# Conduit Fill and Derating



## chris75 (Aug 25, 2007)

jamiedolan said:


> My plan is to run 5 120V circits in each junction box. That would put me at 15 total wires (hot neutral ground for each circuit). For derating, I would use the 4-6 current carring conductor value of 80%. So for my 12 gage, I would derate from 30A to 24A. and for my 14 gage I would derate from 25A to 20A.
> 
> .
> 
> Jamie


 
I'm only gonna help with the derating part for now, first off, you would only need 1 ground wire per conduit, 2nd, if you pull all MWBC ( multiwire branch circuits) then you dont count the grounded (aka : neutral) conductor as current carrying. So, if you ran 3 sets of blk, red, wht, and one ground wire, (10 conductors total, but only 6 are current carrying) Just some usefull INFO thats all.


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## jamiedolan (Sep 2, 2008)

chris75 said:


> I'm only gonna help with the derating part for now, first off, you would only need 1 ground wire per conduit, 2nd, if you pull all MWBC ( multiwire branch circuits) then you dont count the grounded (aka : neutral) conductor as current carrying. So, if you ran 3 sets of blk, red, wht, and one ground wire, you would end up with 6 ckts, and only 6 current carring conductors. Just some usefull INFO thats all.


Thanks. I forget to research the grounding issue. I was wondering with EMT if I even need to run a ground wire at all or if as long as I am bonded to the box if the EMT serves as the ground path? It's not a big deal to run the ground wire, I am just curious if the EMT is a proper ground.

Thanks
Jamie


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## chris75 (Aug 25, 2007)

jamiedolan said:


> Thanks. I forget to research the grounding issue. I was wondering with EMT if I even need to run a ground wire at all or if as long as I am bonded to the box if the EMT serves as the ground path? It's not a big deal to run the ground wire, I am just curious if the EMT is a proper ground.
> 
> Thanks
> Jamie


Yes, EMT is allowed to be used as the grounding conductor.


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## Stubbie (Jan 7, 2007)

Why are you using 4-6 current carrying conductors if you have 5 120 volt circuits and none are MWBC?

I'm also not understanding how your using the emt. It sounds like you are using a "piece" of emt. How long is it?


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## jamiedolan (Sep 2, 2008)

Stubbie said:


> Why are you using 4-6 current carrying conductors if you have 5 120 volt circuits and none are MWBC?
> 
> I'm also not understanding how your using the emt. It sounds like you are using a "piece" of emt. How long is it?


HI;

I have about 18 pieces of NM that need to be junctioned and connected to a panel. They are up in the joist area.

My plan is to install metal 4" boxes, clamp 4-5 pieces of NM to the box, then connect the NM wires to THHN and run the THHN about 5-7 feet through EMT to the electrical panel.

I am using 3/4" EMT with runs that are all less than 10' (I have not done it yet, so I am not positive how long they will end up being, but I am sure none will be over 10'). It is longer than the 24" piece of conduit I have read about in the NEC that has some expemptions.

5 seemed like a reasonable number to junction in a 4" box, and accourding to my understanding of derating cables, 5 circuits in 1 conduit will allow me to maintain 15A with 14 gage and 20A with 12gage. 


Thank You
Jamie


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## Gigs (Oct 26, 2008)

Is this another go at aesthetic improvement?

Needlessly terminating the NM in junction boxes above the panel just for looks is just going to make your life that much harder down the road. You won't be able to figure out what wire goes to what breaker without doing electrical tests, normally you can just see where it's going.


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## jamiedolan (Sep 2, 2008)

Gigs said:


> Is this another go at aesthetic improvement?
> 
> Needlessly terminating the NM in junction boxes above the panel just for looks is just going to make your life that much harder down the road. You won't be able to figure out what wire goes to what breaker without doing electrical tests, normally you can just see where it's going.


The wires need to be extended one way or another to get them out of the old panel and into the new one. So either I patch on about 18 pieces of NM and run them into the new panel or I do it with THHN and conduit.

They currently go into my old pushmatic panel, I installed the new panel a couple feet away and wired it up as a sub panel. Now you going to ask why I didn't just put the new panel in place of the old panel and not have to splice any wire. Well even if I put it in the same place I would have to splice all of the neutrals because they are all cut short and attached at the top of the pushmatic box. I really don't want to have all thoses splices in the panel. So that leaves me with extending the wires some place or another, and I would rather not have it be in the breker box.

Jamie


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## Gigs (Oct 26, 2008)

jamiedolan said:


> The wires need to be extended one way or another to get them out of the old panel and into the new one. So either I patch on about 18 pieces of NM and run them into the new panel or I do it with THHN and conduit.
> 
> They currently go into my old pushmatic panel, I installed the new panel a couple feet away and wired it up as a sub panel. Now you going to ask why I didn't just put the new panel in place of the old panel and not have to splice any wire. Well even if I put it in the same place I would have to splice all of the neutrals because they are all cut short and attached at the top of the pushmatic box. I really don't want to have all thoses splices in the panel. So that leaves me with extending the wires some place or another, and I would rather not have it be in the breker box.
> 
> Jamie


Ah, OK, carry on then.


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## Stubbie (Jan 7, 2007)

5 120 volt branch circuits that are not MWBC's is 10 current carrying wires. You would need to derate at 50%.

I don't like your plan....


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## jamiedolan (Sep 2, 2008)

Stubbie said:


> 5 120 volt branch circuits that are not MWBC's is 10 current carrying wires. You would need to derate at 50%.
> 
> I don't like your plan....


