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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

where in the code book does it say that you have to pound a ground rod for a perminant generateor. i know you have to connect the equipment ground to it that goes to your service.

greg mccrady
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Reply from: David Engelhart
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.
Reply to David Engelhart

Reply from: Mike Holt
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.”
Reply to Mike Holt

Can a 400 amp breaker or fused safety switch be (disconnecting means) installed ahead of a meterbase in a residence? For example; in a 320 meterbase. The load side of the meterbase with split lugs would feed two parallel panels (200 amp & 100 amp) with their own mains breakers. The 400 amp main would then be the first "main" disconnect where the grounding rods and heater bonding would take place?... Is this allowed by code?

Miguel

Miguel Pineda
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Reply from: David Engelhart
Typically, a meter base on a single family residence is not at the load side of a disconnect, but I have heard utility stories. The code does not specify where it goes, but 230.82 allows it on the supply side of the disconnect. The question at hand is if the disconnect can be ahead of the meter base, and that would be up to the utility. I see other potential problems with the installation for which there are not enough details. The review of 240.21(B) would seem to be in order to answer the vagueness. Also, what is "heater bonding"? The grounding would have to take place at the 400 amp disconnect. The equipment grounding would have to begin there also. But, what do you do with it when it goes through the meter base? That is what it is supposed to do. The meter base may have a "tickler" wire that is connected to the grounded neutral termination. Also, the meter base is typically bonded to the grounded neutral termination because the neutral termination is bolted to the housing. But, wouldn't that make the meter housing part of the current carrying grounded circuit? I would think so but it is allowed in 250.142(B). This is typically used when there are more than six main disconnects that are fed after meter enclosures and a main has to be implemented before the individual unit metering. Hope I havn't confused the issue tooo much, but I hope I answered the question.
Reply to David Engelhart

Reply from: Miguel Pineda
I will meet again with the Inspector this coming Wednesday,May 10, 2006 This was just a query, because it sounded odd to me on what he said. I'll hope to square things-away (get an understanding) up with him on Wednesday... Thank you for your reply... I'll see what he is going to specify for this work... Miguel
Reply to Miguel Pineda

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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