GEC and SSBJ on transformer with multiple secondary

[wwhitney]

we are not going to agree on this one

One big problem with your interpretation is that it matches what the column heading "Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors" already says. So there would be zero reason to have a note for the case of multiple sets of SECs or secondary conductors if that is what is intended. The presence of the note indicates a different meaning is intended, and the interpretation I mentioned is the only reasonable understanding of the note's wording.

Cheers, Wayne

 

[Elect117]

I'm going to base t as stated, on the largest of ungrounded supply conductor(s). if you have more than one set your going to have a set with the largest and you will also have a set with the smallest or all sets will have the same size. if there are parallel conductors in one or more sets all base the largest set on the equivalent size as if it was a single conductor by the sum of areas of the conductors in parallel.

i understand your reading of this section i disagree with it, the same as you disagree with my reading of this section

Are we still talking about system bonding jumper?

What size would you have for the SBJ, installed at the transformer, for the following:

(1) 400A CB with parallel 3/0
(2) 80A CB with #3

 

[david]

One big problem with your interpretation is that it matches what the column heading "Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors" already says. So there would be zero reason to have a note for the case of multiple sets of SECs or secondary conductors if that is what is intended. The presence of the note indicates a different meaning is intended, and the interpretation I mentioned is the only reasonable understanding of the note's wording.

Cheers, Wayne

First set a single 1/0
Second set parallel 1/0
Third set parallel 2/0

The largest equivalent size would be the set with 2/0

 

[david]

Are we still talking about system bonding jumper?

What size would you have for the SBJ, installed at the transformer, for the following:

(1) 400A CB with parallel 3/0
(2) 80A CB with #3

Based on the equivalent size of two 3/0's

 

[Ccarlson32]

Are we still talking about system bonding jumper?

What size would you have for the SBJ, installed at the transformer, for the following:

(1) 400A CB with parallel 3/0
(2) 80A CB with #3

For sizing the SBJ, you would size it based the sum circular mil, 3/0+3/0+#3.

Based on the equivalent size of two 3/0's

Agree with david, then how about sizing your GEC then?

 

[david]

When you use the method of multiple disconnects in single enclosures

Verses

A single enclosure with multiple disconnects

The potential fault would be less on the system with the multiple enclosures

But your sizing it bigger than a single supply to single enclosure with multiple disconnects

 

[Elect117]

The NEC adds all three in their example when sizing the SBJ. As I previously stated. And I believe David is referring to a bonding jumper since he was using table250.102(C)(1). Where as Table 250.66 is used for GECs.




I believe Wayne and Augie had two different opinions for how to size your GEC.

 

[wwhitney]

First set a single 1/0
Second set parallel 1/0
Third set parallel 2/0

The largest equivalent size would be the set with 2/0

That is true, but the Note doesn't call for the largest equivalent size. It calls for the largest sum; for each leg of the supply, you sum the conductors for that leg from all sets. So 1/0 + 2 * 1/0 + 2 * 2/0 in the above example, whether using Table 250.66 or Table 250.102(C)(1).

Cheers ,Wayne

 

[wwhitney]

a bonding jumper since he was using table250.102(C)(1). Where as Table 250.66 is used for GECs.

In the 2020 NEC, both tables 250.102(C)(1) and 250.66 have notes about "largest sum of the areas of the corresponding conductors of each set." So the method of dealing with multiple non-parallel sets is the same for both tables.

In the 2023 NEC, that note was deleted for 250.102(C)(1); I'm not sure what to think of that, or how to size the SBJ for an SDS with multiple sets of secondary conductors.

Cheers, Wayne

 

[Elect117]

In the 2020 NEC, both tables 250.102(C)(1) and 250.66 have notes about "largest sum of the areas of the corresponding conductors of each set." So the method of dealing with multiple non-parallel sets is the same for both tables.

In the 2023 NEC, that note was deleted for 250.102(C)(1); I'm not sure what to think of that, or how to size the SBJ for an SDS with multiple sets of secondary conductors.

Cheers, Wayne

The note might be gone from the table 250.102(C)(1) but the example still remains in 250.28(D)(3) and that example is specific to sizing the system bonding jumper for more than one enclosure when installed at the transformer.

"a single system bonding jumper shall be installed at the source and sized in accordance with 250.28(D)(1) based on the equivalent size of the largest supply conductor determined by the largest sum of the areas of the corresponding conductors of each set."

 

[david]

One big problem with your interpretation is that it matches what the column heading "Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors" already says. So there would be zero reason to have a note for the case of multiple sets of SECs or secondary conductors if that is what is intended. The presence of the note indicates a different meaning is intended, and the interpretation I mentioned is the only reasonable understanding of the note's wording.

Cheers, Wayne

That's not true the notes is giving you guidance on how to be in compliance with what the heading says when there are two to six individual enclosures

 

[wwhitney]

That's not true the notes is giving you guidance on how to be in compliance with what the heading says when there are two to six individual enclosures

You are misreading the note, see the example in post #27 (although I'm not clear on the source of that example, maybe it's some enhanced content from the NFPA.) See also the wording from 250.28(D)(3), quoted in post #30.

Cheers, Wayne

 

[Ccarlson32]

So, are we concluding that we take this same process to size our GEC as we would or SBJ? Or do we take the largest of all derived secondary conductors. (As they are not paralleled) ...
(In my case largest is 250cu) and size GEC off that or the sum of the largest in each set. (250cu + 250al)


Glad my first topic posting was one that has brought multiple opinions to the forum!

 

[david]

The note might be gone from the table 250.102(C)(1) but the example still remains in 250.28(D)(3) and that example is specific to sizing the system bonding jumper for more than one enclosure when installed at the transformer.

"a single system bonding jumper shall be installed at the source

and sized in accordance with 250.28(D)(1) based on the equivalent size of the largest supply conductor

it stands alone as the target find the equivalent size of the largest supply conductor

determined by

how do i determine what is yhe size of the (largest) supply conductor

the largest sum of the areas of the corresponding conductors of each set."

The (largest sum)
When you have more than one set you have more than one sum

What is the largest sum what is the smallest sum
Are all having the same sum.
It does not say the sum of the corresponding conductors of all the sets.
It says the largest of each set