 
I might have understood this wrong, does this only apply to the neutral leg of a 240v circuit? 

NEC 310.15:
 
(4) Neutral Conductor.​(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit
shall not be required to be counted when applying the provisions​of 3l0.l5(B)(2)(a).

lf so, then, I will only run 4 circuits per conduit and then I only have to derate 70% and can use the wires at 15 and 20 (14# and 12#).

Thanks
Jamie


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## Stubbie (Jan 7, 2007)

> I might have understood this wrong, does this only apply to the neutral leg of a 240v circuit?



Correct... shared neutrals of MWBC as an example.

120 volt 2 wire circuits require both the hot and neutral to carry the same amps when loaded... so both must be counted.


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## jamiedolan (Sep 2, 2008)

Stubbie said:


> Correct... shared neutrals of MWBC as an example.
> 
> 120 volt 2 wire circuits require both the hot and neutral to carry the same amps when loaded... so both must be counted.


Ok. So I will count the neutrals. Then I will stick with 4 circuits per conduit, derating 70% which will leave my 12# at 20A and my 14# at 15A. 

Sound like a better plan. 

Thanks Much
Jamie


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## frenchelectrican (Apr 12, 2006)

The magic number for most conductor derating is typically 9 so you have 4 circuits which it will have 8 ccc { current carry conductors } so you are one wire below of magic zone.

If you gone with 5 circuit then you will have to use the derating on that conduit that can affect depending on the conductor size.

Merci,Marc


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## InPhase277 (Feb 9, 2008)

Jamie, don't forget about box fill when you are putting all these circuits in. You have more or less got the gist of the pipe fill and derating, but you must keep in mind the volume of the boxes too. Since you are going with the pipe method, you might consider a 24 x 6 x 6 wireway as your junction. The only draw back is you have to drill your own holes in it, but it will likely have more than enough volume. Just keep good track of the circuits and don't mix things up.


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## Stubbie (Jan 7, 2007)

I'm not going to go into my dislike for a bunch of junction boxes so that is your call. Also I'm not sure what your budget is but some of the manufacturers make renovation panels that allow you to get those short neutrals, grounds , hot conductors and service entrance to reach their appropriate termination points in a very workman like manner. I'll give you a link to the cutler hammer one I always liked and hey they have improved it since I last installed one......

http://www.salesmanagementplus.com/ContentOnDemand/MfgContentPieces/Images2/Eaton/CH_Renovation.pdf


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## jamiedolan (Sep 2, 2008)

InPhase277 said:


> Jamie, don't forget about box fill when you are putting all these circuits in. You have more or less got the gist of the pipe fill and derating, but you must keep in mind the volume of the boxes too. Since you are going with the pipe method, you might consider a 24 x 6 x 6 wireway as your junction. The only draw back is you have to drill your own holes in it, but it will likely have more than enough volume. Just keep good track of the circuits and don't mix things up.


No mix -up's nice and organize.








Since I was only putting 4 in each conduit, and after I do some re-wiring, I think I may only end up with 12 circuits, so that will only be 3 boxes, not too bad. My neutrals were just long enough once clamped into the box.

I used 4"x4" boxes, based on the chart I saw in the NEC, I am fine with 4 circuits in it.









Thanks
Jamie


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## rgsgww (Jul 5, 2008)

jamiedolan said:


> No mix -up's nice and organize.
> View attachment 6036
> 
> 
> ...


 
Looks nice, great work! :thumbup1:


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## jamiedolan (Sep 2, 2008)

rgsgww said:


> Looks nice, great work! :thumbup1:


Thanks  Hopefully I can finish up the old connections tommrow, then I am going to start running some new circuits.

Jamie


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## jamiedolan (Sep 2, 2008)

Stubbie said:


> I'm not going to go into my dislike for a bunch of junction boxes so that is your call. Also I'm not sure what your budget is but some of the manufacturers make renovation panels that allow you to get those short neutrals, grounds , hot conductors and service entrance to reach their appropriate termination points in a very workman like manner. I'll give you a link to the cutler hammer one I always liked and hey they have improved it since I last installed one......
> 
> http://www.salesmanagementplus.com/ContentOnDemand/MfgContentPieces/Images2/Eaton/CH_Renovation.pdf


They do look pretty slick. I will keep them in mind if I ever have to do one of these again.

I should have had the meter pulled today, it would have be so much faster. In the pushmatic panels, the mains are in such a bad position, your right next to them when you are taking out the neutrals / grounds. My main is AL and it is covered in some old coating that looks like cheap waxed paper. I was kind of afraid I was going to end up making contact with the main with my pliers or a ground wire and cause a fault. I wondered what happens if you cause a fault on your main prior to your breaker, does the meter have some type of overcurrent / fault protection?

I used live tools and handeled everything properly, just in case the mains energized somethig they shouldn't have.

I am keeping my junction boxes nice and neat, and up in between the joists. 

Thanks
Jamie


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## rgsgww (Jul 5, 2008)

jamiedolan said:


> does the meter have some type of overcurrent / fault protection?


No, the meter does not have any protection other than being able to be pulled. I do not think there is overcurrent/fault protection after the xformer. I know there is one before it. If any person who has worked with poco lines want to fill me in here, Id appriciate it...


xformer diagram (split-phase aka 120/240)


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## chris75 (Aug 25, 2007)

Why didn't you just run two neutrals up and make them multi wire branch circuits? just curious thats all.


