Sizing gounding conductors from a transformer to a panel

[curtiszimmerman]

We have a branch feed from a 200 amp 3 phase 480 volt breaker feeding a 150 kva transformer providing a parallel 208/120 3 phase feed to a 400 amp panelboard. The argument develops from the fact that there is not a breaker immediately after the transformer on the secondary that the actual breaker size is not known and that you would use 250.66 instead of sizing off 250.122 using a 400 amp breaker size.

 

[sandsnow]

Re: Sizing gounding conductors from a transformer to a panel

You have an Equipment Bonding Jumper in the case you described, NOT an Equipment Gronding Conductor. EGC only occur after an OCPD.
Size you EBJ per 250.30(A)(5) which refers you to 250.28 which are the rules for the Main Bonding Jumper. So you use 250.66 unless the derived pjase conductors are over 1100 kCMIL; in which case you would use 12.5% of the largest phase conductor.