Sizing Grounding Electrode Conductor?

[The Iceman]

I just finished Mike Holt's video, Grounding versus Bonding, for the second time. I'm still not sure about the 12 1/2% rule. If I'm sizing the GEC for a metal water pipe, or the grounded neutral conductor from the service to the meter enclosure, and I'm over 1100 Kcmil copper, how does the 12 1/2% rule work? Actually table 250.66 has over 1100Kcmil. Now I'm really confused. When do I use the 12 1/2% rule? And how does it work?

 

[raider1]

The 12 1/2% rule is for sizing the main bonding, system bonding jumper,and equipment bonding jumpers on the line side of the service ect..

When sizing the GEC you use Table 250.66 and the largest GEC that would be required by that table is a 3/0 copper or 250 kcmil aluminum.

Take a look at 250.28(D) for sizing the main bonding jumper, 250.30(A)(1) for system bonding jumpers, 250.102(C) for equipment bonding jumpers on the supply side of a service and for EBJ's for SDS's.

Chris

 

[Dennis Alwon]

Iceman are you referring to 12.5% in art. 250.24(C)(1) for grounded conductors???

 

[The Iceman]

I think I got it now. 12 1/2% rule doesn't apply to the GEC. It applies to the grounded neutral conductor over 1100Kcmil. So if your phase conductors in a parallel service adds up to over 1100Kcmils, then your grounded neutral can't be sized smaller than 12 1/2% of the area of the largest service-entrance phase conductors?

 

[raider1]

I think I got it now. 12 1/2% rule doesn't apply to the GEC. It applies to the grounded neutral conductor over 1100Kcmil. So if your phase conductors in a parallel service adds up to over 1100Kcmils, then your grounded neutral can't be sized smaller than 12 1/2% of the area of the largest service-entrance phase conductors?

Correct, in addition to the size of the grounded neutral conductor also look at the sections that I referenced to the main bonding jumper, system bonding jumper and equipment bonding jumpers, as they also need to be sized at 12 1/2% if the conductors they are sized from exceed 1100 kcmil copper.

Chris

 

[The Iceman]

OK. I'm getting closer :smile: . Now If have 3 sets parallel 600 Kcmil copper. I take one phase conductor from each run and mult. by 600? Example: 3 X 600 = 1800 Kcmil. Then the GEC would be 3/0 copper. The grounded neutral conductor can't be smaller than 12 1/2% of what? Is it the 600Kcmil or the 1800Kcmil?

 

[Dennis Alwon]

OK. I'm getting closer :smile: . Now If have 3 sets parallel 600 Kcmil copper. I take one phase conductor from each run and mult. by 600? Example: 3 X 600 = 1800 Kcmil. Then the GEC would be 3/0 copper. The grounded neutral conductor can't be smaller than 12 1/2% of what? Is it the 600Kcmil or the 1800Kcmil?

Iceman. 250.24(C)(1) says 12.5% of the largest entrance conductor. I take that to mean the 600KCM wire not 1800kcm.

 

[Cow]

Iceman. 250.24(C)(1) says 12.5% of the largest entrance conductor. I take that to mean the 600KCM wire not 1800kcm.

250.24(C)(2) says otherwise.

 

[Dennis Alwon]

250.24(C)(2) says otherwise.

Ah yes, keep reading I should. Another cuppa for me.

 

[The Iceman]

Iceman. 250.24(C)(1) says 12.5% of the largest entrance conductor. I take that to mean the 600KCM wire not 1800kcm.

I'm not sure but it looks like cow is right. 250.24 (B) (2) the size of the grounded conductor shall be based on the total circular mil area of the parrallel conductors. But then it goes on and confuses me even more. I do understand the GEC conductor but the grounded neutral conductor 12 1/2% rule with parrallel conductors I'm not sure about.

 

[Dennis Alwon]

I'm not sure but it looks like cow is right. 250.24 (B) (2) the size of the grounded conductor shall be based on the total circular mil area of the parrallel conductors. But then it goes on and confuses me even more. I do understand the GEC conductor but the grounded neutral conductor 12 1/2% rule with parrallel conductors I'm not sure about.

Yes, I agree-- my post above yours admits that--- Iat least it was suppose to.

 

[The Iceman]

Cool. Now one last question and I think I this. 250.24 (B) (2) Do we size the grounded neutral by adding up the cir. mil. of one ungrounded conductor in each parrallel run? Example: 3 sets of 600Kcmil 3 X 600 = 1800 cir. mil. and take that and multiply by 12.5% to get the neutral size?

 

[Dennis Alwon]

Cool. Now one last question and I think I this. 250.24 (B) (2) Do we size the grounded neutral by adding up the cir. mil. of one ungrounded conductor in each parrallel run? Example: 3 sets of 600Kcmil 3 X 600 = 1800 cir. mil. and take that and multiply by 12.5% to get the neutral size?

No. Since 600 KCM is equal to 600,000 cir. mil then you must multiply 3 x 600,000= 1,800,000

 

[The Iceman]

My bad. So do you mult. 1,800,000 by 12.5% to get the minimum size grounded neutral conductor?

 

[Dennis Alwon]

My bad. So do you mult. 1,800,000 by 12.5% to get the minimum size grounded neutral conductor?

Yep. and you get 225,000 which means a 250mcm is the minimum

 

[The Iceman]

Got It! Thanks for the help.