Circuits sharing a neutral......

[iwire]

I wouldn't think the two wire circuits to be MWBC. would I be thinking wrong?

The two wire circuits are not, but they are supplied from one.

I gotta say I don't like your wiring methods.

Well in my example I 'mixed' the circuit more then I really do.

Usually I will have lighting supplied separately with 480Y/277 MWBCs and power supplied with 208Y/120 MWBCs.

As Roger pointed out it is common and no different electrically then a feeder to a panel, it is also more energy efficient on many fronts.

About the only time I choose not to run MWBCs is in data center....the runs are short anyway the circuits are always being re-worked for new equipment.

 

[infinity]

Realolman, this a very common wiring method and is used industry wide.

Agreed, in commercial work this is the norm and has been for decades. One or more MWBC's to a central pint, typically a homerun box, and then various two wire circuit out to the loads. Saves money on long homerun conductors (using less conductors) and less terminations at the panel.


So you could work on any portion of the two wire circuit beyond where it connects to the MWBC homerun without a need to de-energize the other circuit sharing the same homerun neutral.

 

[realolman]

I am obviously not as experienced with different installations as you, but it seems odd to me to run # 10 or 12 multi conductor cables for any great distance, rather than a sub panel.

I know I'm not telling you anything you don't already know to say that the only energy efficiency is gained in the portion of the circuit actually comprised of the shared neutral.

If the installation is in conduit, it seems to me more difficult to identify the conductors associated with the MWBC and I think the tie is a good idea.

 

[iwire]

but it seems odd to me to run # 10 or 12 multi conductor cables for any great distance, rather than a sub panel.

Next time you happen to be in a Wal-mart or Lowe's or Home Depot, Target, or most any other other large store look around and see if you can tell where the panels are.

Same can be said for large office buildings filled with just 'office cubes'

The large majority of the circuits are more then 100' many longer then 250' and some even longer then that.

Theses types of business do not want panel in the way of any future layouts so they are placed in groups far from the loads they serve.

This is the opposite of what I see in industrial where we likely will have panels everywhere.

Its' just different solutions for different sets of problems.

 

[RichyL]

Does it say in the 08 code if the handle ties must be permanant? If it allows removable handle ties, then when doing work on the mwbc, couldnt you just remove the handle tie flip the breaker off and the other circuit(s) can still be active?

 

[celtic]

Does it say in the 08 code if the handle ties must be permanant? If it allows removable handle ties, then when doing work on the mwbc, couldnt you just remove the handle tie flip the breaker off and the other circuit(s) can still be active?

I don't have an '08....yet.....in the meantime we can both use this:
[SIZE=+1]Online Access to 2008 National Electrical Code? (NFPA 70?) Softbound Edition[/SIZE]

(Sometimes...like 99% of the time, when I click I AGREE, I get bumper to this standard:
Online Access to NFPA 10: Standard for Portable Fire Extinguishers, 2007 Spanish Edition[SIZE=+1]

[/SIZE]The work around is too just "go back", then hit "I agree" again)




[SIZE=+1] [/SIZE]

 

[LarryFine]

Does it say in the 08 code if the handle ties must be permanant? If it allows removable handle ties, then when doing work on the mwbc, couldnt you just remove the handle tie flip the breaker off and the other circuit(s) can still be active?

That would totally defeat the purpose.

 

[iwire]

That would totally defeat the purpose.

I agree with you.

 

[peter d]

I have worked with foremen who would never run multiwire circuits under any circumstances, even if allowed by specs. Let's just say I thought it was ridiculous to have to pull 3 cables to the same area rather than one and put in a j-box and branch off from there.

I think this whole trend away from MWBC's is just a sign of the dumbing down of the industry. I personally find it a little insulting because it seems like the ones making the rules have no confidence in an electrician's ability to understand basic circuitry.

 

[RichyL]

That would totally defeat the purpose.

Is the purpose to trip all mwbc breakers when there is a fault or to make sure the entire circuit is deenergized when working on it. Or maybe it is both. Does the NEC have an explanation?

 

[iwire]

Is the purpose to trip all mwbc breakers when there is a fault or to make sure the entire circuit is deenergized when working on it. Or maybe it is both. Does the NEC have an explanation?

If the intent was to trip all the circuits they requirement would require common trip breakers not handle ties.

Handle ties do not necessarily open the other circuits during a ground fault on one.

 

[George Stolz]

Is the purpose to trip all mwbc breakers when there is a fault or to make sure the entire circuit is deenergized when working on it.

A common trip breaker is not required, handle ties would be equally acceptable. It's intended to force the worker to shut down all two or three poles when working on the system.

Two or three single-pole breakers will not necessarily trip together when their handles are tied.

Does the NEC have an explanation?

Does it ever?

Sometimes, the intent of a section can be determined by looking at the ROPs and ROCs resulting in the change. More detail on the process can be found here in the FAQ.

 

[RichyL]

I understand how this can be a major pain in commercial buildings since a lot of the lighting is on mwbc. It sounds almost counterproductive in commercial work because shutting a circuit down could make you shut 2-3 lighting circuits off, while there is workers in the building, halting progress. This almost forces you to have to work it hot, which imo is counterproductive. As far as residential goes I dont see this code being much of a problem. But i hear where you commercial guys are coming from Thanks again for your support
Rich

 

[peter d]

With the expansion of AFCI's and handle tie requirements, MWBC's will basically disappear from residential wiring for all but a few circuits in the kitchen.

Edit: And that really makes me

 

[stickboy1375]

With the expansion of AFCI's and handle tie requirements, MWBC's will basically disappear from residential wiring for all but a few circuits in the kitchen.

Edit: And that really makes me

Pete, they make 14-2-2 wire, so I don't see what the fuss is about?

 

[peter d]

Pete, they make 14-2-2 wire, so I don't see what the fuss is about?

True, it's just the idea that we won't be able to use MWBC's that bugs me. It's a principle thing.

I've never used the 14-2-2. How much of a $ premium is it? Is it hard to find? I know that EW stocks it, but you probably have to ask them to bring it in.

 

[roger]

Pete, they make 14-2-2 wire, so I don't see what the fuss is about?

It would still be using an extra conductor, natural resources, and we would not be taking advantage of the efficiency of a MWBC.

Roger

 

[stickboy1375]

It would still be using an extra conductor, natural resources, and we would not be taking advantage of the efficiency of a MWBC.

Roger

I didn't ask for afci's,



edit to add...
(hope that didn't sound rude?)

 

[peter d]

It would still be using an extra conductor, natural resources, and we would not be taking advantage of the efficiency of a MWBC.

Roger

You can bet that copper.org had a party when these new code rules were introduced!

 

[stickboy1375]

True, it's just the idea that we won't be able to use MWBC's that bugs me. It's a principle thing.

I've never used the 14-2-2. How much of a $ premium is it? Is it hard to find? I know that EW stocks it, but you probably have to ask them to bring it in.

I use it all the time, but I do not know the price...