# Ground wire when multiple branches run through same conduit



## McSteve (Dec 8, 2009)

I can answer a couple parts of this at least:



Ptron said:


> I have two books on wiring and these seem like basic questions about multiple branches sharing conduit, but damned if I can find the answers.
> 
> ...
> 
> The conduit will of course be grounded at the panel but I assume it should not be used as the ground since at some point the circuits will branch and either use MC or NM cable with a ground wire. Or_ is it_ acceptable to use the conduit as the the ground and bond the the individual branch wires to the junction box?


As long as we're talking about metallic conduit, you can indeed use the conduit as the EGC, and connect your cable ground wires at the junction boxes.



Ptron said:


> Lastly, is there any requirement that multiple hots running through the same conduit be color coded? Hard to find anything other than black and red in small quantities.


From what I've seen, I don't think there's any such requirement, (unless the wires are carrying different voltages?). But it sure is a good idea. At the very least, tag the wires with a number or something at each end, you'll be glad you did later.


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## joed (Mar 13, 2005)

A single ground wire sized to largest circuit in the conduit is fine. For example if there are two 15 amp ciruits and a 30 amp circuit the ground must be #10 to match the 30 amp circuit not #14 to match the 15 amp circuit.
There is no requirement that I know of requiring different colour wires for different hot circuits.


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## Ptron (Nov 28, 2008)

Very helpful replies. Thanks much.

Other than the limits for the number of wires in the conduit and in the junction boxes (I know how to figure those based on AWG, conduit size, box space, etc.) are there any restrictions on the # of circuits that can share a conduit? 

For example for 1/2 EMT, I think (off the top of my head, don't quote me on this) the limit is 9 12awg wires. If the individual ground wires were required that would be 3 20 or 15 amp circuits, but with a single ground wire or conduit as EGC you can get 4 circuits in there. 

I'm running new branches in my basement for a workshop, bandroom, laundry room. Power tools, amplifiers, PAs, Washer/dryer, dehumidifier, etc.


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## nap (Dec 4, 2007)

just the wire count, not the number of circuits when dealing with wire fill 

BUT don't forget to derate if necessary. When derating requires you to upsize the wire, of course, that leads to a recalculation of the wire fill.


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## joed (Mar 13, 2005)

There are two separate issue with wires in conduit.
One is conduit fill - the number of wires that will fit in a conduit. All wires count for this one.
The other is derating. If more than 9 current carrying conductors are in a conduit then you must derate the current carrying capacity of the wires. Ground wires and some of the neutrals don't count for this one among others.


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## darren (Nov 25, 2005)

joed said:


> If more than 9 current carrying conductors are in a conduit then you must derate the current carrying capacity of the wires. .


 Joed is this the american rule because in Canada it is more then 3 current carrying conductors that you have to start to derate.


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## joed (Mar 13, 2005)

Under the NEC when you start to derate you don't start at 15 amps for #14 wires. Derating begins with less conductors but until you go over 9 the derating doesn't bring #14 below 15 amps.


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## nap (Dec 4, 2007)

darren said:


> Joed is this the american rule because in Canada it is more then 3 current carrying conductors that you have to start to derate.





joed said:


> Under the NEC when you start to derate you don't start at 15 amps for #14 wires. Derating begins with less conductors but until you go over 9 the derating doesn't bring #14 below 15 amps.


derating starts at >3 wires here as well. What joed was stating (just in case you didn't quite understand it), is that the derating is based on the the actual ampacity of the wire. When considering #14cu (15 amp OCPD) and #12cu (20 amp OCPD) the code has already derated them to a level that it would take derating for 9 (or more) CCC for the rating to be lower than the limitation already imposed.

#14, #12, and #10 are the only sizes that this applies to as they are the only sizes that are limited by code in this manner.


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## Ptron (Nov 28, 2008)

Ok, I think I get it but let me make sure. I was thinking of running 9 wires, 4 circuits and one ground, so 8 current carrying wires. The chart I have says that for 7-9 current carrying wires you have to derate to 70% ampacity. At least the laundry circuit will be 20A but since the 12awg 90c wire that I'm using has already been derated from 30A max actual capacity to 20A I should be ok, correct?


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## jbfan (Jul 1, 2004)

Yes, as long as you are using thwn/thhn wire you are derating from 30 amps*70% and you get 21 amps.


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## SleepyDog (Jun 20, 2010)

Hey guys, even though I am not the OP I found this thread very useful. I would like to ask another question to test my understanding. Assuming we are talking about 12 AWG THHN/THWN wire for 20 amp circuits and metal EMT conduit which will serve as the ground, I can actually run 3 MWBC's through one piece of conduit which effectively means 6 individual circuits, correct? Wish I knew about this before I redid my kitchen. It would have made a lot neater installation to run EMT from the panel into a JBox near the kitchen and then break into individual runs.


