multiwire branch circuits

[GerryB]

What is the reasoning behind using 2-pole breakers for multiwire branch circuits? I recentlly did a service and they had a sub panel that had to be eliminated. It had 11 circuits, I put a columbia box in the attic and ran 6 3wire cables to it. So now in my new panel these are multiwire branch circuits right? I know you want to put the the circuits on different legs for balance, but marking the panel was a chore (had to lift wires). Also trouble shooting would be the same. Then there was trying to explain to the HO why there was all these 2-pole breakers in his panel. Is it also a safety issue?

 

[templdl]

What is the reasoning behind using 2-pole breakers for multiwire branch circuits? I recentlly did a service and they had a sub panel that had to be eliminated. It had 11 circuits, I put a columbia box in the attic and ran 6 3wire cables to it. So now in my new panel these are multiwire branch circuits right? I know you want to put the the circuits on different legs for balance, but marking the panel was a chore (had to lift wires). Also trouble shooting would be the same. Then there was trying to explain to the HO why there was all these 2-pole breakers in his panel. Is it also a safety issue?

Consider if both lines of a multiwire circuit (home run) land on the same yoke and you want to worl on the yoke, how would you be assure that the breakers for each line landing on the yoke are in the open position? With either a 2p or a (2) 1p w/handle tie you have no choice but to turn off both lines at the same time.

 

[augie47]

It is a safety issue. With MWBC sharing a neutral, someone working on the circuit might turn off only one breaker associated with the MWBC opening up the possibility of a shock from a "back-feed" on the neutral.

 

[ronaldrc]

Here is a pictorial diagram of the danger of a multiwire branch circuit with single pole breakers

And why they started requiring 2 pole circuit breakers.

http://home.comcast.net/~ronaldrc/wsb/Multiple_wire_branch_circuits.htm

 

[infinity]

Here is a pictorial diagram of the danger of a multiwire branch circuit with single pole breakers

And why they started requiring 2 pole circuit breakers.

http://home.comcast.net/~ronaldrc/wsb/Multiple_wire_branch_circuits.htm

Nice graphic.

 

[K_Buz]

On a side note, do they make 2P AFCI breakers? I haven't seen them, and when I look them up, all that comes up is single pole. And if they don't, what about handle ties? Or does THAT depend on the manufacturer?

 

[chris kennedy]

Someone needs to say it...

Qualified Person.

One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved.

 

[K_Buz]

On a side note, do they make 2P AFCI breakers? I haven't seen them, and when I look them up, all that comes up is single pole. And if they don't, what about handle ties? Or does THAT depend on the manufacturer?

Found my answer

 

[iwire]

Someone needs to say it...

Thank you!

 

[kwired]

Someone needs to say it...

TY. Always been my thought on this also. Used to just be required when both lines were connected to same device, I had no problem with that - why would you shut off one but not the other in this situation? There has been some trend to make the NEC accomodate the safety of those that don't having any business messing with things. You can't idiot proof things - a better idiot will always come along:happyyes:

 

[infinity]

TY. Always been my thought on this also. Used to just be required when both lines were connected to same device, I had no problem with that - why would you shut off one but not the other in this situation? There has been some trend to make the NEC accommodate the safety of those that don't having any business messing with things. You can't idiot proof things - a better idiot will always come along:happyyes:

I'm a big proponent of MWBC's but IMO sometimes idiot proofing maybe the only thing to keep someone from harming themselves. And I'm not just referring to Harry Homeowner. A co-worker, who's taken OSHA10, lock out-tag out and has 25 years experience was almost killed a few weeks ago when he went up against a 277 volt neutral from a MWBC. Looks like he might fully recover but he could be out of work for several months if not longer. So maybe the handle-tie rule for MWBC's isn't so bad after all.