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## Stubbie (Jan 7, 2007)

I know what you mean about close quarters, I once had to replace/move an old split buss pushmatic 100 amp panel that had a new 4" main plumbing drain installed by the homeowner....which he ran vertically about 12 inches in front of the panel. He removed the hinged door of the panel since he couldn't open it with the drain pipe in the way. Why do people do this?? Sounds like you needed to move yours too.

It would do you some good (learning curve..) to draw out what Chris was saying about the two multiwires. This would have had some advantages that I think you would have appreciated... to get you started... draw 2 multiwires from the panel to that junction box then split the 4 nm cables between them. Then think of things in terms of how many conductors you used compared to what you have now and also box and conduit fill plus conductor deration. Doesn't make either way any better but it is a good way to see a different approach to what you have now.


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## frenchelectrican (Apr 12, 2006)

rgsgww said:


> No, the meter does not have any protection other than being able to be pulled. I do not think there is overcurrent/fault protection after the xformer. I know there is one before it. If any person who has worked with poco lines want to fill me in here, Id appriciate it...
> 
> 
> xformer diagram (split-phase aka 120/240)


 I will be glad to due I work on high voltage system from time to time but I will keep it simple here

The only protection you will get this from the transfomer is the primary side { high voltage } and there is no protection on secondary side { low voltage } due pretty good percetage of the overhead cans { transfomer(s)} secondary side do feed few home they can go anywhere from one home to many as 10 homes depending on the grid size and setup.

If in case you wondering why there is no protection on secondary side.,,, the secondory side what the term we called short circuit amparage avablitity that mean the X amout of amparage for short cirucit is avable from the transfomer and the X amout of short circuit will varies a bit due so many factors and type of transfomer.

However typically shortcircuit avabilty useally over 8,000 amp or higher depending on the transfomer size the bigger it go the more short circuit capitcy go up.

Myself I ran into few service cable shorted out and they do pretty good damage there { it don't matter if single phase resdentail or triphase large commercal/industrail user } the last one I work on not too long ago it did actally melt the steel riser{ pipe } on the commercal building sorta like out of control welder.

Yeah for the fuse on transfomer ? they useally are oversized and super long delay before they blow normally they sized to either blow the heck out of the transfomer or hevey shortcircircuit kick the fuse out.
That why you useally don't see the fuse blow on light to pretty good overload sistuation.

IF more question either myself or other electricians can expain more if you like to.

Merci,Marc


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## InPhase277 (Feb 9, 2008)

chris75 said:


> Why didn't you just run two neutrals up and make them multi wire branch circuits? just curious thats all.


The only drawback is that with MWBCs you have to use double pole breakers. You have to turn off two circuits just to work on one. Depending on what circuits those are, that can suck.


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## chris75 (Aug 25, 2007)

InPhase277 said:


> The only drawback is that with MWBCs you have to use double pole breakers. You have to turn off two circuits just to work on one. Depending on what circuits those are, that can suck.


Depends on what code cycle your on, CT is not adopting the 2008, full speed ahead!


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## jamiedolan (Sep 2, 2008)

chris75 said:


> Depends on what code cycle your on, CT is not adopting the 2008, full speed ahead!


Wisconsin is on 2005.

Jamie


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## jamiedolan (Sep 2, 2008)

InPhase277 said:


> The only drawback is that with MWBCs you have to use double pole breakers. You have to turn off two circuits just to work on one. Depending on what circuits those are, that can suck.


Just regular double pole breakers that take up 2 slot positions - because you have to make sure each circuit draws power from each phase? 

So they trip in common as well, correct? So a lighting circuit, and your outlet circuit on a branch with a shared neutral will both trip at once if the outlets cause a fault/overdraw, correct?

Jamie


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## jamiedolan (Sep 2, 2008)

Stubbie said:


> I know what you mean about close quarters, I once had to replace/move an old split buss pushmatic 100 amp panel that had a new 4" main plumbing drain installed by the homeowner....which he ran vertically about 12 inches in front of the panel. He removed the hinged door of the panel since he couldn't open it with the drain pipe in the way. Why do people do this?? Sounds like you needed to move yours too.
> 
> It would do you some good (learning curve..) to draw out what Chris was saying about the two multiwires. This would have had some advantages that I think you would have appreciated... to get you started... draw 2 multiwires from the panel to that junction box then split the 4 nm cables between them. Then think of things in terms of how many conductors you used compared to what you have now and also box and conduit fill plus conductor deration. Doesn't make either way any better but it is a good way to see a different approach to what you have now.


Thanks guys for pointing this out. I have not had a lot of experiance with MWBC before and maybe I didn't give it enough consideration...

My understanding of a MWBC is that you use a double pole breaker, that has both poles connected so it is a common trip. Then you just share the same neutral with 2 circuits. Is this correct? 

Also it sounds like I could put 6 circuits in a conduit with MWBC and only have to derate to 70% still. 

The only thing I have a question about is what if these circuits need ground fault or ARC fault protection in the panel? 

Are MWBC compatiable with ground fault or arc fault breakers?

Thanks
Jamie


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## rgsgww (Jul 5, 2008)

jamiedolan said:


> The only thing I have a question about is what if these circuits need ground fault or ARC fault protection in the panel?
> 
> Are MWBC compatiable with ground fault or arc fault breakers?
> 
> ...


I have seen double pole arc fault and gfci breakers, so yes, they are.

Depends on the circuit. If the conduit goes to a bedroom for instance, yes you need afci protection.

MWBC will not work with single pole afcis, but your not allowed to use single poles with MWBC, only common trip dble poles.

