Grounding vs Bonding Online Training

This article was posted 05/05/2006 and is most likely outdated.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

The following answer is based to an installation where the transfer switch does not open the neutral.

Grounding is not required, according to the grounding requirements of the NEC, since it would serve no purpose to do so. But many generator manufactures have boiler plate instructions specifying how they want the generator grounded. Because the instructions specifically require grounding, you have to waste the time and money to drive a ground rod [110.3(B)].

It’s unfortunate that many generator manufactures don’t understand the principles of grounding and the NEC requirements for such equipment. I hope that one day the generator manufactures would simply state that the generator must be grounded in ‘accordance with the NEC.”

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