“Dedicated ground” vs “isolated ground”

brantmacga

Señor Member
Location
Georgia
Occupation
Electrical Monke
Have a project where the plans called for “dedicated ground” on a few circuits. Each circuit itself is dedicated, using MC cable back to the panel. As in, there is either duplex or quad in the box, and single run of 12/2 MC back to the panel. Metallic boxes, but 100% wood framing. Original drawings did show steel trusses and demising walls but changed to wood due to steel lead times.

This tenant space has Panel A, which is a 225/3 sub-panel with 4-wire feeder off the exterior disconnect, and panel A2 which is a 100/3 fed from Panel A.

I reviewed the drawings when asked by the job foreman and saw there was no isolated ground bar specified in either panel, so to me, a dedicated circuit that doesn’t share a ground with other circuits is indeed a dedicated ground. There is no grounding path between any other circuit and the “dedicated ground” circuit other than they share the same grounding bar.

In my opinion the engineer should have specified “isolated ground”, listed an IG ground bar in each panel as both have “dedicated ground” circuits, and required a 2nd EGC in the feeder if they indeed wanted an isolated ground.

What do you think? Lesson for me, or lesson for the engineer?


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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Have a project where the plans called for “dedicated ground” on a few circuits. Each circuit itself is dedicated, using MC cable back to the panel. As in, there is either duplex or quad in the box, and single run of 12/2 MC back to the panel.

I reviewed the drawings when asked by the job foreman and saw there was no isolated ground bar specified in either panel, so to me, a dedicated circuit that doesn’t share a ground with other circuits is indeed a dedicated ground. There is no grounding path between any other circuit and the “dedicated ground” circuit other than they share the same grounding bar.
That describes a "dedicated ground" installation, which means no shared EGCs or conduit.

In my opinion the engineer should have specified “isolated ground”, listed an IG ground bar in each panel as both have “dedicated ground” circuits, and required a 2nd EGC in the feeder if they indeed wanted an isolated ground.
That describes an “isolated ground” installation, which is different from a dedicated ground.



You need to know what is wanted to know what is correct.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
The engineer didn't want an IG system but he does want a separate EGC (waste of money) for each circuit. The two things are not the same.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
On an isolated ground with wood framing and metal boxes like you described can you use regular receptacles or you still have to use IG receptacles?
You have two separate ground wires? One for box, one for receptacle?
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Engineering
I reviewed the drawings when asked by the job foreman and saw there was no isolated ground bar specified in either panel, so to me, a dedicated circuit that doesn’t share a ground with other circuits is indeed a dedicated ground. There is no grounding path between any other circuit and the “dedicated ground” circuit other than they share the same grounding bar.

In my opinion the engineer should have specified “isolated ground”, listed an IG ground bar in each panel as both have “dedicated ground” circuits, and required a 2nd EGC in the feeder if they indeed wanted an isolated ground.

What do you think? Lesson for me, or lesson for the engineer?


Sent from my iPhone using Tapatalk
You are correct.
 

brantmacga

Señor Member
Location
Georgia
Occupation
Electrical Monke
On an isolated ground with wood framing and metal boxes like you described can you use regular receptacles or you still have to use IG receptacles?
You have two separate ground wires? One for box, one for receptacle?


In an isolated ground circuit you would have two EGC’s; one lands to the box, the other to the device.

I directed the guys to use 12/2 MC and standard grounding receptacles.


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infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
In an isolated ground circuit you would have two EGC’s; one lands to the box, the other to the device.

I directed the guys to use 12/2 MC and standard grounding receptacles
I agree with your method of using a separate MC cable for each dedicated circuit. That satisfies the dedicated EGC requirement. As you've mentioned if an IG system were required you would have two EGC's in the cable.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
In an isolated ground circuit you would have two EGC’s; one lands to the box, the other to the device.

I directed the guys to use 12/2 MC and standard grounding receptacles.


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I’ve never worked with IG, so I’m simply asking…

If you use standard grounding receptacles rather than IG receptacles and the box isn’t tied to metal framing what’s the use of the second EGC?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I’ve never worked with IG, so I’m simply asking…

If you use standard grounding receptacles rather than IG receptacles and the box isn’t tied to metal framing what’s the use of the second EGC?
Has no real use if you do not use an IG receptacle.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
A metal box should be directly bonded to an EGC, regardless of the device's configuration.

A truly isolated box and receptacle can be an IG installation without an isolated, second EGC.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
The document at the link below for Starline plug-in busways says they have three versions:

1. Housing or case ground: The busway housing is the only conductor for the ground path.
2. Dedicated ground: A dedicated bus bar is provided for ground and it's bonded to each enclosure for plug-in units.
3. Isolated ground: Provides a bus bar for ground that is not bonded to the enclosures or raceway. Orange receptacles in the plug-ins have ground terminals isolated from the receptacle mountings.

And so in this interpretation, you'd have a dedicated ground if you ran a ground wire thorough metallic conduit and bonded it to metal boxes, etc. as the NEC requires.

https://www.starlinepower.com/wp-content/uploads/2015/11/Tech-Brief-DG-vs-IG.pdf
 
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