This article was posted 05/05/2006 and is most likely outdated.

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250.97 Bonding 277V/480V.

May 5, 2006  

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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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Copyright © 2006 Mike Holt Enterprises,Inc.
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Comments

  • It depends, but it sounds like it would only be for lightning dissipation there at the generator if lightning were to strike it. The equipment grounding conductor gave me that clue. But, if the neutral conductor is disconnected from the utility and completely switched to the generatoer, then it is a separatly derived system, and the neutral would have to be grounded. 250.34, 250.20(D). Maybe 110.3(B), the instructions tell you to do so as well.
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