Multi wire circuits share the neutral, but MUST use both legs of the panel, not just one. Its all about balance. The neutral is the center-tap of the xformer. The output is called the "secendary" of the xformer.

Split-phase tranformer (xformer) used to power most homes.


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## jamiedolan (Sep 2, 2008)

rgsgww said:


> I have seen double pole arc fault and gfci breakers, so yes, they are.
> 
> Depends on the circuit. If the conduit goes to a bedroom for instance, yes you need afci protection.
> 
> ...


HI;

Can you joint together together 2 AFCI or GFCI breakers in a CH panel to create a double pole or doesn't it work that way at all?

The 15 and 20A breakers are around $6 for both the QO and CH panels. However I just looked it up and your right many of the larger CH breakers are cheaper. All of the breakers are MUCH more expensive than the breakers for the panels like the home line. But I really wanted a nice panel with a full copper bus bar. I just looked it up and the CH double pole shared neutral AFCI breakers are about $135 (from the prices I saw online), a bit on the pricey side. 

Jamie


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## chris75 (Aug 25, 2007)

jamiedolan said:


> HI;
> 
> Can you joint together together 2 AFCI or GFCI breakers in a CH panel to create a double pole or doesn't it work that way at all?
> 
> ...


If your not installing new circuits, I wouldn't even install afci breakers.


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## jamiedolan (Sep 2, 2008)

chris75 said:


> If your not installing new circuits, I wouldn't even install afci breakers.


These are old circuits I am hooking up right now, I can't afford to do the AFCI's now. But was planing on try to replace the breakers with AFCI in the future as I could afford to.

Do you think the AFCI's are not worth it? If you have nice solid high quality wiring and outlets and switches in place, do the AFCI's really provide any benefit?
Thank You.
Jamie


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## chris75 (Aug 25, 2007)

jamiedolan said:


> These are old circuits I am hooking up right now, I can't afford to do the AFCI's now. But was planing on try to replace the breakers with AFCI in the future as I could afford to.
> 
> Do you think the AFCI's are not worth it? If you have nice solid high quality wiring and outlets and switches in place, do the AFCI's really provide any benefit?
> Thank You.
> Jamie


 
I'm an electrician with a house built in the 70's and I still dont see a need for them, I have my reasons, for one, not enough testing was done before they were entered into the almighty code book, so I just dont see enough proof that they do anything. except that they will locate shoddy wiring practices. ( shared circuit neutrals, ground to neutral connections.)


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## jamiedolan (Sep 2, 2008)

chris75 said:


> I'm an electrician with a house built in the 70's and I still dont see a need for them, I have my reasons, for one, not enough testing was done before they were entered into the almighty code book, so I just dont see enough proof that they do anything. except that they will locate shoddy wiring practices. ( shared circuit neutrals, ground to neutral connections.)


I am rewiring several parts of my house and replacing every outlet. So I will discover and bad wiring. ( I have found some weird combinations put onto a circuit, all origional wiring put in place in 63 when the house was build. For example: Kitchen lights, Microwave, foyer, closet, outside outlet, out side house lights, lamp post, all on one 15A ). In short, I will catch any wiring problems myself, if thats all the AFCI's are really good for, then I don't need to spend $1,000 to populate my box with expensive devices that tell me my wiring is good. 

Thanks
Jamie


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## chris75 (Aug 25, 2007)

jamiedolan said:


> I am rewiring several parts of my house and replacing every outlet. So I will discover and bad wiring. ( I have found some weird combinations put onto a circuit, all origional wiring put in place in 63 when the house was build. For example: Kitchen lights, Microwave, foyer, closet, outside outlet, out side house lights, lamp post, all on one 15A ). In short, I will catch any wiring problems myself, if thats all the AFCI's are really good for, then I don't need to spend $1,000 to populate my box with expensive devices that tell me my wiring is good.
> 
> Thanks
> Jamie


 
The problem is, once you re-wire, you need to bring everything up to current code.  So for instance, if you add a receptacle in your bedroom, then it has to be AFCI protected, even if the rest of the room is not.


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## chris75 (Aug 25, 2007)

jamiedolan said:


> ( I have found some weird combinations put onto a circuit, all origional wiring put in place in 63 when the house was build. For example: Kitchen lights, Microwave, foyer, closet, outside outlet, out side house lights, lamp post, all on one 15A ). Jamie


 
That was the norm back then, they just didnt have large loads like they do today.


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## jamiedolan (Sep 2, 2008)

chris75 said:


> The problem is, once you re-wire, you need to bring everything up to current code.  So for instance, if you add a receptacle in your bedroom, then it has to be AFCI protected, even if the rest of the room is not.


I am on the 2005 code cycle here in wisconsin. So my understanding is that yes, an addition to a bed room would require an AFCI, but no where else would. I belive that this changes in 2008 to require virtually everything to be AFCI that isn't required to be GFCI.

Are they making AFCI outlet style units or only AFCI breakers?

Jamie


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## chris75 (Aug 25, 2007)

jamiedolan said:


> I am on the 2005 code cycle here in wisconsin. So my understanding is that yes, an addition to a bed room would require an AFCI, but no where else would. I belive that this changes in 2008 to require virtually everything to be AFCI that isn't required to be GFCI.
> 
> Are they making AFCI outlet style units or only AFCI breakers?
> 
> Jamie


 
You are correct with the above statement, were on the 2005 as well, and thankfully skipping right over the 2008, and I've yet to hear about an afci receptacle style unit, but the NEC seems to believe they exist.


