Grounding electrode conductor size

ktm400sx

Member
I have been asked to check a ground electrode conductor size. From the utility, there are parallel 3/0 conductors to the MCC. (Two 3/0 per phase, Two parallel conduits, each with the 3 phases and a #4 ground)
The two #4 grounds from the utility terminate on the MCC ground buss. So far so good. Now, the grounding electrode conductor from the MCC to the ground electrode (the customers ground electrode) is one #2 conductor. From table 250.66 it appears that the grounding electrode conductor should be two #4's. Am I right?

Thanks for any help
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
You do not parallel the grounding electrode conductor. Note 1 states to add the cir. mil of the 3/0 conductors. Table 8 Chapter 9 shows the cir mil for 3/0 at 167,800. Multiple by 2 for the parallel run and you get 335,600-- closest size to that is 350 kcm. Thus you need a 1/0 copper grounding electrode conductor
 

ktm400sx

Member
Just read it again. ..if I understand it right, it means the ground electrode conductor is sized for the sum of the two 3/0 service conductors...but I'm not sure.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I'm a little confused. The OP states there is a #4 "ground" from the POCO. There is no ""ground" from a utility. Is the OP referring to a grounded conductor? If so, parallel 3/0 would require a minimum (assuming there is no neutral load) of #2 copper to serve as the grounded conductor in each raceway be brought to the disconnecting means. As others have stated, the GEC size would depend on the GE type. Am I missing something here?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I'm a little confused. The OP states there is a #4 "ground" from the POCO. There is no ""ground" from a utility. Is the OP referring to a grounded conductor? If so, parallel 3/0 would require a minimum (assuming there is no neutral load) of #2 copper to serve as the grounded conductor in each raceway be brought to the disconnecting means. As others have stated, the GEC size would depend on the GE type. Am I missing something here?
I think it must be an undersized neutral that is paralleled with #4. Of course that is a NO NO..... You are right that the whole setup sounds wrong or is definitely questionable.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
The grounded conductor is required and must be sized according to the conductors in each parallel raceway according to table 250.66. {250.24(C) & 250.24(C)(2)} and must be a minimum of #1/0.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
The grounded conductor is required and must be sized according to the conductors in each parallel raceway according to table 250.66. {250.24(C) & 250.24(C)(2)} and must be a minimum of #1/0.
That was my point. But I'm not sure of your math. 2, 3/0=335,600. That would be a #2 CU.
 

Smart $

Esteemed Member
Location
Ohio
Can't parallel a conductor smaller than 1/0 [310.10(H)].
Getting back to the OP, this parallel run as described would be service conductors. Conduit typically indicates a service lateral. There may not be service entrance conductors. That posits the question where is the service point? The lateral could be under utility control and NEC rules would not apply.
 

Smart $

Esteemed Member
Location
Ohio
I have been asked to check a ground electrode conductor size. From the utility, there are parallel 3/0 conductors to the MCC. (Two 3/0 per phase, Two parallel conduits, each with the 3 phases and a #4 ground)
The two #4 grounds from the utility terminate on the MCC ground buss. So far so good. Now, the grounding electrode conductor from the MCC to the ground electrode (the customers ground electrode) is one #2 conductor. From table 250.66 it appears that the grounding electrode conductor should be two #4's. Am I right?

Thanks for any help
I'm a little confused. The OP states there is a #4 "ground" from the POCO. There is no ""ground" from a utility. Is the OP referring to a grounded conductor? If so, parallel 3/0 would require a minimum (assuming there is no neutral load) of #2 copper to serve as the grounded conductor in each raceway be brought to the disconnecting means. ..
I think it must be an undersized neutral that is paralleled with #4. Of course that is a NO NO..... You are right that the whole setup sounds wrong or is definitely questionable.
The grounded conductor is required and must be sized according to the conductors in each parallel raceway according to table 250.66. {250.24(C) & 250.24(C)(2)} and must be a minimum of #1/0.
As a grounded conductor, still have to question the setup, as the OP says the #4's land on the MCC ground bus. Of course the OPer could be mistaken by thinking it is a grounding conductor because that is also where the GEC terminates... but instead lands on a neutral bus (should be mounted with stand-off insulators and have a main bonding jumper). Could also be an MCC designed for 3? 3-wire (non-compliant).
 
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