GEC sizing question

nizak

Senior Member
I am going to be installing a 400A(320 continuous) single phase meter base on an upcoming single family dwelling job. I will have 2/0 conductors supplying a 200A panel in the house and #3 conductors supplying a separate 100A building located on the property. If the only GE that is present on site is a concrete encased electrode attached to the footing rebar, is #4 CU the largest size conductor I would be required to install? If a metal underground water pipe was present, would #2CU then have to be installed? Also, is a driven ground required at the separate building? It will be a 3 wire feed from the meter base to the building. Thanks.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I am going to be installing a 400A(320 continuous) single phase meter base on an upcoming single family dwelling job. I will have 2/0 conductors supplying a 200A panel in the house and #3 conductors supplying a separate 100A building located on the property. If the only GE that is present on site is a concrete encased electrode attached to the footing rebar, is #4 CU the largest size conductor I would be required to install? If a metal underground water pipe was present, would #2CU then have to be installed? Also, is a driven ground required at the separate building? It will be a 3 wire feed from the meter base to the building. Thanks.
A separate electrode is definitely needed at the separate building. If you do not have a CEE at that building, a driven ground rod (or rods) will be the simplest way to comply.

Note that if the separate building has rebar in its footings which qualify as a CEE but there is no point at which to connect to it, a strict AHJ may require that you do whatever is necessary to attach to the rebar, including chipping out concrete.
 

jumper

Senior Member
Note that if the separate building has rebar in its footings which qualify as a CEE but there is no point at which to connect to it, a strict AHJ may require that you do whatever is necessary to attach to the rebar, including chipping out concrete.
Unless inspector has an amendment, that would be hard to enforce.

250.50 Grounding Electrode System. All grounding electrodes
as described in 250.52(A)(1) through (A)(7) that are
present at each building or structure served shall be bonded
together to form the grounding electrode system. Where
none of these grounding electrodes exist, one or more of
the grounding electrodes specified in 250.52(A)(4) through
(A)(8) shall be installed and used.

Exception: Concrete-encased electrodes of existing buildings
or structures shall not be required to be part of the
grounding electrode system where the steel reinforcing bars
or rods are not accessible for use without disturbing the
concrete.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
You would need to calculate the electrode to the water pipe based on 250.66 if you needed one. IMO, even though 100 amps is going to the other building the entire service size conductors must be used to calculate the grounding electrode conductor. Look at Note 1 to T. 250.66.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Unless inspector has an amendment, that would be hard to enforce.
It is not clear in the OP's post whether the 100A building is an existing building or a new building.
If existing then the exception does save the OP.
But existing for how long?
From the time the slab was poured to the time the first electrical service was installed even if that is just a few days?
Never occupied between the time the slab was poured and the first electrical service was installed?
Is the OP replacing an existing service or supplying the first service to the building?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
You would need to calculate the electrode to the water pipe based on 250.66 if you needed one. IMO, even though 100 amps is going to the other building the entire service size conductors must be used to calculate the grounding electrode conductor. Look at Note 1 to T. 250.66.
No, the GEC would be sized for the service conductors supplying that disconnect. See 250.64 (D)(2).
While what the OP proposes is NEC compliant he would be advised to make sure the POCO will allow it as some require service disconnects to be adjacent to the meter. Then he would need to run a 4 wire feeder to the garage.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
No, the GEC would be sized for the service conductors supplying that disconnect. See 250.64 (D)(2).
While what the OP proposes is NEC compliant he would be advised to make sure the POCO will allow it as some require service disconnects to be adjacent to the meter. Then he would need to run a 4 wire feeder to the garage.

NO? I think it depends on where the grounding electrode conductor is run to. Around here we go to the meter so IMO that serves both buildings and my answer, I believe is correct. I agree if you go to the disconnect that would be different. Now if there are two disconnects at the main building one which controls the second building then I would not go to each disconnect. If there is one disconnect at the first building and one at the second building then I believe you are correct if you go to the disconnects
 

jumper

Senior Member
NO? I think it depends on where the grounding electrode conductor is run to. Around here we go to the meter so IMO that serves both buildings and my answer, I believe is correct. I agree if you go to the disconnect that would be different. Now if there are two disconnects at the main building one which controls the second building then I would not go to each disconnect. If there is one disconnect at the first building and one at the second building then I believe you are correct if you go to the disconnects
Dennis, I agree with all, but given that the OP said a 3W service to the other building I was going for the section I bolded.
 
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