Snap switches for Multiwire Branch Circuits

[mivey]

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.

I'll shoot. Limit the circuit to a 16 amp 100 foot run to get an acceptable voltage drop. Then you have 3.2 volts on the neutral. The body resistance calculated to 24 kohms. This gives you a current of 0.13 mA through your body. Other normal configurations should keep you below 0.5 mA.

It is still best to treat it as you would an ungrounded conductor lest you get careless and get between neutrals or run across an abnormal configuration.

 

[hurk27]

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.

Well even with a 10 volt drop across the neutral return path to the panel, it would be a very good guess that there would be very little current flow across the body:-?

for the OP:
A switch after the common neutral point I see no problem if within site of the work being preformed, before the common point, do yourselves a favor go turn off the common trip breaker for the MWBC:roll:

 

[220/221]

Seems like just more work to me too.

Unless it's a special application, how often are you going to use something like that. If you want to disconnect one of the circuits, pull off the panel cover and use your screwdriver.

 

[roger]

If you want to disconnect one of the circuits, pull off the panel cover and use your screwdriver.

Another good example of what will be taking place.

Roger

 

[220/221]

Of course I would shut down the panel first. :grin:

 

[roger]

Of course I would shut down the panel first. :grin:

Of course.

Roger

 

[brother]

Of course I would shut down the panel first. :grin:

Seeing that you would FOLLOW OSHA/NFPA 70e rules on this, how much time/expense and sheduling and hangling with the customer on big jobs will you be spending doing this, compared to just having that 'snap switch' to turn off the circuit (ie. a receptacles)

 

[Fulthrotl]

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.

yes, and like the song goes... "you give them a short, sharp shock, and they
don't do it again".

Larry, pardon me while i have a brief diatribe here.....

accidentally or purposefully opening the neutral will have the same effect.:smile:
if someone takes apart a neutral, without understanding the nature of
polyphase electricity, their job description should rightly be changed from
inside wireman, to resistive load. and it will be.

the darkness will not necessarily be sudden. i got across a loose neutral in
an attic, with my elbow on a gas pipe, and my wrist on a loose wire laying
in the insulation. the wire went to a 120 volt lamp in the attic. it flickered
with the current that went thru my arm. what was sudden was the swearing.

however, i agree that darkness usually comes quickly to an electrician.
the unexpected part i'm having trouble with..... what would the electrician
EXPECT to happen when he separates wires?

sometimes the shock hazard can be at your fingertips, sometimes it can be
at your elbow, sometimes it can be at the hand of your predecessor, who left
a bare hot wire laying buried in blow in insulation, just for you....

forgive me, i'm just venting here..... but after i LOTO my 3 pole MWBC, plunging
the entire 9th floor of the chase manhatten bank building into darkness, put on my
five point harness so i can climb up on a 10' stepladder, staying off the top
6 steps, put on my protective goggles over my glasses, impairing my vision
with steam, put my hard hat on, and hearing protection, so i won't hear the
people sitting below me cursing me in the darkness, i can then slide a number
#12 wire under a wire nut, and heat up a power pole so Stacy in human
resources can plug in a 1,500 watt space heater in, and trip out the circuit,
killing 5 terminals and a local fileserver, because the dumb bimbo is so fat and
lazy that reaching an extra foot to the black plug instead of the orange plug
is just SO HARD TO REACH.... and the reason she has to plug in a !,500 watt
bun warmer in the first place is she hasn't excercised since the age of eleven,
and she's 80 pounds overweight....

sorry... i'm just burnt out with some of the ever more stringent OSHA mania
coming down the chute....

 

[iaov]

I thoroughly enjoyed your diatribe!!

 

[brother]

yes, and like the song goes... "you give them a short, sharp shock, and they
don't do it again".