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## InPhase277 (Feb 9, 2008)

jamiedolan said:


> Just regular double pole breakers that take up 2 slot positions - because you have to make sure each circuit draws power from each phase?


That's some of it. The code makers wanted to ensure that the neutral would not be overloaded, for one. The other reason is they wanted to ensure that if you are working on an MWBC that you can't open a neutral and be electrocuted by voltage from the other half.



> So they trip in common as well, correct? So a lighting circuit, and your outlet circuit on a branch with a shared neutral will both trip at once if the outlets cause a fault/overdraw, correct?
> 
> Jamie


Yes that would happen as well. I don't usually recommend that DIYr's mess around with multiwire circuits, because they can confound even seasoned professionals. But if the circuit terminates in a junction box near the panel, and you are careful, this can reduce your pipe and box fill. Since you are not on the 2008 Code, you don't have to necessarily put them on double poles. But you must be mindful that each leg lands on a different phase in the panel.

BTW, as far as AFCIs go, I believe they are a waste of money. In my little pocket of Alabama, where we have no enforcement of codes, I don't use them. I only use them in jurisdictions where they are required. In one instance, I followed behind a handyman that wired some rooms before he was fired. I placed his crap work on AFCIs, but other than that, they exist because of the manufacturers' reps on the code making panels, IMO.


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## frenchelectrican (Apr 12, 2006)

jamiedolan said:


> Wisconsin is on 2005.
> 
> Jamie


 
Yes it is still on 2005 code cycle until next spring { 2009 } we will take in 2008 Code cycle however there are few changes along the way myself and few Wisconsin sparkys and Inspectors just got the info not too long ago.

AFCI requirement still optional the mantory usesage is pushed back to 2011 

TamperResenst receptales yes .,, go ahead with it 

Removed GFCI extempts but sump pump receptale still have extempt unless local code stated else.

There are few other changes as well almost parallel with full 2008 code unchanged expect few words change along the way.

Merci, Marc


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## jamiedolan (Sep 2, 2008)

InPhase277 said:


> That's some of it. The code makers wanted to ensure that the neutral would not be overloaded, for one. The other reason is they wanted to ensure that if you are working on an MWBC that you can't open a neutral and be electrocuted by voltage from the other half.


I went and got some double pole breakers, I'll need more breakers by the time I am done anyway. 

Is there anything you need to do to make sure the neutral isn't over loaded? Are factors that apply? Or is it as simple as 2 circuits that land on different legs of the panel both share one neutral wire that is the same gage as the circuits? i.e. 2 -20A 12 gage hot circuits from 1 double pole breaker run from panel to destination. Both use the same #12 white ground wire. Is it that simple?

Thanks
Jamie


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## InPhase277 (Feb 9, 2008)

jamiedolan said:


> I went and got some double pole breakers, I'll need more breakers by the time I am done anyway.
> 
> Is there anything you need to do to make sure the neutral isn't over loaded? Are factors that apply? Or is it as simple as 2 circuits that land on different legs of the panel both share one neutral wire that is the same gage as the circuits? i.e. 2 -20A 12 gage hot circuits from 1 double pole breaker run from panel to destination. Both use the same #12 white ground wire. Is it that simple?
> 
> ...


On a properly installed MWBC, the neutral can carry no more current than any one leg. If you use a double pole breaker, you can't overload the neutral. If you use two different breakers, you must be sure to place the breakers on different phases.


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## jamiedolan (Sep 2, 2008)

InPhase277 said:


> On a properly installed MWBC, the neutral can carry no more current than any one leg. If you use a double pole breaker, you can't overload the neutral. If you use two different breakers, you must be sure to place the breakers on different phases.


As long as they are on different phases in the panel, that ensures that the neutral can not be overloaded? I know that the neutral is a return path for electricity, but I don't know how much or what amount of power it exactly carries.

So if both of thoses 20A circuits ran with say a 18A load, the one 12 gage neutral would still be adequate protection?

Does that shared neutral pose any potential problems with GFCI devices? I read some things a while back that gave me the impression that a shared neutral could cause a problem with GFCI because the outlet senses load on the neutral from the other circuit and trips. Or did I understand this incorrectly?

Thanks again. I have to get my hook up finished tommorw. I had to take a break today, That was a ton of work yesterday to get that old panel cleared out and everything) but currently only have 4 circuits hard wired, and a couple of things jumped into a open box (yea not safe).

If the MWBC's will work for a couple of these, it will save me a lot of time, Ill be able to do all of the old circuits in 2 4x4" boxes up there in the joists and the rest of them can be done elsewhere like in the crawl space.

Thanks again
Jamie


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## jamiedolan (Sep 2, 2008)

InPhase277 said:


> On a properly installed MWBC, the neutral can carry no more current than any one leg. If you use a double pole breaker, you can't overload the neutral. If you use two different breakers, you must be sure to place the breakers on different phases.


So does this in effect mean that 2 -20A circuits only really have the capicity for 20A total and not 20A each for a total of 40A? I am confused about how the double pole breaker ensures that the neutral is not overloaded.

I must be missing something.

Jamie


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## rgsgww (Jul 5, 2008)

jamiedolan said:


> So does this in effect mean that 2 -20A circuits only really have the capicity for 20A total and not 20A each for a total of 40A? I am confused about how the double pole breaker ensures that the neutral is not overloaded.
> 
> I must be missing something.
> 
> Jamie



This is what the transformer looks like as a diagram

The red and black connecting to the secendary are your two hots

They share a wire (neutral) going back to the center of the tranformer.