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## darren (Nov 25, 2005)

Ptron said:


> Ok, . I was thinking of running 9 wires, 4 circuits and one ground, so 8 current carrying wires.


Do you plan on running a neutral with each hot, if so there is nothing wrong with this plan.

Could you not use multiwire branch circuits and save yourself some wire. This would also reduce your current carrying conductors(if the rules are the same in the USA) down to 4 instead of 8. Won't make a difference on wire size but with a couple less wires in the pipe it may be easier to pull the wire in.


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## joed (Mar 13, 2005)

When running a MWBC the three wires are only counted as two for derating purposes. The neutral is not counted as it only carries the unbalanced current. So three MWBCs only counts as 6 wires for derating but 9 wires for conduit fill calculations.


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## Proby (Jul 17, 2010)

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:thumbup:


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## Ptron (Nov 28, 2008)

darren said:


> Do you plan on running a neutral with each hot, if so there is nothing wrong with this plan.


That was the plan.



> Could you not use multiwire branch circuits and save yourself some wire. This would also reduce your current carrying conductors(if the rules are the same in the USA) down to 4 instead of 8. Won't make a difference on wire size but with a couple less wires in the pipe it may be easier to pull the wire in.


I guess there's no big reason I couldn't do this. I'd have to rearrange stuff in the service panel since one phase (sorry if that's not the right term) is filled up and the the other has all the open spots. But since it's not particularly well load balanced between the two phases as is maybe that's not a bad thing.


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## joed (Mar 13, 2005)

Phases alternate down each side. So unless you have every other space empty you have room on both phases.


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## secutanudu (Mar 15, 2009)

Don't forget to use handle ties or a 2-pole breaker if you plan to do a MWBC.


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## Jim Port (Sep 21, 2007)

There is little point to try to "balance" a panel in a residential setting. The location of the loads shift too much and you would need to realign the breakers at different time period due to changes in use based on time of day.


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## Ptron (Nov 28, 2008)

joed said:


> Phases alternate down each side. So unless you have every other space empty you have room on both phases.


Ah yes, thanks. heh. Otherwise a double pole breaker wouldn't have 240 across it. anyway I'll probably just do the original plan unless I have trouble pulling that many wires through.


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## Ptron (Nov 28, 2008)

I have another question I can't find the exact answer to in my books. In the case of EMT to a junction box and then MC from the junction box to an outlet box, do I still have to pigtail the EGC and attatch it to the outlet box? In other words does the wrap of MC cable count as a connection for the outlet box. [Note that I'm only talking about the ground connection of the box, not the outlet. The EGC would of course get connected directly to the outlet.]

I managed to pull 9 wires through but I went with 3/4 EMT to do it. Probably would have been very hard with 1/2".


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## jbfan (Jul 1, 2004)

Anytime you make a connection inside a box, the box must be grounded.


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## Ptron (Nov 28, 2008)

jbfan said:


> Anytime you make a connection inside a box, the box must be grounded.


Understood, but what I'm wondering is if the connection via the metal jacket of the MC cable is acceptable as the ground connection to the box or if it's mandatory to connect the EGC to the box even though there will also be a ground connection via the jacket. I presume the latter since the jacket can't be used as the EGC but I don't see this spelled out in the books I have.


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## Proby (Jul 17, 2010)

The armor of MC is not considered an EGC. 

The armor of BX is considered an EGC because it has the bonding strip inside.


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## Ptron (Nov 28, 2008)

Proby said:


> The armor of MC is not considered an EGC.
> 
> The armor of BX is considered an EGC because it has the bonding strip inside.


I understand that, which is why I said it's presumably not an acceptable connection for the box. To put it another way I'm wondering if the NEC differentiates between the ground connection required for the box and that for the outlet (I don't know why it would but it seems possible.) Obviously the EGC of the MC goes to the outlet as always.


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## nap (Dec 4, 2007)

Ptron said:


> I understand that, which is why I said it's presumably not an acceptable connection for the box. To put it another way I'm wondering if the NEC differentiates between the ground connection required for the box and that for the outlet (I don't know why it would but it seems possible.) Obviously the EGC of the MC goes to the outlet as always.


not sure what the confusion is. When you bring MC into a metal box, you must bond the egc in the MC to the box. If you have an EGC coming from your feed side, that too must be bonded to the box. If you are using conduit as your EGC, the connector bonds it to the box.



> In other words does the wrap of MC cable count as a connection for the outlet box.


NO. The sheathing on MC cable is not considered to be an EGC.

Now, if you actually have AC cable instead of MC, then the sheath and the bonding wire within the sheath is considered to be an EGC as long as you use listed connectors.


and proby; I sure wish people would use the proper term. BX, as far as I know, was never an actual listed wiring method. Even when it had problems, I believe it was considered to be AC cable. I know what we use today is called AC cable. I believe that leads to a misunderstanding of the two different types of cable that look virtually the same externally (MC, AC)


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