2008 Soares Book - Chapter 21

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finhead

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Tables 21-1 through 21-5 in the 2008 Soares book seem to support the conclusion that the values in Table 250.66 should be increased where the length of the grounding electrode conductor (GEC) exceeds 100 feet in length. This may or may not be true, but perhaps someone can help me clarify a few points.

1. Table 21-2 (column 4) shows the ampacity of the GEC to be based on the 75 degree column. Since this conductor is permitted to be bare, why limit the ampacity to the 75 degree column ?

2. Table 21-2 (asterick) tells us these values are based on the assumption that the GEC is the sole connection between the grounded system conductor and the electrode. What about the parallel path that consists of the grounded system conductor, the utility transformer, the grounding electrode at the transformer, and the earth. What about a third parallel path where a supplemental electrode is required?

3. Table 21.5 mysteriously arrives at two questionable conclusions. First, the voltage drop of the GEC, based on short-time current rating, should not exceed 40 volts. Why not 30 volts or 50 volts? Second, that the resistance of the GEC, when it exceeds 100 ft, should not exceed the resistance of 100 ft of the conductor specified in the table. Why not consider the total resistance of the path to ground where parallel paths exist?

4. Exactly what does the short-time current rating signify? Using the voltage drop formulas from the UGLY Book, I am unable to duplicate the author's voltage drop for the various conductors.

Thanks for your comments and
Happy New Year!
Brian Dolan
 
I make no claim to be an authoratative, but here are my comments:

finhead said:
Tables 21-1 through 21-5 in the 2008 Soares book seem to support the conclusion that the values in Table 250.66 should be increased where the length of the grounding electrode conductor (GEC) exceeds 100 feet in length. This may or may not be true, but perhaps someone can help me clarify a few points.

1. Table 21-2 (column 4) shows the ampacity of the GEC to be based on the 75 degree column. Since this conductor is permitted to be bare, why limit the ampacity to the 75 degree column ?
Click to expand...

There has to be some temperature as a basis. I suspect NEC 110.14 plays into this. "Permitted to be bare" does not mean the conductor must be bare. Since the conductor may be insulated there should be a reasonable limit on temperature. Also, the GEC is frequently run exposed. If people can touch a conductor I think it's reasonable to limit the conductor temperature. On the technical side, higher temperature means higher resistance in the conductor. Limiting the temperature rating has the effect of capping the GEC resistance.

2. Table 21-2 (asterick) tells us these values are based on the assumption that the GEC is the sole connection between the grounded system conductor and the electrode. What about the parallel path that consists of the grounded system conductor, the utility transformer, the grounding electrode at the transformer, and the earth. What about a third parallel path where a supplemental electrode is required?
Click to expand...
How would you model the parallel path? How do you measure the resistance or impedance of the parallel path? Can you guarantee that the parallel path stays the same? Since the GEC alone is the minimum acceptable installation it has to meet all requirements by itself.

3. Table 21.5 mysteriously arrives at two questionable conclusions. First, the voltage drop of the GEC, based on short-time current rating, should not exceed 40 volts. Why not 30 volts or 50 volts? Second, that the resistance of the GEC, when it exceeds 100 ft, should not exceed the resistance of 100 ft of the conductor specified in the table. Why not consider the total resistance of the path to ground where parallel paths exist?
Click to expand...
Mysteriously? I suspect someone spent a long time doing research to reach those values. I'm not sure that it would be appropriate in Soares to detail the research behind every value stated. As to the parallel path question, how do you model the parallel path and how do you guarantee that the parallel path will not change?

4. Exactly what does the short-time current rating signify? Using the voltage drop formulas from the UGLY Book, I am unable to duplicate the author's voltage drop for the various conductors.

Thanks for your comments and
Happy New Year!
Brian Dolan
Click to expand...
I don't have the 2008 Soares so I can't address this directly. I also don't know how far off your numbers are. There are many different methods for calculating voltage drop. Without seeing your calcs I would assume that you used a calculation method with different assumptions that the method used by the author. I don't have Ugly, but I suspect that the calculation there is "ballpark" and on the conservative side.

Martin
 
finhead said:
Tables 21-1 through 21-5 in the 2008 Soares book seem to support the conclusion that the values in Table 250.66 should be increased where the length of the grounding electrode conductor (GEC) exceeds 100 feet in length. This may or may not be true, but perhaps someone can help me clarify a few points.

