Separate Grounding Conductor

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darren71

Member
Hi all. I have a question about a grounding conductor to a subpanel. I have a project where I want to replace an existing 800 amp 480/277V subpanel in a building that is currently fed from the building service via (2) 4" buried conduits with (4) 750 MCM in each. I cannot see where any ground conductor has been installed in the raceways. Question is, how do I get a ground conductor to the subpanel, as the existing feeder is 300-400 feet long approximately, routes under a wing of the building, and has no pull boxes? Can I provide a ground via another route from the main to the subpanel, or will I cause an issue because the ground is not with the parallel feeder? I believe I could physically route a ground in a conduit from the service to the subpanel through the building. Thanks
 

KevinVost

Senior Member
Location
Las Vegas
What are the buried conduits? PVC, RMC? If the conduit in the ground meets the requirements of 250.118 then you don't need to run a ground wire. From the sound of the install, you should have metal conduit or you would have a ground wire installed already.
 

A/A Fuel GTX

Senior Member
Location
WI & AZ
Occupation
Electrician
What do you have for the feeder conduits? If they meet 250.118, you already have an equipment grounding conductor.
 

darren71

Member
Sorry, I left that out. The conduit is PVC unfortunately. I think the subpanel was originally supposed to be a 2nd service, but they changed their mind when the addition it feeds was built.
 

KevinVost

Senior Member
Location
Las Vegas
You will have to install a ground somehow, that ties back to the original service. Can you get a ground from the original feed panel to the new location through the building? How is the panel getting its ground now? If it is rebonded to the grounded conductor I would think that you may have a case of objectional current in the building (this was a recent topic you can look up). That situation would be very dangerous.
 

darren71

Member
I think I can route a conduit and separate ground conductor from the main to the subpanel through the building. Is this accepted by code, even though the grounding conductor is not with the circuit conductors, and does not even follow the same route as the ground conductors?
 

darren71

Member
Well, 250.134 says to ground the equipment by means of 250.134 A) OR B). If "B" is not available due to existing conditions, am I not permitted to go to "A", by running a separate ground, one that does not have to be with the circuit conductors? Currently, the subpanel is bonded to the building steel, I'm not sure if the neutral is bonded to the ground in the panel.
 

KevinVost

Senior Member
Location
Las Vegas
IMO 250.134 (A) refers to the types of EGC avalible (250.118), but then 250.134 (B)requires the EGC (if a wire) to be routed with the circuit conductors. The 2005 hand book has a good explanation of the requirement for routing the EGC with the circuit conductors, in order to produce a low impedance path and help cancel out the magnetic flux in the curcuit conductors during a fault, which keeps the time of operation of the OCD to a minimum.

That being said, if the panel is bonded to the building steel and the steel is part of the grounding electrode system you have achieved the same ends, just not to code. I would pose this question to the AHJ and get their opinion on the installation.

As far as bonding the grounding and grounded conductor, the only place for this is at the main service disconnect.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
You'll need to install the EGC's within the conduits containing the feeder conductors. Or maybe you can use the four existing conductors and make them A,B,C and EGC. Now you can feed Delta/Wye transformer and develop a neutral at the subpanel. The one potential hitch in that plan is re-identifying the neutral if the conductor is white or gray and not taped.
 
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