# MWBC > AFCI & conduit fill



## sarness (Sep 14, 2010)

I don't do much conduit work, but I was thinking of a situation the other day.

Lets say you have a 3/4" metal conduit with 7 20a 12awg circuits, 6 of which are MWBC's, this gives you 8 current carrying conductors. So far this install would be compliant (if you don't count the neutrals as being current conducting) but what if down the road you switch some circuits to AFCI?

Sure the 3/4" can handle the extra neutral wires, but wouldn't you have to install 15a OCPD on all the circuits entering that conduit due to derating or replace the #12's with #10?


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## NolaTigaBait (Oct 19, 2008)

Why are you adding neutrals because of AFCI breakers?...

Im not sure if they make afci 2 pole breakers....Is this what you mean?


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## NolaTigaBait (Oct 19, 2008)

Yes, they do...

http://www.sea.siemens.com/us/Produ...AFCI/Pages/2_pole__Combination_Type_AFCI.aspx


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## HARRY304E (Sep 15, 2010)

sarness said:


> I don't do much conduit work, but I was thinking of a situation the other day.
> 
> Lets say you have a 3/4" metal conduit with 7 20a 12awg circuits, 6 of which are MWBC's, this gives you 8 current carrying conductors. So far this install would be compliant (if you don't count the neutrals as being current conducting) but what if down the road you switch some circuits to AFCI?
> 
> Sure the 3/4" can handle the extra neutral wires, but wouldn't you have to install 15a OCPD on all the circuits entering that conduit due to derating or replace the #12's with #10?


The Neutral is a current carrying conductor.
Look in capter 9 for the type of conduit your using..



> *ARTICLE 100 Neutral conductor*.The conductor connected to the neutral point of a system that is intended to carry current under normal conditions


.


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## HARRY304E (Sep 15, 2010)

sarness said:


> I don't do much conduit work, but I was thinking of a situation the other day.
> 
> Lets say you have a 3/4" metal conduit with 7 20a 12awg circuits, 6 of which are MWBC's, this gives you 8 current carrying conductors. So far this install would be compliant (if you don't count the neutrals as being current conducting) but what if down the road you switch some circuits to AFCI?
> 
> Sure the 3/4" can handle the extra neutral wires, but wouldn't you have to install 15a OCPD on all the circuits entering that conduit due to derating or replace the #12's with #10?


 It sounds like your in a commercial building so you don't need to use AFCI breakers..


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## sarness (Sep 14, 2010)

Can o worms can o worms!

AFCI yes, residential, you can't share neutrals on AFCI, just like GFCI, or I'm I thinking wrong?

Four triple headers, lack of room to bore holes, conduit better choice?


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## Dennis Alwon (May 9, 2009)

sarness said:


> Can o worms can o worms!
> 
> AFCI yes, residential, you can't share neutrals on AFCI, just like GFCI, or I'm I thinking wrong?
> 
> You can share neutrals with arc fault on a mwbc if you use dp afci breakers. Some manufacturers make them.


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## NolaTigaBait (Oct 19, 2008)

Dennis Alwon said:


> sarness said:
> 
> 
> > Can o worms can o worms!
> ...


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## Dennis Alwon (May 9, 2009)

NolaTigaBait said:


> Yeah, see post #3.


Yes Seimens makes them but I was pointing out that others do also. GE actually uses their standard AFCI SP breakers with a breaker tie. No other manufacturer does that. Not saying good or bad about it just that it is done differently then others.


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## randas (Dec 14, 2008)

HARRY304E said:


> The Neutral is a current carrying conductor.
> Look in capter 9 for the type of conduit your using..
> 
> .


Really? In Canada the neutral in a MWBC isn't current carrying


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## raider1 (Jan 22, 2007)

HARRY304E said:


> The Neutral is a current carrying conductor.
> Look in capter 9 for the type of conduit your using..
> 
> .


The neutral of a MWBC does not count as a current carrying conductor for ampacity adjustment purposes in accordance with 310.15(B)(4)(a).

Chris


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## manchestersparky (Mar 25, 2007)

The original poster did not tell us enough information to just dismiss the neutral conductors as non-current carrying conductors.
One needs all the information prior to making that call.

Is the OP dealing with a 4 wire 3 phase wye circuits ?
The MWBC's, are they 2 phase conductors and a neutral or 3 phase conductors and a neutral?
What are those circuits for ?
Do those circuits have non-linear loads ?

310.15 (4)(b)
In a 3- wire circuit consisting of two phase conductors and the neutral conductor of a 4 wire, 3 phase, wye connected system, a common conductor carries approximately the same current as the line - to - neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15 ( B) (2) (a)

310.15(4)(c) 
On a 4 wire, 3 phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor, the neutral conductor shall therefore be considered a current carrying conductor.


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## farlsincharge (Dec 31, 2010)

randas said:


> Really? In Canada the neutral in a MWBC isn't current carrying


Not yet anyway, they'll have us running one conductor per conduit here soon.