1. Table 21-2 (column 4) shows the ampacity of the GEC to be based on the 75 degree column. Since this conductor is permitted to be bare, why limit the ampacity to the 75 degree column ?
Click to expand...
I would guess offhand that it would be related to the rating of any connection/terminal in the link.
finhead said:
2. Table 21-2 (asterick) tells us these values are based on the assumption that the GEC is the sole connection between the grounded system conductor and the electrode. What about the parallel path that consists of the grounded system conductor, the utility transformer, the grounding electrode at the transformer, and the earth. What about a third parallel path where a supplemental electrode is required?
Click to expand...
My guess here is that there would be too many variables involved for a Table to contemplate all these parallel paths.
finhead said:
3. Table 21.5 mysteriously arrives at two questionable conclusions. First, the voltage drop of the GEC, based on short-time current rating, should not exceed 40 volts. Why not 30 volts or 50 volts? Second, that the resistance of the GEC, when it exceeds 100 ft, should not exceed the resistance of 100 ft of the conductor specified in the table. Why not consider the total resistance of the path to ground where parallel paths exist?
Click to expand...
My guess is the Table does not contemplate all the variables. It would be a useful Table if it had such calculations.
finhead said:
4. Exactly what does the short-time current rating signify? Using the voltage drop formulas from the UGLY Book, I am unable to duplicate the author's voltage drop for the various conductors.
Click to expand...
Resistances vary widely between wire manufacturer's conductors which may account for some of the discrepancies.

Thanks for your comments and
Happy New Year!
Brian Dolan[/quote]
 
Brian,
I would not really be concerned with the GEC as it should not be carrying fault current...I would be more concerned with the voltage drop in the EGC which is intended to carry fault current.
 
finhead said:
Tables 21-1 through 21-5 in the 2008 Soares book seem to support the conclusion that the values in Table 250.66 should be increased where the length of the grounding electrode conductor (GEC) exceeds 100 feet in length. This may or may not be true, but perhaps someone can help me clarify a few points.

Happy New Year!
Brian Dolan
Click to expand...

I have read that book. As I recall the 100 ft and the 40 volts was with regard to the EGC not the GEC.
 
Martin
Thanks for your comments.

There has to be some temperature as a basis. I suspect NEC 110.14 plays into this. "Permitted to be bare" does not mean the conductor must be bare. Since the conductor may be insulated there should be a reasonable limit on temperature. Also, the GEC is frequently run exposed. If people can touch a conductor I think it's reasonable to limit the conductor temperature. On the technical side, higher temperature means higher resistance in the conductor. Limiting the temperature rating has the effect of capping the GEC resistance.

I can 't imagine current on the GEC lasting long enough to heat the conductor to a level dangerous to touch. I also can't imagine that when the size of the GEC was determined, a long long time ago, that insulation was a factor in the decision.

How would you model the parallel path? How do you measure the resistance or impedance of the parallel path? Can you guarantee that the parallel path stays the same? Since the GEC alone is the minimum acceptable installation it has to meet all requirements by itself.

Let me start with your last statement. The GEC alone is not the minimum acceptable installation. On a grounded system, which the Soares book specifies, the GEC and the system grounded conductor are both connected to electrodes in the earth. One via the utility transformer and one via some electrode within the building. This is going to be the case whether or not a supplemental electrode is required.
The resistance of the GEC is not equal to the resistance of the path. There is also the resistance of the connection to the electrode and the resistance of the electrode connection to earth. I do not know how to measure the impedance of this parallel circuit, but I can't see how 0.068 ohms - the difference between 1,000 ft and 100 ft of 3/0 AWG is going to significantly alter the final result. In addition, neither of the paths can be assumed to remain unchanged.


Mysteriously? I suspect someone spent a long time doing research to reach those values. I'm not sure that it would be appropriate in Soares to detail the research behind every value stated. As to the parallel path question, how do you model the parallel path and how do you guarantee that the parallel path will not change?

Ok, mysteriously may not have been the best word, but a footnote or works cited notation would have been helpful. The more I look at the tables, the more it appears that the author crunched a lot of numbers based on 100 feet of GEC and then suggests that the resulting voltage drop and resistance values should not be exceeded.

Thanks again for your comments
Brian
 
don_resqcapt19 said:
Brian,
I would not really be concerned with the GEC as it should not be carrying fault current...I would be more concerned with the voltage drop in the EGC which is intended to carry fault current.
Click to expand...

Don
What concerns me are rumers of proposals intending to change Table 250.66 to reflect the conclusions of Chapter 21 in the Soares book. Also of concern is my 401K, which is now a 201K.

Brian
 
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