer neutral.



I'm not afraid of MWBC and have been specifying them all my career. The cost savings in copper and labor is real, and customers should benefit from this reality. But 1pole breakers have always been acceptable.

With the new NEC requirement that multipole CB's be used for MWBC, I'm not for common tripping of 2 additional circuits when only 1 is the problem. Nor am I for turning off 3 circuits when only 1 is troublesome.



Installations needing to meet the NEC 2008, MWBC are required to be connected to CB with a common trip. Isn't that what all the hub bub is all about.
No, Mwbc are required to have a 'common disconnect' not a 'common trip' there is a difference. Read NEC 2008 210.4(B) again.
 
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roger

Moderator
Staff member
Location
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Electrician
Installations needing to meet the NEC 2008, MWBC are required to be connected to CB with a common trip. Isn't that what all the hub bub is all about.
Mark, this is a common misconception, there is no requirement for the breaker(s) to be common trip, the requirement is for all the ungrounded conductors to be disconnected simultaneously.

BTW, I don't mean to be argumentative, but that won't stop me either. :D

Roger
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Do they make breakers with easily removable handle ties? Seems like it would be much easier to simply install 1 pole breakers and remove a handle tie,
Removing the handle tie for manual operation would totally defeat the purpose of the handle tie.
 

mxslick

Senior Member
Location
SE Idaho
No, Mwbc are required to have a 'common disconnect' not a 'common trip' there is a difference. Read NEC 2008 210.4(B) again.

And what pray tell is the availability of a "Common Disconnect" and NOT "Common Trip" breaker? I have never seen any such animal as a "Common Disconnect" breaker in any of the catalogs I've looked at. :roll:

I for one like the idea of the snap switches being installed, and agree that it is a good compromise to avoid people working hot or having to kill all 3 (or both) circuits.

I don't use or allow MWBC's in my line of work (and please, let's not start the debate on why not) but in those situations where MWBC's are used I'd gladly take the time and materials to install those switches.

Bet it would be cheaper than the "Common Disconnect" breaker which probably doesn't exists or is a special order item.
 
This is one of the indirect things I believe that rule will cause. Many violations from just not doing it, to removing handle ties. its a mess in my opinion.

I still see there may be some cost savings under certain conditions for installing the snap switches close to the panel. But like you said it does defeat the intent. Anyone think this rule will be over turned in the next code cycle??
 

iwire

Moderator
Staff member
Location
Massachusetts
And what pray tell is the availability of a "Common Disconnect" and NOT "Common Trip" breaker?
The NEC does not even require that a breaker be used as the disconnecting means.

210.4(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates
A 2 or 3 pole switch located beside the panel meets the requirement.

2 or 3 single pole breakers connected by handle ties meet the requirement.



I don't use or allow MWBC's in my line of work (and please, let's not start the debate on why not)
Don't you watch Law & Order?

Once you bring something up you don't get to ignore questions about it later. :D
 

roger

Moderator
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Location
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Occupation
Electrician
And snap switches would not? Same difference....
No, it is not the same, installing switches in a circuit is totally legal, removing parts from or altering equipment is a different story, although as we have discussed here, it is going to happen.

Roger
 

hurk27

Senior Member
IMO This doesn't seem like a big deal if we follow the code elsewhere, ballasted fixtures will have the required disconnects, and incandescent fixtures most likely will have a wall switch close by or could have one installed where the circuit ceases to be a MWBC.
Much safer in my book
 

LarryFine

Master Electrician Electric Contractor Richmond VA
And snap switches would not? Same difference....
I never said I think the switches are a good idea. They're no different than using the individual wall-mounted light switch as a disco.

Now, if you made a handle tie for the three switches, I'd be all for it. (But then, the handle-tied breakers do the same thing, no?)
 

mxslick

Senior Member
Location
SE Idaho
Now how did I know Bob would bring this up?

Now how did I know Bob would bring this up?

I don't use or allow MWBC's in my line of work (and please, let's not start the debate on why not).
Don't you watch Law & Order?
Nope. :) But did stay at a Holiday Inn. :)

Once you bring something up you don't get to ignore questions about it later. :D
LOL let's see if anyone else cares to know why and if they do I'll start another thread about it.. :)
 
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