Bonding jumper sizing confusion

[codequestion]

I have 3000 amps service 480/277v. In the service equipment enclosure what size main bonding jumper and what size bonding jumper which bonds to the service equipment enclosure be used per NEC 250 table 250.102? I am bit confuse about 12.5% thing.

 

[Smart $]

What is the sum of the ungrounded service conductor kcmil values (per phase)? Take that sum and multiply it times 0.125. The result is the size MBJ and other jumpers you must use... unless greater than the size of the largest ungrounded service conductor in each set. If that is the case, you can reduce the MBJ and other jumpers to that size. FWIW, this only happens when you have more than 8 sets.

 

[infinity]

I have 3000 amps service 480/277v. In the service equipment enclosure what size main bonding jumper and what size bonding jumper which bonds to the service equipment enclosure be used per NEC 250 table 250.102? I am bit confuse about 12.5% thing.

If you look at T250.102(C)(1) you'll see that it stops at 1100 kcmil. When your total kcmil for the parallel sets exceeds the 1100 kcmil the main bonding jumper needs to be large enough to carry the fault current so the NEC has established a minimum size of 12.5% of the total kcmil size of the parallel SEC's.

So in your case if we use 600 kcmil SEC's you would need 8 sets (3000 amps/420 amps per set = 7.14 or 8 sets):

8*600 kcmil = 4800 kcmil * 12.5% = 600 kcmil main bonding jumper.

You could also use 8 sets of 500 kcmil and the MBJ would only need to be 500 kcmil even for the same 3000 amp service.