Main Bonding Jumper in service Equipment

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vicdelta

Member
I have a 3 Phase 4 W 1600A Over Head Service going into a Pull Section then
there are 2 service Over Current Device,one is 1200A and the other 400A.
The largest phase conductors are set of (3)750 Kcmil per phase.
I applied 12.5% x 3x 750 Kcmil which equals to 281,250 Kcmil,because phase conductors are over 1100 Kcmil per 250-28(D).I came up with a MBJ size 300 Kcmill. Is this right approach or based on Table 250-66 the max size shall be 3/0 regardless of service phase conductor.
Thanks
 

raider1

Senior Member
Staff member
Location
Logan, Utah
Is this right approach or based on Table 250-66 the max size shall be 3/0 regardless of service phase conductor.

You are using the right approach, the main bonding jumper must be at least 12.5% of the largest phase conductor if the phase conductor is larger than 1100 Kcmil copper.

Chris
 

M. D.

Senior Member
250.28 (d) is the right method in my opinion. here is some back up from an ECM article

Main bonding jumper. The main bonding jumper is the link between the neutral and the equipment ground bars within the service. This vital connection allows the ground-fault current to return to the source. Without the main bonding jumper, the fault would have to travel through high-impedance earth rather than low-impedance copper. This high-impedance path would most likely limit the current and prevent circuit breakers from tripping ? or at least prevent them from tripping soon enough to avoid equipment damage.

Size the main bonding jumper per Table 250.66. Many people assume this table indicates the maximum size main bonding jumper is 3/0 AWG, but that's another common misconception. The bonding jumper must be at least 12.5% of the equivalent area of the phase conductors [250.28(D)]. If you're running 11 sets of 500 kcmil conductors (a 4,000A service, for example), the main bonding jumper would need to be 700 kcmil minimum, not 3/0 AWG.

Janof, P.E., is an associate and senior project manager at Sparling, an electrical engineering and technology consulting firm with offices in Seattle and Portland.
 
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