250.64 (D) (1)

[augie47]

I always assumed it was addressing something like buss feeds where you don't have a kcmil area but can base you GEC on the equivalent area if it was conductors.

 

[david]

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.

I'm going to digest this further. If I was to circle each set, it would be the conductors supplying the main/service disconnect at each enclosure.
I never seen a service disconnect with 500k on A, 250k on B and 250k on C

I understand your illustration to be mathematical sets not a set of conductors at a serviceservice disconnect

 

[david]

I always assumed it was addressing something like buss feeds where you don't have a kcmil area but can base you GEC on the equivalent area if it was conductors.

I think that would true to go to the table with a circular mil area as if wire type conductors where present

 

[david]

If I was to have a single phase service and a three phase service grouped at one location and I only wanted to run one single grounding electrode conductor to my water main. I could not use the common location method.

I obviously i would not use the single location method.

Here are my sets for the single phase, line one and line two (L1 500k, L2 500k) the second set (L1 500k, L2 500k) and the 3 phase three sets A,B,&C in that order.
(250,250,250) (500,500,500) (600,600,600)
I'm thinking there is no corresponding conductors between the single phase service and the three phase service.
The three phase sets have a sum of 1350k
The single phase sets have a sum of 1000k

However the table note dosnt work for a common grounding electrode conductor because there is not a corresponding ungrounded conductor between the single phase and three phase services.

The total circular mils Is 1350k plus 1000k

2350k works for the text equation because it dosnt stipulate corresponding conductors of each set

 

[david]

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.

After thinking through a lot of possibilities I will agree you could have service breaker or main feeder breaker where you only needed 500k supply conductors and you run out of 500k so you complete the service or feeder using 600k you had as left over stock

(500,500,600)(500,500,600)(500,500,600)A&B phase would both be summed at 1500k while C phase would be 1800k

 

[wwhitney]

If I was to have a single phase service and a three phase service grouped at one location and I only wanted to run one single grounding electrode conductor to my water main. I could not use the common location method.

If by common location method you mean 250.64(D)(1), sure you could, and your post goes on to say how.

I'm thinking there is no corresponding conductors between the single phase service and the three phase service.

I agree that this is the reason for the different text in 250.64(D)(1) vs the Note 1 to Table 250.66. The latter applies only when the multiple sets have a common upstream connection, so there are corresponding conductors within each set. If you have multiple services to a building, there may not be corresponding conductors between the sets, and the text of 250.64(D)(1) handles that possibility by being broader.

Cheers, Wayne

 

[jaggedben]

I wonder if the rather duplicative language is because 250.64(D)(1) applies to buildings served by both feeders and services, but the Table Note 1 only applies to services.

If by common location method you mean 250.64(D)(1), sure you could, and your post goes on to say how.

I think he means 250.64(D)(3). And I agree with him that one can't use that method if there are multiple grounded conductors from different systems.

 

[david]

Thank you, for working through this with me