Isolated ground

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splinetto

Senior Member
Location
Missouri
What is the proper way to install an iso ground at a sub panel? Do you isolate the grounds in the sub and then drive a ground rod or do you run a seperate ground wire form the sub to the main panel?
 

walkerj

Senior Member
Location
Baton Rouge
The only way is to have an isolated(from regular EGC's) grounding conductor all the way to the service or SDS.

A Ground rod has nothing to do with an isolated ground.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
The isolated ground wire should be run to the service equipment. It should be kept isolated at the sub panel from the other ground wires.

Chris
 

raider1

Senior Member
Staff member
Location
Logan, Utah
The isolated grounding conductor would be sized based on the overcurrent device for the circuit the isolated ground serves.

Chris
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
What if you're running the IG back to the XO of an transformer. Would it be required to be sized based on the size of the secondary conductors?
 

raider1

Senior Member
Staff member
Location
Logan, Utah
What if you're running the IG back to the XO of an transformer. Would it be required to be sized based on the size of the secondary conductors?

Good question.

IMHO, if the isolated ground is for a 20 amp circuit on the secondary side of a transformer the isolated ground that is run back to the XO at the transformer need not be larger than a #12.

I don't think that the code really address this.

Chris
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
In a nutshell, you wire the IG as if it were another non-bonded neutral conductor, except a single conductor can be used for each pathway, and tied together (i.e., shared, or run as you would run conduit) and need be only as large as the regular EGC does.
 

tryinghard

Senior Member
Location
California
Isolated ground circuits are a poor attempt to eradicate stray current; when installed they compromise the integrity of the equipment ground fault path. If circuitry is installed correctly there is no need to separate these equipment grounds (bonds)!

I say the proper way to install isolated ground circuits is not to use them. Think about how the fault current will travel to its supply if the equipment shorts. If required on your drawings question their need.
 

tryinghard

Senior Member
Location
California
What is the proper way to install an iso ground at a sub panel? Do you isolate the grounds in the sub and then drive a ground rod or do you run a seperate ground wire form the sub to the main panel?

An isolated ground circuit simply includes an additional equipment ground conductor that remains insulated from the other required equipment grounding; it routes to the equipment from the circuit?s particular supply. Again it is additional to the still required equipment grounding of 250.118. It is also the only effective ground fault path for the supplied equipment!

If your circuit originates in a sub panel supplied by a transformer (separately derived system) the isolated grounds would bond with all other equipment grounds at this particular sub and not at any supply ahead of this subpanel, this is in keeping with 250.118 ?The equipment grounding conductor run with the circuit conductors?. If your circuit originates in a subpanel not supplied by a transformer the isolated grounds may remain isolated there connecting to a larger isolated EGC to its supply where all will connect.

The number one reason for stray current is incorrect neutral bonding; if this is qualified as correct there really is no need for isolating equipment grounding. Stray or induced current is not trying to return to earth it is trying to return to its source.
 

iwire

Moderator
Staff member
Location
Massachusetts
What if you're running the IG back to the XO of an transformer. Would it be required to be sized based on the size of the secondary conductors?

IMO no per the wording of Table 250.122.

It most likely will not have to be larger than 12 AWG, not many IG circuits above 20 amps.
 

brian john

Senior Member
Location
Leesburg, VA
Isolated ground circuits are a poor attempt to eradicate stray current; when installed they compromise the integrity of the equipment ground fault path. If circuitry is installed correctly there is no need to separate these equipment grounds (bonds)!

I say the proper way to install isolated ground circuits is not to use them. Think about how the fault current will travel to its supply if the equipment shorts. If required on your drawings question their need.


We think a like, both ski and are in the same profession. We'd get along fine (as long as you learn to say YEP YOU're RIGHT)

But the IG way sells more material good for Kennecott, EC's and workers.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If your circuit originates in a sub panel supplied by a transformer (separately derived system) the isolated grounds would bond with all other equipment grounds at this particular sub and not at any supply ahead of this subpanel, this is in keeping with 250.118 ?The equipment grounding conductor run with the circuit conductors?.
I agree, but wouldn't a transformer-supplied panel technically not be a sub-panel?
 

iwire

Moderator
Staff member
Location
Massachusetts
If your circuit originates in a sub panel supplied by a transformer (separately derived system) the isolated grounds would bond with all other equipment grounds at this particular sub and not at any supply ahead of this subpanel, this is in keeping with 250.118 ?The equipment grounding conductor run with the circuit conductors?.

Change that to might stop at the sub panel but I make up my bonding n the transformer so the IG will go all the way back to XO at the SDS serving that panel(s).

If you install a TVSS the instructions typically call for the IG of that to go to XO as well.



If your circuit originates in a subpanel not supplied by a transformer the isolated grounds may remain isolated there connecting to a larger isolated EGC to its supply where all will connect.

The IG is not required to be increased in size at the sub panel. Typically IG circuits are all 20 amp so the IEGC could remain 12 AWG.



The number one reason for stray current is incorrect neutral bonding; if this is qualified as correct there really is no need for isolating equipment grounding. Stray or induced current is not trying to return to earth it is trying to return to its source.

I agree incorrect neutral bonding is a problem but was not the reasons IGs came to be. If I recall correctly IGs became necessary when the technology and cables connecting data equipment included a ground reference along with the data signals. This would make a loop between equipment.

Modern systems like Ethernet carry no ground references. I believe Gar makes optical solutions for equipment that still use the ground reference.
 

tryinghard

Senior Member
Location
California
Change that to might stop at the sub panel but I make up my bonding n the transformer so the IG will go all the way back to XO at the SDS serving that panel(s).

If you install a TVSS the instructions typically call for the IG of that to go to XO as well.

The IG is not required to be increased in size at the sub panel. Typically IG circuits are all 20 amp so the IEGC could remain 12 AWG.

In my opinion these equipment grounds do not "run with" their circuit conductors causing a violation of 250.118. Also with longer length than its circuit conductors it may not accomodate VD, just as if the circuit conductors ran the whole way with would cause 250.122(B) to be applied.

I agree incorrect neutral bonding is a problem but was not the reasons IGs came to be. If I recall correctly IGs became necessary when the technology and cables connecting data equipment included a ground reference along with the data signals. This would make a loop between equipment.

Modern systems like Ethernet carry no ground references. I believe Gar makes optical solutions for equipment that still use the ground reference.

This might be true and it has cause great mistakes in equipment grounding!
 

iwire

Moderator
Staff member
Location
Massachusetts
In my opinion these equipment grounds do not "run with" their circuit conductors causing a violation of 250.118.

Of course they do, they run with the circuit conductors all the way back to the source just like a normal EG does.:smile:

Also with longer length than its circuit conductors it may not accommodate VD,

No different than a regular EGC.:smile:

just as if the circuit conductors ran the whole way with would cause 250.122(B) to be applied

There is no need to apply 250.122(B) to the portion that runs with the feeder any more then you would with a normal EGC.:smile:
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Bob,
The isolated grounding conductor could be a lot longer than the associated branch circuit conductors in the cases where the system bonding jumper or main bonding jumper is not in the same panel as the branch circuit OCPD. I don't think the code directly addresses the sizing of the isolated grounding conductor in this case.
 
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