12 and 1/2 % rule

[Bob Clukey]

Can someone explain the following? Thanks!

Bonding conductors upstream of an overcurrent device, such as the supply side of a service or between a transformer and panelboard, are typically sized using Table 250.66 and the 12?% rule discussed in 250.102(C).

 

[augie47]

When sizing bonding jumpers you use Table 250.66 based on your phase conductor size, however, 250.66 drops off at 1100 kcmil(cu). If you phase conductors exceed 1100, you must use 12-1/2% of their size to determine the bonding conductor size.
An example would be (6) 4" conduits with 500's. If your were using a single bonding jumper to bond the conduits you would need to size it a (6) x (500,000) x 12-1/2" or a 400 kcmil bond.

 

[Little Bill]

When sizing bonding jumpers you use Table 250.66 based on your phase conductor size, however, 250.66 drops off at 1100 kcmil(cu). If you phase conductors exceed 1100, you must use 12-1/2% of their size to determine the bonding conductor size.
An example would be (6) 4" conduits with 500's. If your were using a single bonding jumper to bond the conduits you would need to size it a (6) x (500,000) x 12-1/2" or a 400 kcmil bond.

Augie, that's an awful lot of "ciphering" for 1 day!:grin:

 

[infinity]

Here's a photo of the system bonding jumper (bottom green conductor), a 350 kcmil for 5 sets of 500 kcmil secondary conductors. Also in the photo is the #3/0 GEC. Although the bonding jumper is required to be sized at 12.5% the GEC is not required to be larger than #3/0. Also noteworthy is the fact that the SBJ is sized on the 5 sets of ungrounded condcutors and not the 10 sets (200% neutral) of the neutral conductor.