GEC - Transformer - separetly derived AC Service

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Eng

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I have a project where we show a 225KVA 480Vpri/120/208 3p 4w sec transformer on the 4th floor penthouse. I have a note on the documents stating to ground the transformer per the NEC and coordinate with the local AHJ.

The AHJ will not make the call as to how to ground the transformer. The building is concrete, no structural steel, no water piping nearby on the 4th floor.

What are my options to ground this transformer? I do not see any economical options here other than providing a GEC back to the main building service GEC.
 

iwire

Moderator
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Location
Massachusetts
You provide a separate GEC all the way back to the service, we run into this all time in the brick and wood mill buildings in this area.

See 250.30(A)(3) or (3)
 

infinity

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Staff member
Location
New Jersey
Occupation
Journeyman Electrician
You provide a separate GEC all the way back to the service, we run into this all time in the brick and wood mill buildings in this area.

See 250.30(A)(3) or (3)


We have the same issue with poured concrete buildings. We run a #3/0 back to the service, install a grounding plate and make all of the connections there.
 

Eng

Member
We have the same issue with poured concrete buildings. We run a #3/0 back to the service, install a grounding plate and make all of the connections there.

I proposed this solution. The contractor fired back and wants to provide a GEC down a concrete shaft, pound a 10' ground rod and cover it with concrete. Is this code compliant?

I am looking for a code reference that requires the entire GES to be bonded together.

Assistance is appreciated.
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
A ground rod that isn't below grade???? I don't think that will work. I believe Bob gave the reference.
 
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iwire

Moderator
Staff member
Location
Massachusetts
250.58 Common Grounding Electrode. Where an ac system
is connected to a grounding electrode in or at a building
or structure, the same electrode shall be used to ground
conductor enclosures and equipment in or on that building
or structure.
Where separate services, feeders, or branch
circuits supply a building and are required to be connected
to a grounding electrode(s), the same grounding electrode(
s) shall be used.
Two or more grounding electrodes that are bonded together
shall be considered as a single grounding electrode
system in this sense.
 

Eng

Member
250.58 Common Grounding Electrode. Where an ac system
is connected to a grounding electrode in or at a building
or structure, the same electrode shall be used to ground
conductor enclosures and equipment in or on that building
or structure. Where separate services, feeders, or branch
circuits supply a building and are required to be connected
to a grounding electrode(s), the same grounding electrode(
s) shall be used.
Two or more grounding electrodes that are bonded together
shall be considered as a single grounding electrode
system in this sense.

Great, thanks
 

Eng

Member
What about 250.50 exception? this is an existing building. Is what the contractor proposing OK if he eliminates the ground rod and connects the GEC to the concrete encased electrode?

The concrete is in direct connection with earth.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
What about 250.50 exception? this is an existing building. Is what the contractor proposing OK if he eliminates the ground rod and connects the GEC to the concrete encased electrode?

The concrete is in direct connection with earth.

That exception states that you don't have to dig up the footings to use the rebar as an electrode for existing buildings. Not sure how that would apply.
 

Eng

Member
New information on this situation.

The contractor found out that the GE is located at the site located pad mounted utility transformer. The code does not clarify between where the GE is located, only that the GEC's must all be bonded together to create the GES.

Are we required to install this penthouse GEC back to the site located pad mounted utility transformer?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The code says we can't use the primary EGC as the grounding electrode conductor for the secondary, but can we use the primary EGC as the bonding conductor to bond the two electrodes together if the driven rod is used as the secondary grounding electrode?

This is a perfect example of a case where we should be able to upsize the primary EGC and use it as both the EGC and GEC. There is no technical reason why we can't....just a code rule that says we can't because the do different things.
 
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