When these two potentials are balanced on the neutral, it is not overloaded.

The neutrals purpose is to balance out the circuit. With two different potentials, such as the red and black, you do not overload the conductor.

The red goes to one leg in the panel, the black goes to the other.
7.2kv is the poco's input at a default of 7200 volts.
The double pole makes sure that the hots stay on two different legs, so you don't overload.
Single poles cannot be used because if you turned off only one breaker, half of the circuit will still be live.
Lets say a fault occured, like above, half of the circuit will still be live.


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## Gigs (Oct 26, 2008)

jamiedolan said:


> So does this in effect mean that 2 -20A circuits only really have the capicity for 20A total and not 20A each for a total of 40A? I am confused about how the double pole breaker ensures that the neutral is not overloaded.
> 
> I must be missing something.
> 
> Jamie


Yes I think so. Lets call the two sides of the MWBC (a) and (b).

Lets say that side (a) is has 5 amps load, and side (b) has 15 amps load.

The current flowing through the neutral will be 10 amps.

This scenario is using 20 amps at 120v or 2400 watts.

Now consider a balanced scenario. 10 amps per side. Neutral carries zero current. You are using 20 amps at 120v or 2400 watts.

The tricky stuff come in if the neutral is broken for any reason. Then what was a MWBC becomes a *series* circuit, and you have what amounts to loads in series across 240v circuit. This is a bad situation and you'll have 120v devices with higher and lower voltages than normal. Or even worse, if the phases are perfectly balanced, you might not notice it for a while, then you turn on an appliance and your TV explodes. It can really look like a haunted house!










Oh I realized I didn't actually fully answer your question. Because a 2 pole breaker is adjacent slots, it is always pulling from both phases. If you were to use two single pole breakers, and you skipped a slot, then they would both be on the same phase. Instead of currently flowing from (a) to (b), it would add up. so 10 on (a) and 20 on (b) becomes 30 on the neutral if you accidentally pulled from the same side.


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## jamiedolan (Sep 2, 2008)

Gigs said:


> The tricky stuff come in if the neutral is broken for any reason. Then what was a MWBC becomes a *series* circuit, and you have what amounts to loads in series across 240v circuit. This is a bad situation and you'll have 120v devices with higher and lower voltages than normal. Or even worse, if the phases are perfectly balanced, you might not notice it for a while, then you turn on an appliance and your TV explodes. It can really look like a haunted house!


Thanks for the explaination. An open neutral on a MWBC isn't really any different than an open neutral on a regular circuit is it? Because at the receptacle of a MWBC you still only have one hot and one neutral, so you would never get a 240v connection, just a regular open neutral condition such as can happen with any circuit that has an open neutral.

Correct?

Jamie


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## chris75 (Aug 25, 2007)

jamiedolan said:


> Thanks for the explaination. An open neutral on a MWBC isn't really any different than an open neutral on a regular circuit is it? Because at the receptacle of a MWBC you still only have one hot and one neutral, so you would never get a 240v connection, just a regular open neutral condition such as can happen with any circuit that has an open neutral.
> 
> Correct?
> 
> Jamie


 
Not correct, an open neutral on a MWBC will automatically create a seriese 240v circuit. This can only happen on multiwire branch circuits, and just so you know, your service coming into your home is basically a multi wire branch circuit.


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## chris75 (Aug 25, 2007)

jamiedolan said:


> I am confused about how the double pole breaker ensures that the neutral is not overloaded.
> 
> I must be missing something.
> 
> Jamie









...................


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## jamiedolan (Sep 2, 2008)

chris75 said:


> Not correct, an open neutral on a MWBC will automatically create a seriese 240v circuit. This can only happen on multiwire branch circuits, and just so you know, your service coming into your home is basically a multi wire branch circuit.


So the danger is if the neutral between the 2 circuits were to be intact, but the neutral to the panel gets disconnected. Then the neutral between the 2 devices bridges the 2 legs if it isn't connected to the panel.

But if the neutral were severed at a point where it cut the path between the 2 circuits, and you just had a hot wire each going directly to an outlet and there was no in series neutral, then there could be no series circuit created.?

Jamie


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## jamiedolan (Sep 2, 2008)

I used double pole breakers. I just made the change about 20 minutes ago. Thanks to my obsessive labeling of each and every wire, it only took about 5 minutes to change it over to 2 MWBC. I just put a meter on one of the circuits and read 120v. 

So if I put a 10A load on each "leg" of my MWBC and out a clamp meter on the neutral wire, I should see a very low load, or technically none if the circuits were perfectly balanced with there load.

Jamie


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## rgsgww (Jul 5, 2008)

jamiedolan said:


> So if I put a 10A load on each "leg" of my MWBC and out a clamp meter on the neutral wire, I should see a very low load, or technically none if the circuits were perfectly balanced with there load.
> 
> Jamie


Yes, it will never overload the neutral.

Between the two "legs" you should get 240 and from leg to neutral 120. That will tell you if its done right.


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## Gigs (Oct 26, 2008)

jamiedolan said:


> So the danger is if the neutral between the 2 circuits were to be intact, but the neutral to the panel gets disconnected. Then the neutral between the 2 devices bridges the 2 legs if it isn't connected to the panel.


I think you got it now, it turns into a series circuit and the relative resistance of the two circuits determine what voltage each side "sees". 