Larry, pardon me while i have a brief diatribe here.....

accidentally or purposefully opening the neutral will have the same effect.:smile:
if someone takes apart a neutral, without understanding the nature of
polyphase electricity, their job description should rightly be changed from
inside wireman, to resistive load. and it will be.

the darkness will not necessarily be sudden. i got across a loose neutral in
an attic, with my elbow on a gas pipe, and my wrist on a loose wire laying
in the insulation. the wire went to a 120 volt lamp in the attic. it flickered
with the current that went thru my arm. what was sudden was the swearing.

however, i agree that darkness usually comes quickly to an electrician.
the unexpected part i'm having trouble with..... what would the electrician
EXPECT to happen when he separates wires?

sometimes the shock hazard can be at your fingertips, sometimes it can be
at your elbow, sometimes it can be at the hand of your predecessor, who left
a bare hot wire laying buried in blow in insulation, just for you....

forgive me, i'm just venting here..... but after i LOTO my 3 pole MWBC, plunging
the entire 9th floor of the chase manhatten bank building into darkness, put on my
five point harness so i can climb up on a 10' stepladder, staying off the top
6 steps, put on my protective goggles over my glasses, impairing my vision
with steam, put my hard hat on, and hearing protection, so i won't hear the
people sitting below me cursing me in the darkness, i can then slide a number
#12 wire under a wire nut, and heat up a power pole so Stacy in human
resources can plug in a 1,500 watt space heater in, and trip out the circuit,
killing 5 terminals and a local fileserver, because the dumb bimbo is so fat and
lazy that reaching an extra foot to the black plug instead of the orange plug
is just SO HARD TO REACH.... and the reason she has to plug in a !,500 watt
bun warmer in the first place is she hasn't excercised since the age of eleven,
and she's 80 pounds overweight....

sorry... i'm just burnt out with some of the ever more stringent OSHA mania
coming down the chute....

So I will take this as a vote for installing the snap switches right after the handle tie 3 pole disconnect.

 

[cycotcskir]

Sorry to dig up a dead horse, just to kick it again, but I haven't read any response comparing the cost of the solutions.

500' of #12 THHN from Grainger is only $108. How much would the cost be to install a 1900 with 2 switches? How about one in every room or above the ceiling in a random location?

I'll assume and in-line installation with no additional conduit with all prices from Grainger for consistency and only because they list their prices online.

1900 Box - $3.02
2G Raised Toggle cover - $4.31
2 Commercial Duty Sw. - $9
2 Zinc EMT Conn. - 2.50
1 Handle Tie - $2.72
Forget the labor and mounting hardware or using a 2 pole breaker and we are up to $21.55.
100' of #12 THHN - $21.60

The labor would be the difference.

Comparing adding switches to simply adding a neutral seems to me ridiculous. You are already there to pull the wires, just add another and now you don't even need a 2 pole or handle tie. Plus, on many installations the length of the additional noodle will be limited to the point at which the circuit splits.

I think that for the majority of situations, this would be the easiest and cheapest solution if someone is concerned about turning off Suzies computer.

 

[markstg]

I vote installing single pole disconnects after the 3 pole breaker violates the intent of the code, since adding a nuetral conductor for each circuit is the method for being able to use 1-pole breakers.

IMO, I see this as the death knell to MWBC, as unless the 3 circuits are associated, and are all able to be disconnected at the same time to fix one circuit, (probably not viable in many cases), the 1-pole breaker with individual circuit neutrals is where the code is leading us. May not be a bad idea with all of those Non-Linear loads these days on branch circuits, which show up in the nuetrals, not in the hot conductor.

 

[roger]

I vote installing single pole disconnects after the 3 pole breaker violates the intent of the code, since adding a nuetral conductor for each circuit is the method for being able to use 1-pole breakers.

IMO, I see this as the death knell to MWBC, as unless the 3 circuits are associated, and are all able to be disconnected at the same time to fix one circuit, (probably not viable in many cases), the 1-pole breaker with individual circuit neutrals is where the code is leading us. May not be a bad idea with all of those Non-Linear loads these days on branch circuits, which show up in the nuetrals, not in the hot conductor.

