Grounding and Bonding - Questions and Answers

By Mike Holt for EC&M Magazine

Q1. Neutral and Ground Connections
At service equipment the NEC permits the grounded (neutral) conductor, the equipment enclosure, and the grounding electrode conductor to all terminate together. Is it permissible to land the equipment grounding conductors on the neutral bus bar with the grounded (neutral) conductors, or am I required to set a ground bar for the equipment grounding conductors?

A1: Equipment grounding conductors can terminate on the same terminal bar with the grounded (neutral) conductors, but this is only permitted if the panelboard is rated service equipment and it is used as service equipment in accordance with 230.71(A). However, each grounded (neutral) conductor must terminate within the panelboard in an individual terminal [408.21].

Article 100: Service Equipment. The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s) and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control and cutoff of the supply.

Q2. Generator Grounding 250.34
I have a question regarding small 5 kW generator. Let us say that the generator supplies only equipment mounted on the generator, cord-and-plug-connected equipment through receptacles mounted on the generator, or both. Should the generator have a ground rod connection? There is a point on the frame of the generator marked "ground" so it appears that it may be intended, or at least an option.

A2. No, a ground rod is not needed, not required, and would serve no purpose, see 250.34(A). So why does the manufacture provide the ground lug on the generator? Because they do not know any better.

Q3. Transformer Grounding 250.30
I have recently moved to a new project were some of the Journeyman electricians believe that the neutral and grounds should be connected together in a panel that is being fed from a 480V to 120/208V transformer. I have always made this neutral-to-ground connection at the transformer, and kept the grounds and neutrals separated. Which is more correct?

A3. A neutral-to-ground connection shall be made at one location on the secondary of the transformer at any point from the source to the first system disconnecting means or overcurrent device [250.30(A)(1)].

Caution: A bonding jumper connection at both the source and the first disconnecting must be avoided because doing so would create a parallel path where dangerous neutral current could flow over the grounding conductors or grounding paths in violation of 250.6.

Q4. Grounding Remote Building and Structures
NEC 250.32 allows the grounded (neutral) circuit conductor supplying a second building to be bonded to the equipment grounding conductor at the remote building. grounding electrode system of the second building. If the two buildings are 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). Is this a trivial concern? Do you know of any problems that can arise from such a situation such as GFI relays in the main service? All those whom I have discussed this with have dismissed it as trivial. They site residential houses connected via a common water line. I would like to hear your comments.

A4. The practice of bonding the grounded (neutral) conductor to the equipment enclosure is only permitted by 250.32(B)(2) where

(1) an equipment grounding conductor is not run with the supply to the building or structure, and

(2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved, and

(3) ground-fault protection of equipment has not been installed on the common ac service, and

(4) the size of the grounded conductor shall not be smaller than the larger of:

That required by 220.22 (maximum unbalanced neutral load), or

That required by 250.122 (equipment grounding conductor size).

When an equipment grounding conductor is not run to a separate building or structure, the grounded (neutral) conductor must be used 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)].

CAUTION – The use of the grounded (neutral) conductor for equipment bonding is permitted by the NEC. This practice should only be done after careful consideration and a review of the potential safety hazards. Even if the initial installation will not result in an unacceptable parallel path for neutral current flow, there remains the possibility that a future installation of, say, metal piping between the separate buildings or structures, could reverse that situation.

Q5. Grounding Patient Care Areas 517.13(A)
Wiring in patient care areas must be provided with a ground path for fault current by the installation of the circuit conductors in a metal raceway system, or a cable armor or sheath assembly that qualifies as an equipment grounding return path in accordance with 250.118. Can flexible metal conduit or surface metal raceways be used for branch circuits in these areas?

A5. Flexible metal conduit in lengths not exceeding 6 ft can be use for branch circuit wiring in patient care areas of a health care facility, if the circuit conductors contained in the raceway are protected by overcurrent devices rated at 20 amperes or less. Surface metal raceways are also considered suitable for grounding unless there were some marking, UL information that would prohibit its use as a sole grounding path.

Mike Holt's Comment: If you have any comments or feedback, please let me know,

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