This can't happen with regular circuits unless the main service neutral at the panel became disconnected, which is less likely.

So these are the two main risks, a broken neutral leading a series circuit, and incorrect feeding from the same phase causing an overloaded neutral. Since you used double pole breakers you have eliminated the possibility of the latter risk, but the former risk is something to be careful of.


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## Stubbie (Jan 7, 2007)

Jamie

In your situation the only place your neutral is shared is from the junction box to the circuit breaker panel. At the junction box you essentially split it into four 120 volt circuits that do not share the neutral. A 240 volt circuit can only occur if you open the 'shared' neutral at the junction box or breaker panel. If the neutral opens on the two 120 volt loops you will only lose that branch of the split multi wire.. the other branch will work normally. For example one advantage this configuration allows is gfci receptacles to be installed where ever you want them because the neutral is not shared after the junction box. the simple diagram below shows your neutrals entering the JB and joined with the shared neutral to the circuit breakaer box. If a double pole breaker is used then two neutrals will be on leg A and two on Leg B and will cancel each other as shown 20A - 18A = 2A on the shared neutral. The neutrals after they leave the JB will carry the full return amperage of there particular hot wire just like any 2 wire (H-N)120 volt circuit.


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## jamiedolan (Sep 2, 2008)

Stubbie said:


> Jamie
> 
> In your situation the only place your neutral is shared is from the junction box to the circuit breaker panel. At the junction box you essentially split it into four 120 volt circuits that do not share the neutral. A 240 volt circuit can only occur if you open the 'shared' neutral at the junction box or breaker panel. If the neutral opens on the two 120 volt loops you will only lose that branch of the split multi wire.. the other branch will work normally. For example one advantage this configuration allows is gfci receptacles to be installed where ever you want them because the neutral is not shared after the junction box. the simple diagram below shows your neutrals entering the JB and joined with the shared neutral to the circuit breakaer box. If a double pole breaker is used then two neutrals will be on leg A and two on Leg B and will cancel each other as shown 20A - 18A = 2A on the shared neutral. The neutrals after they leave the JB will carry the full return amperage of there particular hot wire just like any 2 wire (H-N)120 volt circuit.


Sounds like I got it right. Thank You for your help and for the diagrams. Here are the updated photos. I am going to pull some thhn for that 2nd box soon, I just have to figure out if one of the circuits in there is suppose to be 15 or 20A ( in this house the feeder cable into the panel is not necessarly the same gage as the rest of the circuit ).

















































Thanks again!
Jamie


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## jamiedolan (Sep 2, 2008)

Blue = 15 Amp Circuits
Yellow = 20 Amp Circuits
Green = Ground
White = Neutral

All of the Blue and Yellow are MWBC, I may run in some different colors for regular circuits (non MWBC). Althought I plan to make most of my new runs MWBC to there junction boxes in the atic that EMT is going to run to. Lots of EMT to use up, I bought 35 pieces last night and already had over a dozen. Yikes!

Jamie


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## Gigs (Oct 26, 2008)

I didn't know you could use any color you want for hots, but I looked it up and it looks like it's OK. Don't let a european electrician in there though or he might zap himself!


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## Stubbie (Jan 7, 2007)

Here is a diagram more closely resembling what you have but I think you have it figured out.

Also I can't remember if you said you removed the metal strap connecting the neutral bars together. You need to do that then reinstall the bonding jumper to the ground bar that the feeder insulated green equipment ground is connected to.


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## jamiedolan (Sep 2, 2008)

Gigs said:


> I didn't know you could use any color you want for hots, but I looked it up and it looks like it's OK. Don't let a european electrician in there though or he might zap himself!


I remember reading about the colors used in other countries, but don't remember what they do differently...jamie


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## jamiedolan (Sep 2, 2008)

Stubbie said:


> Here is a diagram more closely resembling what you have but I think you have it figured out.
> 
> Also I can't remember if you said you removed the bonding jumper from the ground bar to the metal strap that goes over to the neutral bar if not you need to do that.


Yep, I am running isolated neutral bar, clip is gone. Everything appears to be working just fine. I'll double check voltage reading tommrow, and look at the neutrals with a clamp meter to make sure all is well, which I am sure it is. I'll have the last of the old circuits I am hooking up done in the am, then I need to start running EMT so I can re-wire several circuits.

I am really happy with how the MWBC worked out. It allowed me to junction everything right up there in the joists, hidden nice and neat in those 2 boxes that connect to the panel with EMT. Makes the panel look really neat and organize with just THHN in it. 

Thanks again
Jamie


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## InPhase277 (Feb 9, 2008)

Jamie, I count 10 conductors entering that pipe. Since these are MWBCs to 2-wire NM, that means you have 10 + 12 = 22 conductors in that box (counting only one ground). The box looks like a 4 x 4 x 2-1/8". I do believe you have exceeded the volume limit for the box. You might look into an extension ring.


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## joey b (Oct 18, 2008)

Although it may not be as cost efficient, could one run a shared neutral from 2 MWBCs that were located on the same leg, assuming that amperage allowance was not violated? for example, run 2 20 amp circuits (from the same supplying leg) with 12 guage wire for the hot and 8 gauge for the shared neutral? Seems this would also eliminate the possibility of the broken neutral situation.


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## Gigs (Oct 26, 2008)

joey b said:


> Although it may not be as cost efficient, could one run a shared neutral from 2 MWBCs that were located on the same leg, assuming that amperage allowance was not violated? for example, run 2 20 amp circuits (from the same supplying leg) with 12 guage wire for the hot and 8 gauge for the shared neutral? Seems this would also eliminate the possibility of the broken neutral situation.


