Sizing a GEC to a ground rod for a generator

[cohosilver]

I have a question going on here that I could use some input on. If there is a generator with a 1200A MDP and there are (3) 4x500mcm ER-TC cables to a 120OA MDP and there is an MBJ at the MDP, what size GEC do I need for the generator and for the MDP when the only grounding electrodes available are ground rods?

It has always been my contention that the GEC to ground rods, when the GEC is only going to the ground rods is 6 AWG. Everywhere I looked, specifically under 250.30, I was refered back to 250.66 and I do not see where 250.66(A) would not apply.

The only available electrodes will be ground rods, it is in the middle of the desert.

 

[petersonra]

I agree that max required size is #6.

 

[jtester]

I agree that the maximum size to a driven ground rod is #6, but if the GEC is to be sized per 250.66, that doesn't seem to comply. What prevents you from installing a 3/0 ground ring? Then you comply with everything.

Jim T

 

[petersonra]

250.66A is part of 250.66 so you are complying with 250.66 by taking advantage of 250.66A.

 

[augie47]

side question:
Do your TC cables include a EGC ?

 

[cohosilver]

Excellent question and thanks for asking. They in fact do contain an EGC, but that EGC inside of the cables is too small to comply with T250.122 based on the OCD protecting the cables (1200A) so additional EGC conductors needed to be run with with the cables and tie-wrapped to each cable so that it was clear which cable went with each EGC.

Did it seem that everyone agreed with 6 AWG being the largest sized EGC that needed to be run to the ground rods since that was the only EGC being abailable (no building steel or water pipe)?

I liked the comment about the ground ring, in some of these installations that may be a good solution, in others however, it just is not practical.

Respectfully,

 

[infinity]

Did it seem that everyone agreed with 6 AWG being the largest sized EGC that needed to be run to the ground rods since that was the only EGC being abailable (no building steel or water pipe)?

I liked the comment about the ground ring, in some of these installations that may be a good solution, in others however, it just is not practical.

Respectfully,

I agree, #6 is the largest GEC required to the rod(s).

 

[augie47]

I concur, #6

 

[Dennis Alwon]

Did it seem that everyone agreed with 6 AWG being the largest sized EGC that needed to be run to the ground rods since that was the only EGC being abailable (no building steel or water pipe)?

Respectfully,

I don't see how 250.66(A) can be any clearer. #6 is all that is needed to the rod. BTW I believe you mean GEC not EGC. So easy to do.

 

[petersonra]

Excellent question and thanks for asking. They in fact do contain an EGC, but that EGC inside of the cables is too small to comply with T250.122 based on the OCD protecting the cables (1200A) so additional EGC conductors needed to be run with with the cables and tie-wrapped to each cable so that it was clear which cable went with each EGC.

I am not sure that is code.

 

[jtester]

250.66A is part of 250.66 so you are complying with 250.66 by taking advantage of 250.66A.

I've always had trouble understanding why ground rods, alone, are sufficient. I must be overlooking a code section that says you can use rods only.

Jim T

 

[RUWired]

I am not sure that is code.

It's not and article 250.122(F) would support that statement.

There also seems to be a problem with the ampacity of three runs of 500 kcmil on a 1200 amp breaker. 240.4(C) requires equal or greater when over 800 amps.

Rick

 

[RUWired]

I've always had trouble understanding why ground rods, alone, are sufficient. I must be overlooking a code section that says you can use rods only.

Jim T

Article 250.50 allows the use of rods (and other electrodes)when all the other electrodes are not present.

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.

Rick

 

[jtester]

Article 250.50 allows the use of rods (and other electrodes)when all the other electrodes are not present.

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.

Rick

I know I am beating a dead horse, but it seems to me that if I have a requirement to run a 1/0 GEC, and there is an electrode, (A)(4) that will accept it, how do I get by by choosing one that only accepts #6?

Jim

 

[iwire]

I know I am beating a dead horse, but it seems to me that if I have a requirement to run a 1/0 GEC, and there is an electrode, (A)(4) that will accept it, how do I get by by choosing one that only accepts #6?

Because there is nothing that says you have to do that.

 

[jtester]

Because there is nothing that says you have to do that.

I understand, but it seems grey to me. I practice in New Mexico, and the New Mexico Electrical Code seems to have addressed it somewhat.

"Article 250 ? Grounding and bonding. See this article of the NEC except as provided below.
(1) Section 250.50 Grounding electrode system. See this section of the NEC and add: on new construction a concrete encased electrode shall be considered available and installed in compliance with NEC 250.52(A) (3). If a concrete encased electrode is not present, then at least 20 feet of No. 2 bare copper in direct contact with the earth at a depth below the earth?s surface of not less than thirty (30) inches shall be installed in a continuous trench that is at least twenty (20) feet in length, augmented with a minimum of two (2), eight (8) foot grounds rods spaced a minimum of six (6) feet apart."

I still often see only two driven ground rods.

Jim T

 

[edavis2293]

3 500 kcmil and generator

3 500 kcmil and generator

Three (3)500 kcmil conductors are permitted to supply a 1200A OCP in the 2011 NEC but not in the 2008

Grounding electrode conductors or grounding electrodes are not required if the generator meets the requirement for a separately derived system.

 

[RUWired]

Three (3)500 kcmil conductors are permitted to supply a 1200A OCP in the 2011 NEC but not in the 2008

What article changed to allow this?

Grounding electrode conductors or grounding electrodes are not required if the generator meets the requirement for a separately derived system.

Did you look through 250.30(A)4,5 and 7 along with the exceptions. IMO they are required.

Rick

 

[edavis2293]

What article changed to allow this?




Did you look through 250.30(A)4,5 and 7 along with the exceptions. IMO they are required.

Rick

Section 240.91(B) 2011 NEC

 

[RUWired]

Section 240.91(B) 2011 NEC

That article is limited in application and not for general use.

what about the SDS requirement?
Rick