A closer look at bonding grounding electrodes (NEC 2011, article 250 requirements)

This paper describes the grounding design of a 480V, 3-phase, 4-wire power distribution system for a rail transit facility consisting of a passenger station and two separate parking garages. The authority having jurisdiction (AHJ) made an interpretation of NEC Section 250.50 that the grounding electrodes of the power panels at three different locations should be bonded to the main switchboard grounding electrodes by installing underground bonding conductors. Designers disagreed with AHJ and were opposed to such installation. Due to physical locations of the panels and the switchboard within the facility, such installation was impractical. To meet NEC requirements and to satisfy AHJ, a discussion of bonding of facility grounding electrodes is included in this paper. At locations where underground bonding conductors were not practical to install, bonding of structural steel members with metallic conduits for feeder cables that carry equipment grounding conductors between the main switchboard and various power panels act as bonding conductors.