Here, #12's would only be able to handle 20 amps if there were only three current carrying conductors.

To have eight at 20 amps they would have to be #10's.


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## BuzzKill (Oct 27, 2008)

We should have a "Stump Chris on the Code" contest.


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## jwjrw (Jan 14, 2010)

BuzzKill said:


> We should have a "Stump Chris on the Code" contest.




We would be playing a long time I think....:whistling2::laughing:


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## RePhase277 (Feb 5, 2008)

The neutral of a three-wire circuit is counted as a current carrying conductor when it is derived from a 3-ph 4-wire system.


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## Dennis Alwon (May 9, 2009)

BuzzKill said:


> We should have a "Stump Chris on the Code" contest.


That, IMO, would be a great challenge. I would bet on Chris. :laughing:


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## BuzzKill (Oct 27, 2008)

Yeah I figure BarBQ, DA and Raider could have this huge, no holds barred, no time limit, no disqualifications "cage match" to find the winner.


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## Dennis Alwon (May 9, 2009)

BuzzKill said:


> Yeah I figure BarBQ, DA and Raider could have this huge, no holds barred, no time limit, no disqualifications "cage match" to find the winner.


It would definitely not be me.  I could only hope to know as much as either of those two.


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## sarness (Sep 14, 2010)

Sorry, I thought I gave enough info, its single phase and neutrals are not considered current carrying.

So it would be at the limit unless you changed conductor size which doesn't make sense. You would need to run a larger conduit to reduce fill if you stayed with #10.

My guess then it would be easier to just run multiple 3/4" or 1/2" and stay with #12.


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## raider1 (Jan 22, 2007)

BuzzKill said:


> We should have a "Stump Chris on the Code" contest.





jwjrw said:


> We would be playing a long time I think....:whistling2::laughing:





Dennis Alwon said:


> That, IMO, would be a great challenge. I would bet on Chris. :laughing:





BuzzKill said:


> Yeah I figure BarBQ, DA and Raider could have this huge, no holds barred, no time limit, no disqualifications "cage match" to find the winner.





Dennis Alwon said:


> It would definitely not be me.  I could only hope to know as much as either of those two.


Thanks for the vote of confidence guys.

I can just see it now, BBQ wielding an old beat up NEC and hitting me over the head with it.:laughing::laughing::laughing:

Chris


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## guest (Feb 21, 2009)

raider1 said:


> Thanks for the vote of confidence guys.
> 
> I can just see it now, BBQ wielding an old beat up NEC and hitting me over the head with it.:laughing::laughing::laughing:
> 
> Chris


I can see that too...I've been whacked by BBQ enough times to relate...:laughing:

In relation to the op's situation, I have 6 20 amp circuits and one 15 amp circuit, no MWBC's, with 2 #12 grounds, in a single 3/4" conduit run of about 45 feet feeding my apartment. The 20 amps are #12 and the 15 amp is #14. 

According to the Code, that's a no-no. :no:

But the Code misses out on one big factor, especially in residential: load diversity (or better yet, lack of probability of loading all the circuits to the max at the same time.)

I don't lose sleep over it. 

In the Op's situation I would, however, go with the DP AFCI's to avoid having to add/change conduits.


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## BBQ (Nov 16, 2010)

mxslick said:


> But the Code misses out on one big factor, especially in residential: load diversity (or better yet, lack of probability of loading all the circuits to the max at the same time.)


You can't really write a safety code based on 95% of the typical installations, you have to write it with an eye for the 5% that will be worst case installations.


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## guest (Feb 21, 2009)

BBQ said:


> You can't really write a safety code based on 95% of the typical installations, you have to write it with an eye for the 5% that will be worst case installations.


Very true.  Because we all know that the worst case ones are the ones that end up as a blurb on the evening news.


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## sarness (Sep 14, 2010)

It was just hypothetical going between the difference of max fill and derating of a pre-AFCI installation.


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## guest (Feb 21, 2009)

sarness said:


> It was just hypothetical going between the difference of max fill and derating of a pre-AFCI installation.


No need to get your boxers in a knot, we frequently get into deep discussions here. :laughing::thumbsup:


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## sarness (Sep 14, 2010)

No offense taken, I read enough on here to know tone of answers.

I should just stop over thinking it, nah!


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## sarness (Sep 14, 2010)

Inspector was over for my inspections and I asked about conduit fill and de-rating, he agreed that you would have to jump wire size when over 9 ccc. But it is residential and most households are not running circuits at full ampacity. So he said I could run more the 9 and not de-rate, but he also said don't stuff it to the max either.


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## Dennis Alwon (May 9, 2009)

sarness said:


> Inspector was over for my inspections and I asked about conduit fill and de-rating, he agreed that you would have to jump wire size when over 9 ccc. But it is residential and most households are not running circuits at full ampacity. So he said I could run more the 9 and not de-rate, but he also said don't stuff it to the max either.


Well what he says and what is code compliant is two different things. BTW, I mostly disagree with the inspector.


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## sarness (Sep 14, 2010)

I hear ya.


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