Snap switches for Multiwire Branch Circuits

[LarryFine]

I don't think the purpose was no current in the neutral.

I do.

If you're worried about multi sources in the box, put the switches in seperate boxes.

Then you no longer have MWBC's, because once the neutral is separated (as it would have to be to follow the hots), you can't recombine it.

I would like to start a pole, how many of you would install snap switches after the handle tie breaker on a multi wire bcs. ??

It's a waste of labor and materials and it serves no useful purpose.

Same for installing the snap switches. :wink:

. . . installing switches down stream of the OCPD will at least allow someone to turn off a part of the MWBC.

Perhaps even more ironically, the existing wall light switches function exactly that way now.

The switches do not have to be at the panel, I could strategically place switches ... at the entrance to rooms, etc....

... a disconnect for the one leg or phase insight of say fixtures that it serves would actually be code compliant.

Like the switch by the door to the room? Ironic.

Seriously, why do the existing switches not qualify for that now. Would two switches in series be okay? How far apart would they have to be?

The problem I have with this rule is, a qualified person looking in a box with one neutral and more than one ungrounded conductor would have high suspicions that the neutral could very well be carrying current from the ungrounded conductor that is still on and would not open this neutral until he / she was sure that both were shut off.

Agreed.

BTW, I should clarify that we do not do residential and I don't have a problem with the rule in residential settings.

Agreed.

I never stated you cannot install switches. What I said was you cannot install the switches and think you do not have to install the two pole breaker on the MWBC. These posts always seem to get away form the OP's questions. Now if you install a 2 poll switch at the panel and run both ckts to it you may be able to convince the inspector you met the intent of the code. But the intent of the code is to turn off BOTH ckts so you wasted time and money on the DP switch.

I believe the intent was to install the handle-ties as required, and then install single-pole switches to allow individual circuit de-energization.

 

[M. D.]

Perhaps even more ironically, the existing wall light switches function exactly that way now.

and ironically ,.. this is exactly how the ballast disconnect functions as well:smile:

 

[LarryFine]

If you turn off only one leg, and go into that junction box (or outlet box) and remove the wire nut connecting the neutral wires to each other, and then accidently touch one of the neutral wires, then the other leg can send current through its load, and through you. That is why they require us to turn off all ungrounded conductors at the same time.

Charlie, this is where I agree with you and the handle-tie need. While any qualified wink electrician should know the risk of the MWBC, I've had more than one wirenut pop off and the conductors spring apart.

Even accidentally opening the neutral with only one circuit de-energized can damage equipment, plunge the electrician in sudden, unexpected darkness, and leave him with a real shock hazard at his fingertips.

 

[Dennis Alwon]

Here is the problem as I see it. Suppose you install a MWBC to a JB and from there run two 2 wire circuits to there respective loads. Now you install a switch at the point where the circuit is a two wire circuit. Unless the switch is a dp switch that disconnects the neutral is there no a possibility that someone is working on the lighting cir. , for instance , thinking the cir. is safe and disconnects the neutral wire. If that person makes a path between the neutral and ground will he/she not get some voltage travelling on the neutral from the other cir.?

 

[iwire]

Dennis do you think about that danger when you turn off a single pole breaker to work on a two wire circuit?:smile:

Electrically it is the same thing, as on the supply side of the breaker the circuit is a multi wire feeder sharing a neutral.


If I was to install a single pole switch in the blue wire where the circle says 'two wire' and locked it open how could I be injured working on outlets 5a, 5b or 5c?

 

[infinity]

Dennis do you think about that danger when you turn off a single pole breaker to work on a two wire circuit?:smile:

Electrically it is the same thing, as on the supply side of the breaker the circuit is a multi wire feeder sharing a neutral.


If I was to install a single pole switch in the blue wire where the circle says 'two wire' and locked it open how could I be injured working on outlets 5a, 5b or 5c?

You can't. As it's been said a few times here in this thread, a MWBC neutral may not pose any shock hazard depending on where in the circuit you're working on it. Bob's illustration shows one example of that.

 

[roger]

Here is the problem as I see it. Suppose you install a MWBC to a JB and from there run two 2 wire circuits to there respective loads. Now you install a switch at the point where the circuit is a two wire circuit.

Then there is no problem as shown in Bob's illustration.

Roger

 

[roger]

Like the switch by the door to the room? Ironic.

That was my point.

Roger

 

[charlie b]

If I was to install a single pole switch in the blue wire where the circle says 'two wire' and locked it open how could I be injured working on outlets 5a, 5b or 5c?

The answer is that as you remove the wire nut that connects the three neutral wires to the left side of outlet 5a (the three wires being the one coming into the box, the one feeding the receptacle in the box, and the one leaving the box), then the one coming into the box can be energized via the red circuit breaker, via the load plugged into outlet 3a, via the neutral coming off to the left of outlet 3a, to the junction box in which the neutral is split amoungs the 3 circuits, and via the neutral run leading to outlet 5a.