What about the efficiency of the MWBC over the two wire circuit, not to mention saving on natural resources in the way of fuel and copper? (Think of the big picture)


Just curious, how many cases do you personally know of where a neutral conductor has had damage due to additive harmonics?

Roger

 

[cycotcskir]

Save on fuel and copper, but use steel box, steel or zinc connectors, plastic for the switches, ......... wire nuts, ground screws, anchors.


BTW: Harmonics is a big issue where I am

 

[benaround]

Save on fuel and copper, but use steel box, steel or zinc connectors, plastic for the switches, ......... wire nuts, ground screws, anchors.


BTW: Harmonics is a big issue where I am

What is less expensive, one pipe run with nine circuits (mwbc) or three pipe runs with 3 each?

 

[markstg]

What about the efficiency of the MWBC over the two wire circuit, not to mention saving on natural resources in the way of fuel and copper? (Think of the big picture)


Just curious, how many cases do you personally know of where a neutral conductor has had damage due to additive harmonics?

Roger

Regarding harmonics showing up in neutrals, there was a building with 5-30KVA 480V-120/208 transformers, each transformer feeding a panelboard for 120V lighting and receptacles (a commercial office building, built prior to personal computers). A few months after PC's had been installed in all the offices, a transformer neutral opened up. At first I attributed this to a bad connection. Then a few weeks later a different transformer neutral opened. HMMM, now I'm scratching my head, couldn't be bad connection. Thought about all those PC's with non-linear power supplies added, and figured the harmonics not cancelling in the neutral was the cause. Inspected the other 3transformer nuetrals and 2 were showing immenent signs of openeing.

Changed all transformers to 45KVA, with upsized neutral from transformer to panelboard. Problem solved.

MWBC weren't used in this installation, but figured if it hit the big nuetral it could also effect the MWBC neutrals.

I was wondering, should I expect to see a bunch of toggle switches installed next to lighting/receptacle panelboards in the future so MWBC with 3 pole breakers can be used.

 

[roger]

Changed all transformers to 45KVA, with upsized neutral from transformer to panelboard. Problem solved.

But we are not talking about transformers

I was wondering, should I expect to see a bunch of toggle switches installed next to lighting/receptacle panelboards in the future so MWBC with 3 pole breakers can be used.

It's very possible, especially if I'm involved. :wink:

Roger

 

[Cow]

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, I'm aware it violates code until you'd reinstall it. I'm trying to be realistic here though and snap switches sound like a big PIA to install especially if the whole panel is lighting circuits.

Remove handle tie, lock breaker out and go to work....

 

[markstg]

But we are not talking about transformers

Roger

You asked my experience with harmonics in neutrals. Sorry it was not on the Branch circuit conductors, which had individual neutral conductors, therefore could not show itself there.

But if 1 neutral was carrying the return for 3 branch circuits with predominatly non linear loads it certainly could show up in the branch circuit neutral. In my case it showed up in the common neutral of the transformer/panel feeder.

It's very possible, especially if I'm involved.
Roger

Hmmmm, may have to add to the Electrical Specification that MWBC aren't allowed for so that 1pole circuit breakers will be used. Also, I'm not sure owners would like to see three circuits go down when only 1 is faulted.

 

[roger]

You asked my experience with harmonics in neutrals.

No, I specifically asked about neutral conductors.

But if 1 neutral was carrying the return for 3 branch circuits with predominatly non linear loads it certainly could show up in the branch circuit neutral.

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

In my case it showed up in the common neutral of the transformer/panel feeder.

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

Hmmmm, may have to add to the Electrical Specification that MWBC aren't allowed for so that 1pole circuit breakers will be used. Also, I'm not sure owners would like to see three circuits go down when only 1 is faulted.

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:

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

Roger