Supply Side Bonding Jumper vs EGC

[akazici]

The Supply Side bonding jumper for seperately derived systems ( with bonding at source ) is sized according to 250.102.C.

The equipment ground conductor is sized according to 250.122 ( according to the overcurrent device ).

I have a 5000A transformer ( seperately derived source ), from the secondary of the transformer to the first disconnect I need to pull 15 sets of conductors in 15 raceways.

Sizing the bonding jumper according to the 250.102.C, the biggest conductor is 500 MCM so I choose 1/0 to run in each raceway.

Leaving the 5000A breaker, I run to another switchgear with 5000As. Again I have 15 sets of conductors in 15 raceways but this time to size the EGC i use 250.122, I end up with 700 kcmil in each raceway.

How come the ground fault at the load ( that will pass through the EGC than the SSBJ than the system bonding jumper to return to the source ), needs a 700 kcmil than downsizes to 2/0 on the way ? Am I getting it wrong ?

Any help appreciated.

 

[jumper]

I think note 3 of the table would apply to sizing your jumper.

3. If multiple sets of service-entrance conductors are used as permitted
in 230.40, Exception No. 2, or if multiple sets of ungrounded supply
conductors are installed for a separately derived system, the equivalent
size of the largest ungrounded supply conductor(s) shall be determined
by the largest sum of the areas of the corresponding conductors
of each set.

 

[infinity]

You're correct that the SSBJ is based on the conductors in each parallel raceway in this case #1/0. EGC's are sized according to the OCPD ahead of them whether they're in single or parallel raceways.

 

[Buck Phases]

I know this is an old thread, but I find the nature of the question interesting.

How come the ground fault at the load (that will pass through the EGC [then] the SSBJ [then] the system bonding jumper to return to the source), needs a 700 kcmil [that] downsizes to [1/0] on the way? Am I getting it wrong?

Because the supply-side conductors are 'upstream' of the main bonding jumper, there is no grounding conductor. Instead the supply-side grounded neutral conductor is the intentional fault current path to the transformer. Your 1/0 bonding jumper is there to ensure continuity of conductive parts, but it is not designed to carry the fault current of the 5000A service. The service neutral is designed to handle that function.

For the service OCPD to function properly any one EGC 'downstream' of the disconnect must be sized the carry the full fault current. Any fault 'upstream' of the service disconnect will not be protected by the service OCPD and must blow clear, therefore it does no good to size each paralleled service neutral conductor to carry the full fault current.