 

[iwire]

The answer is that as you remove the wire nut that connects the three neutral wires to the left side of outlet 5a

Charlie, when is it ever safe to work on the supply side of the switch that has been opened to work on a circuit? :-?

You example is the equivalent of opening one single pole breaker in a panel and then trying to open the feeder neutral ........ it goes against all common sense and against anything a qualified person would do.

 

[iwire]

it goes against all common sense and against anything a qualified person would do.

Let me amend that to 'against anything a qualified person should do.'

 

[Dennis Alwon]

I guess I am over thinking. If the neutrals are all connected together in the JB won't current flow and all available paths back to the panel. If circuit one is energized the neutral current will flow on the neutral down on cir. #5. If I disconnect the neutral wire at the recep. on cir #5 wont some current flow down the white wire of cir #5 to ground thru my body if I am grounded. I have never experienced it but current does take all paths. Learn me something--- :smile: or is so small I won't feel it.

 

[480sparky]

I guess I am over thinking. If the neutrals are all connected together in the JB won't current flow and all available paths back to the panel. If circuit one is energized the neutral current will flow on the neutral down on cir. #5. If I disconnect the neutral wire at the recep. on cir #5 wont some current flow down the white wire of cir #5 to ground thru my body if I am grounded. I have never experienced it but current does take all paths. Learn me something--- :smile: or is so small I won't feel it.

If you touched the neutral on any of the circuits, open or not, and are grounded, you do become part of a circuit. Some current will flow. If the neutral is solidly grounded (as it should be) and properly terminated, then the current flow through you will be negligeble. You are part of a parallel path, but your resistance is likely to be so high compared to the neutral that you'll never know it.

 

[iwire]

IIf circuit one is energized the neutral current will flow on the neutral down on cir. #5. If I disconnect the neutral wire at the recep. on cir #5 wont some current flow down the white wire of cir #5 to ground thru my body if I am grounded.

IMO yes due to VD on the neutral that is entirely possible and likely.

And it is exactly the same situation as when you touch the neutral of any two wire circuit as they are all tied together at the panel and the feeder neutral will have some amount of voltage drop.

 

[roger]

Dennis, the same case scenario would exist in any connected grounded conductor that you might touch, even a single pole breakered circuit.

The grounded conductor of a single pole circuit is still connected to the service neutral which is common to more than one leg or phase.

Roger

 

[Twoskinsoneman]

Who you talking to? :grin:

OK I may not be 'young' but I still work with the tools. At 1AM last night I was in a supermarket hunting down lighting circuits I could tap onto for a few new fixtures.

Ok throwing the young in there might not be fair. But I believe a lot of the guys maybe upset with new rules aimed at making it safer to work because they are so experienced and so "qualified" that they may not have a lot of patients with "newer" techs

 

[Dennis Alwon]

Dennis, the same case scenario would exist in any connected grounded conductor that you might touch, even a single pole breakered circuit.

The grounded conductor of a single pole circuit is still connected to the service neutral which is common to more than one leg or phase.

Roger

You are right I never thought about that. I got hung up on the parallel path even tho I never experienced it in that situation I was wondering if one could feel it. Once the current went all the way back to the panel I knew the current would be negligible but I wasn't certain when you were further from the source.

Thanks all for the brain lift. :smile:

 

[roger]

Thanks all for the brain lift. :smile:

Brain lifts are why we are all here. :smile:

Roger

 

[480sparky]

You are right I never thought about that. I got hung up on the parallel path even tho I never experienced it in that situation I was wondering if one could feel it. Once the current went all the way back to the panel I knew the current would be negligible but I wasn't certain when you were further from the source.

Thanks all for the brain lift. :smile:

Do the math, using Ohms law. Assume your body has the resistance to induce a 5 ma flow between you and a hot 120v wire. (Typical for a GFI to sense & trip).

Use that resistance in parallel with, say, a 120v circuit neutral using #12, and use a length long enough to provide some resistance.

Assume that the ground path taken by the parallel path after your body has 0 ohms, just for giggles & grins, and see what amp flow you would experience.

 

[Dennis Alwon]

Do the math, using Ohms law. Assume your body has the resistance to induce a 5 ma flow between you and a hot 120v wire. (Typical for a GFI to sense & trip).

Use that resistance in parallel with, say, a 120v circuit neutral using #12, and use a length long enough to provide some resistance.

Assume that the ground path taken by the parallel path after your body has 0 ohms, just for giggles & grins, and see what amp flow you would experience.

That's too much work. I am tired. Worked in the rain in a trench today. I feel like Al only his muscles don't ache and they are bigger than mine.