Question about Grounding Electrode size

Justin101

Member
Location
United States
Today in class, I was handed back a test i took on the NEC, 2011. I got all question correct but 2, one of them i tried to fight the teacher on because I am almost 100% sure i'm positive.

The generalization of the question is you have a service that needs to be grounded, it does not tell anything else, it says what is the minimum size of grounding electrode required to handle 3,000AMPS 460 Volts.
the possible answers were
A. 4
B. 6
C. 3/0
D. 400kcmil

I answered D, 400kcmil, I knew I had found it somewhere and how to do it in the NEC at the time, but he said I had it wrong and it was C. 3/0. I argued with him, and ever since i have returned from class today I've been researching where I had found this answer. I have a strong belief that my answer "D" was correct not "C". can anyone give me references to the tables, or the area of the code book and explain to me how to get the answer.
 

Justin101

Member
Location
United States
I went and took a look at table 250.122 considering the only given information was the amperage and the voltage. it says 3,000 amps to 400 for copper and 600 for aluminum or copper-clad aluminum.
 

roger

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Electrician
I went and took a look at table 250.122 considering the only given information was the amperage and the voltage. it says 3,000 amps to 400 for copper and 600 for aluminum or copper-clad aluminum.
Forget 250.122 and think about what equivilant size of conductors it would take for a 3000 amp service. As Bob said, 250.66 is the appropriate article.

Roger
 

roger

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BTW, considering the generalization of the question, I don't agree that the answer was "C" either but we won't go there right now. I have a feeling there was some specific wording as to what the electrode was.

Roger
 

Justin101

Member
Location
United States
It was a very tricky question, there was no conductor sizes so i could not use the other table. essentially all the question asked was what size grounding electrode do you need for a 3,000 amp 460 v. If there was more information such as the size of the largest service entrance conductor or the area for parallel conductors then I would have revised my answer, but based solely on the information I was given I came to that conclusion. in my opinion the answer was subjective, there are many other factors that could fall into play and change the outcome tremendously.
 

infinity

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New Jersey
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Journeyman Electrician
It was a very tricky question, there was no conductor sizes so i could not use the other table. essentially all the question asked was what size grounding electrode do you need for a 3,000 amp 460 v. If there was more information such as the size of the largest service entrance conductor or the area for parallel conductors then I would have revised my answer, but based solely on the information I was given I came to that conclusion. in my opinion the answer was subjective, there are many other factors that could fall into play and change the outcome tremendously.
The question was vague because they're trying to trick you. Bottom line is T250.66 ends at 1100 kcmil and #3/0. Obviously a 3000 amp service would require more than 1100 kcmil so you would need to use the largest GEC which is #3/0. In reality the answer could be A, B or C because a CEE would need a #4 minimum and a rod a #6 minimum.
 

GoldDigger

Moderator
Staff member
Today in class, I was handed back a test i took on the NEC, 2011. I got all question correct but 2, one of them i tried to fight the teacher on because I am almost 100% sure i'm positive.

The generalization of the question is you have a service that needs to be grounded, it does not tell anything else, it says what is the minimum size of grounding electrode required to handle 3,000AMPS 460 Volts.
I hope that you are just misquoting the original question, since what you are trying to determine is the size of the Grounding Electrode Conductor, GEC (the wire) not the size of the Grounding Electrode itself.

I too am 100% sure that you are positive, but I am also 100% sure that you are wrong. Although it is also a poorly written question, as infinity said, the answer you gave is not even one that could come from other interpretations. :)
 

don_resqcapt19

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Illinois
In my opinion, they are looking for an answer based on 250.66(A) as that would be the absolute minimum size permitted and if there was no other information in the question, that is what my answer would be.
 

don_resqcapt19

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Staff member
Location
Illinois
.... essentially all the question asked was what size grounding electrode do you need for a 3,000 amp 460 v. ....
That is not what you told us the question asked. You told us that it asked for the minimum size of the EGC. In my opinion, all the information you need to answer the question is in the question...that is, if it really asked for the minimum size.
 

roger

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Fl
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Electrician
That is not what you told us the question asked. You told us that it asked for the minimum size of the EGC. In my opinion, all the information you need to answer the question is in the question...that is, if it really asked for the minimum size.
And this is the reason for my post #5

Roger
 

iwire

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Location
Massachusetts
I think it is impossible to make comments about the validity of the question without seeing precisely how it was worded.

Justin, when working with the NEC it is absoluty critical that you pay very close attention to detail.
 

O'donisR

Member
Location
Miami
Today in class, I was handed back a test i took on the NEC, 2011. I got all question correct but 2, one of them i tried to fight the teacher on because I am almost 100% sure i'm positive.

The generalization of the question is you have a service that needs to be grounded, it does not tell anything else, it says what is the minimum size of grounding electrode required to handle 3,000AMPS 460 Volts.
the possible answers were
A. 4
B. 6
C. 3/0
D. 400kcmil

I answered D, 400kcmil, I knew I had found it somewhere and how to do it in the NEC at the time, but he said I had it wrong and it was C. 3/0. I argued with him, and ever since i have returned from class today I've been researching where I had found this answer. I have a strong belief that my answer "D" was correct not "C". can anyone give me references to the tables, or the area of the code book and explain to me how to get the answer.
See Table 250.66

1100 KCMIL OR OVER use #3/0 Copper for grounding ELECTRODE. The correct answer is (d) use #3/0 copper GEC
 

roger

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Fl
Occupation
Electrician
See Table 250.66

1100 KCMIL OR OVER use #3/0 Copper for grounding ELECTRODE. The correct answer is (d) use #3/0 copper GEC
And that is your answer if the electrode is a rod, pipe, or plate?

Roger
 

O'donisR

Member
Location
Miami
And that is your answer if the electrode is a rod, pipe, or plate?

Roger
There is not specification of what type of electrodes will be use, the only info provided was : (what is the minimum size of grounding electrode required to handle 3,000AMPS 460 Volts), table 250.66 show for a 1100 KCMIL OR OVER use #3/0 Copper for grounding ELECTRODE. That would be my answer..

For ground rod?? I will use #6 copper


Thank you
 

roger

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Location
Fl
Occupation
Electrician
There is not specification of what type of electrodes will be use, the only info provided was : (what is the minimum size of grounding electrode required to handle 3,000AMPS 460 Volts),
And without knowing what the electrode is we would have to take note of the word "minimum" and a rod, pipe, or plate would need nothing larger than a #6.
table 250.66 show for a 1100 KCMIL OR OVER use #3/0 Copper for grounding ELECTRODE. That would be my answer..
Read 250.66(A) before you go to the Table, the answer is (B)

Roger
 

O'donisR

Member
Location
Miami
And without knowing what the electrode is we would have to take note of the word "minimum" and a rod, pipe, or plate would need nothing larger than a #6. Read 250.66(A) before you go to the Table, the answer is (B)

Roger

You right... I missed the key-word Minimum Thank you,, very helpful
 
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