You'd wind up using more copper. 12AWG is (6529.95cmils), and 8AWG (16509.65cmils).

So in addition to creating a hazardously confusing wiring situation, you wouldn't save anything.


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## InPhase277 (Feb 9, 2008)

Gigs said:


> You'd wind up using more copper. 12AWG is (6529.95cmils), and 8AWG (16509.65cmils).
> 
> So in addition to creating a hazardously confusing wiring situation, you wouldn't save anything.


What he said. But yes, you could. You could put every neutral on one big wire as long as it had the ampacity to carry the current.


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## jamiedolan (Sep 2, 2008)

InPhase277 said:


> Jamie, I count 10 conductors entering that pipe. Since these are MWBCs to 2-wire NM, that means you have 10 + 12 = 22 conductors in that box (counting only one ground). The box looks like a 4 x 4 x 2-1/8". I do believe you have exceeded the volume limit for the box. You might look into an extension ring.


I'll pick up an extension for the box. I don't see extensions listed on the chat 314.16(A). How do I know how much capicity the extentions adds and if it is adequate for the number of wires I have?
Thanks
Jamie


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## Gigs (Oct 26, 2008)

jamiedolan said:


> I'll pick up an extension for the box. I don't see extensions listed on the chat 314.16(A). How do I know how much capicity the extentions adds and if it is adequate for the number of wires I have?
> Thanks
> Jamie


Do the general volumetric calculation.


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## jamiedolan (Sep 2, 2008)

Gigs said:


> Do the general volumetric calculation.


 
Ok I see it now. I will just have to figure out what volume the extension adds, hopefully it is labeled on the box extension.
Thanks
Jamie


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## jamiedolan (Sep 2, 2008)

*Box Fill Calculation?*

12 gage is listed as needing 2.25 cu in X 22 = 49.5 cu in. That box is 30.3 cu in. So does that mean I need to find a box extension that adds almost 20 cu in to that box?

It didn't even seem that tight in the box at all, 50 cu in seems like a huge box for these wires. I guess I am getting better at making nice neat connections that take up less space.

Jamie


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## Stubbie (Jan 7, 2007)

Jamie 

Here is a link to get your volumes on extension rings and boxes. 

http://www.tnb.com/contractor/docs/sc_indoorboxes.pdf

I am little confused at what you ended up with... for some reason I was thinking we were talking 2 14/2 cables connected to a 14/3 multiwire and 2 12/2 cables connected to a 12/3 multiwire. Under those considerations you would have 32 cu. in. of box fill.... 7 14's and 7 12's plus one 12 ground.

A 4x 4x 2 1/8 square has 30.3 cu in so you would have been over even with that. 
Right now you have 46 cu in of box fill. So you will need an extension ring of at least 16 cu. on those 4x4's. 

Here is another link to help you in the future


http://www.hubbellcatalog.com/raco/RACO_boxes.asp?FAM=RacoBoxes

Remember on box fill you have different allocations depending on the wire size 14 = 2 counts per wire and 12 = 2.25 counts per wire and one count for the grounds equal to the largest conductor in your situation.



I apologize for not getting this calculated for you as to the size boxes you needed but I was thinking somewhere in all this it had been mentioned but I guess i as wrong. I'll look back in the thread and see where I missed what you ended up with.


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## Stubbie (Jan 7, 2007)

I believe what your saying is you used all 12 awg so you would be 49.5 cu in.

Are all the black cables 14's 12's or mixed? I'm worried you connected the 12's to 14 awg cables and put them on a 20 amp breaker.


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## jamiedolan (Sep 2, 2008)

Stubbie said:


> I believe what your saying is you used all 12 awg so you would be 49.5 cu in.
> 
> Are all the black cables 14's 12's or mixed? I'm worried you connected the 12's to 14 awg cables and put them on a 20 amp breaker.


12/20's are on yellow THHN 12 gage

14/20's are on blue THHN 14 gage.

They are not all 12 gage, there is a mix, I just used that as a rough number since there is only .25 difference between the 2 wires.

I think what happened is that I was only counting the wire once when it came into the box and went out, I wasn't counting that as 2. So I was figuring out my box fill with half of the number of wires I should have been figuring it out with.

They are 12-2 and 14-2 cables that come into the box. Then a hot from each black (that comes into the junction on NM) runs back into the panel. Then 2 hots that share the same breaker have there neutrals tied together and a pig tail run back into the panel. 

Jamie


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## jamiedolan (Sep 2, 2008)

Stubbie said:


> I believe what your saying is you used all 12 awg so you would be 49.5 cu in.


I just got some double gang mud rings, 15 cu in. So that gives me 45.3 cu in. 

I do have a mix with some 12 and some 14 in all the boxes. Actually the box that is the most full is 6-14gage and 7-12gage plus 6 ground wires are bonded in that box from the NM, but they are 16gage ground wires in the NM for some reason. 

I come up with a number of 34.77 for the amount of required space. So unless I am figuring out something incorrectly, with the mud ring, I will have more than enough room to satisify the box fill requirments. (althought physically there was plenty of room to work in the 4"x4" box.). I clamped in the wires in good locations (top feed as much as possiable).

Jamie


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## jamiedolan (Sep 2, 2008)

*Updated Photos.*

I just posted updated photos of my electrical projects, and the panel in the project catagory.

http://www.diychatroom.com/showthread.php?t=32169

Jamie


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