Single ground conductor in conduit when fed from multiple NM cables

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It is often stated an Equipment Grounding Conductor (EGC) can be shared
among multiple circuits residing in a conduit or raceway as long as
the EGC is sized to the maximum of all the circuits.

It is also true that over current protection devices depend on low
impedance for proper operation. Low impedance is achieved by
co-locating the conductors (e.g. in the same conduit, raceway, sheath,
etc.).

My question is how does the single EGC allowance in a conduit apply when
portions of the circuits are NOT within the same conduit, sheath,
etc.?

This arises in a fairly common (shop/garage) situation. NM cable runs
to a junction box in the exposed ceiling which drops via EMT conduit
to lower box(es) containing receptacles. The EMT provides physical
protection for the individual THHN conductors running between the upper
and lower box(es).

The single EGC allowance would suggest it's fine to run just one EGC
in the EMT drop. However, because the EMT drop is fed from individual NM
circuits the entire circuit is NOT contained in one conduit, raceway,
sheath, etc.

Does this mean multiple EGC's need to be run through the EMT for each
circuit fed by an individual NM cable instead of just a single EGC connected
to the largest EGC entering the upper box?

P.S.: I do understand the box bonding must be done with the largest
EGC in the conduit).

Pointers the NEC sections would be appreciated (I've read 250.135(b)
but I'm not sure it applies to this situation).
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Long ago when the moon was new we were allowed to run a single EG to a bonded water pipe to ground a replacement for a 2 wire device. IDR when that exception or practice was no longer allowed. Your installations may have been compliant if they are old enough.

Another look at your post.

You have multiple NM circuits running to one j-box. Tie all EGs together, bond the box with the largest EG used and run 1 EG down each conduit sleeve. Doesn't matter if you have one or four branch circuits in it.

Still not sure if that fits your description, draw a picture or take one.
 
Here 2 diagrams

Here 2 diagrams

Here are 2 simple diagrams, only the grounding conductors are shown. In the first all EGC's are tied together in the top box where the NM cable enters and a single 10 AWG ECG travels the conduit drop to the boxes. In the second diagram 2 ECG's travel the conduit drop to the boxes, each individually connected to their respective circuits. In both cases the box bonding is done with the 10 AWG. Which is correct? I understood the first option with all the ground wires in the top box tied together with a single EGC that services both circuits in the conduit drop is correct, but I'm not sure. Also omitted for clarity are the pigtail bonding jumpers.
 

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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Long ago when the moon was new we were allowed to run a single EG to a bonded water pipe to ground a replacement for a 2 wire device. IDR when that exception or practice was no longer allowed. Your installations may have been compliant if they are old enough.

Another look at your post.

You have multiple NM circuits running to one j-box. Tie all EGs together, bond the box with the largest EG used and run 1 EG down each conduit sleeve. Doesn't matter if you have one or four branch circuits in it.

Still not sure if that fits your description, draw a picture or take one.

You do not even need an EGC to go down the EMT. The EMT can serve as the EGC.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
.... Which is correct? I understood the first option with all the ground wires in the top box tied together with a single EGC that services both circuits in the conduit drop is correct, but I'm not sure....
I agree the first option as correct. One equipment ground wire is all that is needed. Alternately, you could just use the EMT as an equipment grounding conductor and eliminate any worry.
 
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