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This article was posted 05/05/2006 and is most likely outdated.

250.97 Bonding 277V/480V.

May 5, 2006

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PART V. BONDING

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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:
• 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).
• 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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Figure 250–139
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Figure 250–140
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Figure 250–141
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Comments

How does one tell if a box, with ringed knockouts, is listed for use without bonding jumpers? I don't recall ever seeing such an NRTL label on any boxes used at 277/480V without jumpers.

Tom

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