GEC Sizing & what is the GEC

[Ron Poehlmann]

My question has to do with the sizing of the grounding electrode conductor, and what is the grounding electrode conductor in my situation. There may be other items spur off from this question.

The service is 4,000 Amp, 208V, 3 Phase using 11 - 500 kcmil CU per phase.
I called for three electrodes from 250.52(A): 1) Water pipe, 2) a concreete encased electrode of 4 AWG Cu, and 3) a counterpoise electrode consisting of 3 10', 3/4", driven ground rods in a triangular arrangement.

If I require all of these to be tied directly back to the neutral bus, then what is the GEC that needs to be sized per table 250.66?
From 250.66(A) the electrode conductor from the ground rods to the busbar are not requied to be large than #6 AWG Cu; 250.66(B) only requies a #4 Cu from the concrete encased electrode. Is there any place that the code specifies the size of the electrode conductor to the water pipe? The size of these conductors just don't seem to be substantial enough for a system of this size.

I believe the bonding jumper between the ground bus and the neutral bus will be required to be sized per 250.66.

I have read many useful comments on different threads, but have not seen my specific questions answered. I would appreciate all the help I can get.

 

[charlie]

. . . what is the GEC that needs to be sized per table 250.66? . . Is there any place that the code specifies the size of the electrode conductor to the water pipe? . . I believe the bonding jumper between the ground bus and the neutral bus will be required to be sized per 250.66. . .

All of them are GECs (grounding electrode conductors) and all of them are to be sized in accordance with 250.66. The GEC to the ground rods and to the concrete encased electrode are both for lightning, contact with higher voltage lines, and to stabilize the voltage to earth so the 6 AWG and 4 AWG GECs are sized correctly for the amount of expected current. The water piping system is bonded within 5' of where it comes into the building in accordance with 250.50 which tells you to go to 250.52.

The MBJ (main bonding jumper) is a different animal since it will actually carry the fault current back to the source under fault conditions. 250.28 gives the requirements for the MBJ and 250.28(D) gives you the sizing requirements. The minimum size is 687.5 kcmil based on these requirements.

 

[infinity]

If I require all of these to be tied directly back to the neutral bus, then what is the GEC that needs to be sized per table 250.66?

The largest copper conductor in table 250.66 is a #3/0 for service conductors above 1100 kcmil. That would be the size of the GEC to the water pipe. As Charlie has calculated the main bonding jumper is sized according to 12.5% of the size of the service entrance conductors. Also you could run the #3/0 to the water pipe (GEC) and tap off to the other electrodes (bonding jumpers).

Although it won't hurt to use it the "counterpoise electrode consisting of 3 10', 3/4", driven ground rods in a triangular arrangement" is not required.

 

[charlie]

250.4(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
FPN: An important consideration for limiting the imposed voltage is the routing of bonding and grounding conductors so that they are not any longer than necessary to complete the connection without disturbing the permanent parts of the installation and so that unnecessary bends and loops are avoided.

The whole thrust of this section is for the purposes (mostly lightning) indicated and not for fault current. Large conductors are not required for short term use to generally poor electrodes. Just look at the difference between the GEC and the MBJ that is expected to carry fault current returning to the source (transformer). The largest GEC required is 3/0 AWG and the MBJ will be 750 kcmil for the fault current.

 

[Ron Poehlmann]

Thank you gentlemen!
I learned a lot today. It makes so much sense now that I don't know how I missed it. Somehow, I thought there was text like 250.25(D) that applied to the GEC, and I just wasn't finding it. The understanding of the difference between protection for lighting and fault current makes so much sense and will help in future designs and explaining to contractors, inspectors, and other engineers (no one is an expert on everything).

Thank you again.

 

[charlie]

My definition of expert

My definition of expert

. . . no one is an expert on everything . . .

It depends on your definition of expert. My definition is a person who has a good understanding of how much they don't know about a particular subject. I have been around long enough to have a pretty good idea of what I don't know so that makes me an expert. I also have a good idea where to go to get answers for areas outside of my own expertise.

As far as the dictionary's definition, I agree with your first statement.

 

[Skyline Electric]

All of them are GECs (grounding electrode conductors) and all of them are to be sized in accordance with 250.66. The GEC to the ground rods and to the concrete encased electrode are both for lightning, contact with higher voltage lines, and to stabilize the voltage to earth so the 6 AWG and 4 AWG GECs are sized correctly for the amount of expected current. The water piping system is bonded within 5' of where it comes into the building in accordance with 250.50 which tells you to go to 250.52.

The MBJ (main bonding jumper) is a different animal since it will actually carry the fault current back to the source under fault conditions. 250.28 gives the requirements for the MBJ and 250.28(D) gives you the sizing requirements. The minimum size is 687.5 kcmil based on these requirements.

It would be useful to note that 250.52(a)(1) excludes commercial and industrial buildings from this requirement....

 

[mweaver]

Skyline,

If you are referring to the Exception following Section 250.52(A)(1) in the 2008 NEC, I do suggest you read the exception quite carefully.

This is exception is quite restrictive in its application and cannot be blanketly applied to commercial and industrial occupancies...

mweaver

 

[Skyline Electric]

Skyline,

If you are referring to the Exception following Section 250.52(A)(1) in the 2008 NEC, I do suggest you read the exception quite carefully.

This is exception is quite restrictive in its application and cannot be blanketly applied to commercial and industrial occupancies...

mweaver

Actually the exception is a model of leniency. "In Industrial and commercial buildings or structures where conditions......."

Essentially, the "5 foot rule" is not applicable in industrial and commercial buildings, this exception is clear and unambiguous.

 

[JohnJ0906]

Actually the exception is a model of leniency. "In Industrial and commercial buildings or structures where conditions......."

Essentially, the "5 foot rule" is not applicable in industrial and commercial buildings, this exception is clear and unambiguous.

In many commercial buildings I have seen, this exception would not apply.

It does not meet "... where conditions of maintenance and supervision ensure that only qualified persons service the instalation...."

IMO, maintenance men are NOT qualified, yet they often work on both the plumbing and electrical systems.

 

[Mike01]

Thank you gentlemen!
(no one is an expert on everything).

Sure there is I am an expert..I know everything about nothing and nothing about everything....

 

[Ron Poehlmann]

In many commercial buildings I have seen, this exception would not apply.

It does not meet "... where conditions of maintenance and supervision ensure that only qualified persons service the instalation...."

IMO, maintenance men are NOT qualified, yet they often work on both the plumbing and electrical systems.

I agree with John that for many commercial buildings this exception would not apply. In many industrial plants there is a maintenance crew with the different trades, but just because a commercial building has a maintenance man today that knows what to do with an electrode conductor does not mean that the next maintenance man will.

Thank you for your comment.