Sizing bonding jumpers

[Bballfan1700]

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Sorry first time using forum and I don't know how to post pictures. But basically the other night in class we had an issue sizing bonding jumpers. I had the understanding that you added up your largest ungrounded conductor size in each pipe together, then look at table 250.102 and size the bonding jumper based off that. Or size off the amperage of the main and go to table 310.16, and get the size there. That being said I still don't understand why the answer to 3 is D and not E. And why the answer to 6 is C and not D. Thank you in advance.

 

[LarryFine]

Welcome to the forum.

When you have paralleled sets of feeders, each set's EGC must be sized as if it was the only one for all of them.

 

[infinity]

For this example (#7) you would add the secondary conductor sizes together 500+500 kcmil=1000 kcmil and look up 1000 kcmil in T250.102(C)(1) to find the SBJ size. The answer is #2/0 copper.​

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[texie]

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Sorry first time using forum and I don't know how to post pictures. But basically the other night in class we had an issue sizing bonding jumpers. I had the understanding that you added up your largest ungrounded conductor size in each pipe together, then look at table 250.102 and size the bonding jumper based off that. Or size off the amperage of the main and go to table 310.16, and get the size there. That being said I still don't understand why the answer to 3 is D and not E. And why the answer to 6 is C and not D. Thank you in advance.

Because these are supply side bonding jumpers (IE no OCPD ahead of them) you size them for the ungrounded conductors in each raceway per 250.102(C)(2). Also look at Informational Note No. 1.
The system bonding jumper for question 7 would be based on the sum of the ungrounded conductors.

 

[hitehm]

As for #6, Per 250.102(c2), I think it depends on if you're using "individual" bonding jumpers for each conduit or a "single" bonding jumper sized for all conduits. The question only says your using the transformer enclosure to bond everything. Not sure that's enough to determine which. 250.102(c2) states:

(2) Size for Parallel Conductor Installations in Two or More
Raceways or Cables. Where the ungrounded supply conductors
are paralleled in two or more raceways or cables, and an
individual supply-side bonding jumper is used for bonding
these raceways or cables, the size of the supply-side bonding
jumper for each raceway or cable shall be selected from Table
250.102(C)(1) based on the size of the ungrounded supply
conductors in each raceway or cable. A single supply-side bonding
jumper installed for bonding two or more raceways or
cables shall be sized in accordance with 250.102(C)(1).