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Old 08-05-2010, 01:56 PM   #1
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Ground wire when multiple branches run through same conduit


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.

When multiple cirucuits travel a ways through the same conduit should each branch have it's own ground wire or can they share a ground wire for that portion that they run together? (yes I'm talking about ground not neutral)

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?

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.
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Old 08-05-2010, 02:24 PM   #2
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Ground wire when multiple branches run through same conduit


I can answer a couple parts of this at least:

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Originally Posted by Ptron View Post
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.

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Originally Posted by Ptron View Post
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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Old 08-05-2010, 02:51 PM   #3
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Ground wire when multiple branches run through same conduit


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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Old 08-05-2010, 03:23 PM   #4
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Ground wire when multiple branches run through same conduit


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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Old 08-05-2010, 03:27 PM   #5
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Ground wire when multiple branches run through same conduit


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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Old 08-05-2010, 06:17 PM   #6
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Ground wire when multiple branches run through same conduit


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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Old 08-05-2010, 08:19 PM   #7
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Ground wire when multiple branches run through same conduit


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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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Old 08-05-2010, 09:22 PM   #8
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Ground wire when multiple branches run through same conduit


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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Old 08-05-2010, 10:30 PM   #9
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Ground wire when multiple branches run through same conduit


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Joed is this the american rule because in Canada it is more then 3 current carrying conductors that you have to start to derate.
Quote:
Originally Posted by joed View Post
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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Old 08-05-2010, 11:21 PM   #10
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Ground wire when multiple branches run through same conduit


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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Old 08-06-2010, 05:13 AM   #11
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Ground wire when multiple branches run through same conduit


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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Old 08-06-2010, 07:00 AM   #12
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Ground wire when multiple branches run through same conduit


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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Old 08-06-2010, 07:23 AM   #13
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Ground wire when multiple branches run through same conduit


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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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Old 08-06-2010, 08:09 AM   #14
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Ground wire when multiple branches run through same conduit


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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Old 08-06-2010, 08:33 AM   #15
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