250.64 (D) (1)

[david]

Which of these two equations would have the largest sum

1) based on the sum of the of the circular mil area of the largest ungrounded conductor(s) of each set of conductors that supplies the disconnecting means.

2) the equivalent size of the (largest conductor shall be determined by the "largest" sum of the areas of the corresponding conductors of each set.

I have Reached out to several individuals who are really bright at math regarding the above two equations.

So far the responses I received back in comparing the two equations.
"They are confusing and hard to discern what you are being ask to do.
There are a lot of sites out there titled ask a math expert.
I have one more resource that I am waiting to hear back from, he teaches math at slippery rock university slippery rock PA

 

[LarryFine]

I read them as saying the same thing.

 

[david]

I read them as saying the same thing.

Yes some that I have ask say they seem to be saying similar things but recognizing in the text under one circumstance the text tells you to use the first equation but under the other circumstance use the second equation

 

[david]

I was mistaken my friend John teaches behavioral science at slippery rock, it's his wife that has a masters in mathematics, they are looking at the equations together. He said he will get back to me

 

[LarryFine]

I don't see two different equations.

 

[don_resqcapt19]

Where did those two come from? Are they actual code sections?

 

[david]

Where did those two come from? Are they actual code sections?

250.64 (D)(1)

 

[david]

250.64 (D)(1)

Yes

 

[LarryFine]

Okay, after reading that section, it seems the first one applies to the common GEC based on the conductors supplying the group of disconnects, and the second one applies to the individual GECs based on the conductors supplying each individual disconnect.

 

[david]

It gives the first equation in the text

It than says if the service entrance conductors conect directly to the overhead service conductors ........table 250.66 note 1 as the second equation

 

[david]

Okay, after reading that section, it seems the first one applies to the common GEC based on the conductors supplying the group of disconnects, and the second one applies to the individual GECs based on the conductors supplying each individual disconnect.

If we consider the service equipment outside it's easier to illustrate.

Go from a meter to a through and supply the disconnects separately from a through
Use the first equation

As you said use the second equation if they go direct

 

[wwhitney]

A simple example:

Service point is a Termination Box from an underground lateral. From the Termination Box you supply 3 sets of SECs, one 500 kcmil, one 350 kcmil, and one 250 kcmil. For the common GEC you base the size off of 500 + 350 + 250 = 1100 kcmil. For the taps to the disconnects, you size each GEC tap based on the conductors supplying that disconnect.

Cheers, Wayne

 

[david]

Okay, after reading that section, it seems the first one applies to the common GEC based on the conductors supplying the group of disconnects, and the second one applies to the individual GECs based on the conductors supplying each individual disconnect.

Not exactly it still a common grounding electrode conductor being taped in both circumstances

 

[david]

There are two separate equations in this section all I am asking is all circular mil sizes being equal in each set ,witch of the two equations results in a conductor having the larger circular mil area

 

[david]

A simple example:

Service point is a Termination Box from an underground lateral. From the Termination Box you supply 3 sets of SECs, one 500 kcmil, one 350 kcmil, and one 250 kcmil. For the common GEC you base the size off of 500 + 350 + 250 = 1100 kcmil. For the taps to the disconnects, you size each GEC tap based on the conductors supplying that disconnect.

Cheers, Wayne

Are you saying you are using the first equation under those circumstances?

 

[don_resqcapt19]

250.64 (D)(1)

sorry, I never read post titles

 

[Dennis Alwon]

Isn't what is being present in 250.64(D)(1) the same as the notes to T.250.66

 

[wwhitney]

Which of these two equations would have the largest sum

1) based on the sum of the of the circular mil area of the largest ungrounded conductor(s) of each set of conductors that supplies the disconnecting means.

2) the equivalent size of the largest conductor shall be determined by the "largest" sum of the areas of the corresponding conductors of each set.

So are you comparing the phrase (1) from 250.64(D)(1) with phrase (2) from Table 250.66 note 1?

In both of the above expressions, the word "set" refers to set of conductors supplying one disconnect, and has nothing to do with parallel sets. I assume that with any parallel sets, we consider the parallel set to be a single conductor of the equivalent cross-sectional area, then apply the above.

As far as I can see, NEC-wise, there's no reason for the two different wordings, and no reason for 250.64(D)(1) to include the sentence "If the service-entrance conductors connect directly to the overhead service conductors, service drop, underground service conductors, or service lateral, the common grounding electrode conductor shall be sized in accordance with Table 250.66, note 1." I don't see how the situation described in the beginning of that sentence is a reason to treat things any differently from phrase (1), which immediately precedes this sentence.

However, taking these two phrases out of context, formula (1) is always larger than formula (2). That is, the "sum of the largest member of each set" is always bigger than the "largest of the sum of the corresponding members of each set."

For the case that within each set all the ungrounded conductors are the same size, there's no difference. But say you have 3 sets, one set is 500 kcmil for leg A and 250 kcmil for legs B and C; the second set is 500 kcmil for leg B and 250 kcmil for legs A and C; while the last set is 500 kcmil for leg C and 250 kcmil for legs A and B.

Then for (1) the largest ungrounded conductor of each set is 500 kcmil, and the sum of that for the 3 sets is 1500 kcmil. While for (2) the 3 sums of corresponding coductors from each set are 500+250+250 ; 250+500+250; and 250+250+500. Each of these sums is 1000 kcmil, so for (2) the answer is 1000 kcmil.

I seriously doubt that this is an intentional distinction, and I think (1) is just a sloppy way of trying to say (2).

Cheers, Wayne

 

[don_resqcapt19]

There are two separate equations in this section all I am asking is all circular mil sizes being equal in each set ,witch of the two equations results in a conductor having the larger circular mil area

The are both the same if all of the conductor sizes are equal.
However in the text, if you had multiple sets with different size ungrounded conductors in each set, you might get a larger size from the text than from the table note.

In the text you add the largest sizes across all of the phases, If one set has A Φ the largest. the second set had B Φ the largest and the third had C Φ the largest, you would add A from the first set with B from the second, and C from the third and use that sum.

In the table you would individually sum all of the As. all of the Bs and all of the Cs and take the largest of those three sums.

I do not believe that is the intent, but that is how I read the code language. It is my opinion that they are both intended to mean the same thing.

 

[david]

The are both the same if all of the conductor sizes are equal.
However in the text, if you had multiple sets with different size ungrounded conductors in each set, you might get a larger size from the text than from the table note.

In the text you add the largest sizes across all of the phases, If one set has A Φ the largest. the second set had B Φ the largest and the third had C Φ the largest, you would add A from the first set with B from the second, and C from the third and use that sum.

In the table you would individually sum all of the As. all of the Bs and all of the Cs and take the largest of those three sums.

I do not believe that is the intent, but that is how I read the code language. It is my opinion that they are both intended to mean the same thing.

I appreciate that, we have three differant types of of grounding electrode conductor conection points in the overall code section.
A)The common location.
B) The common grounding electrode conductor and taps
C)the individual grounding electrode from individual service disconnects.
In the text equation it takes us to a installation that you could have used the common location mythod, and maybe you do for a portion of the grounding electrode system.

But from the text we have our Service entrance conductors originate from a common location to individual service disconnects and we install a common grounding electrode conductor and taps.

By the text language it directs us to use the fist equation, and by reading the text it seems deliberate.

By the text language it also seems deliberately sending you to the table note only when you have direct individual supply conductors from the utility to individual service disconnects grouped at one location.

On one hand we can speculate that the deliberate distinction was unintentional and basically giving you the same sum under two separate installations mythods.

Or we can except that it is deliberate language directing us to two separate sums for reasons we currently do not understand