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Grounding versus Bonding



250.97 Bonding 277V/480V.

Metal raceways or cables, containing 277V or 480V circuits, terminating at ringed knockouts must be bonded to the metal enclosure with a bonding jumper sized in accordance with Table 250.122, based on the rating of the circuit overcurrent protection device [250.102(D)]. Figure 250.139

Author's Comments:
a Bonding jumpers for raceways and cables containing 277V or 480V circuits are required at ringed knockout terminations to ensure that the effective ground-fault current path has the capacity to safely conduct the maximum ground-fault current likely to be imposed on it back to the electrical supply source, in accordance with 110.10 and 250.4(A)(5).
a Ringed knockouts aren't listed to withstand the heat generated by a 277V ground fault because a 277V ground fault generates five times as much heat as a 120V ground fault. Figure 250.140
Exception: A bonding jumper isn't required where ringed knockouts aren't encountered, or where the box is listed to provide a permanent and reliable electrical bond. Figure 250.141

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