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Code Q&A

Jun 1, 2002 12:00 PM, By Mike Holt, NEC Consultant

Knowing how important NEC issues are to our readers, we're introducing a new online Code Q&A column. Updated weekly, visit this link for answers to your latest Code questions and concerns.

Q. I know 250.32 allows you to bond the grounded (neutral) circuit conductor that supplies a building to the equipment grounding conductor at an accompanying remote building. This is the case at the campus I’m currently working at, but the grounding electrode system of the second building is also connected to the grounded (neutral) conductor. If the two buildings were electrically connected via a conductive path, such as the shielding of a coax cable or water pipe, this would provide an alternate path for neutral current to flow during normal operation (like resistors in parallel). Do you know of any problems that can arise from a situation such as GFI relays in the main service? Everyone I’ve discussed this with has dismissed it as trivial. Are they right?

A. The practice of bonding the grounded (neutral) conductor to the equipment enclosure is only permitted by 250.32(B)(2) where all of the following conditions exist:

An equipment grounding conductor is not run with the supply to the building or structure.
There are no continuous metallic paths bonded to the grounding system in both buildings or structures involved.
There is no ground-fault protection of equipment on the common AC service.

In addition, the grounded conductor may not be smaller than the larger of that required by 220.22 (maximum unbalanced neutral load) or 250.122 (equipment grounding conductor size).

When an equipment grounding conductor doesn’t run to a separate building or structure, you must use the grounded (neutral) conductor to provide the effective ground-fault current path required to clear any ground-faults (line-to-case faults) in addition to carrying any unbalanced neutral current [250.4(A)(3)].