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2005 NEC Changes Summary Article 250

The title of Article 250 was changed from "Grounding" to "Grounding and Bonding" so that it's consistent with the scope of Article 250.

Article 250. Grounding and Bonding

No other article can match Article 250 for misapplication, violation, and misinterpretation. In fact, we commonly see Article 250 violations as requirements! For example, many industrial equipment manuals require violating 250.24(A)(5) as a condition of warranty. The manuals insist on installing an "isolated grounding electrode," which is an electrode without a low-impedance path back to the source neutral, other than through the earth itself. That means the ground-fault current return path to the source neutral (utility transformer) is on the order of several ohms rather than a fraction of an ohm that the typical NEC-compliant installation would provide.

If you apply basic physics and basic electrical theory, you can clearly see Article 250 is right and equipment manuals that require isolated grounding are wrong and other references agree. IEEE-142 and Soares Book on Grounding use the same physics and electrical theory as Article 250. This article isn't a "preferred design specification." As with the rest of the NEC, it serves the purpose stated in Article 90 to be sure the installation is, and remains, SAFE!

Article 250 covers the requirements for providing paths to divert high voltage to the earth, requirements for the low-impedance fault current path to facilitate the operation of overcurrent protection devices, and how to remove dangerous voltage potentials between conductive parts of building components and electrical systems.

Over the past two Code cycles, this article was extensively revised to make it better organized and easier to implement. It's arranged in a logical manner, so it's a good idea to just read through Article 250 to get a big picture view-after you review the definitions. Then study the article closely so you understand the details. The illustrations will help you understand the key points.

Author's Comment: The grounding and bonding requirements contained in this textbook apply to systems that operate at not more than 600V, such as 120/240V, 120/208V, and 277/480V. The NEC permits other system configurations, such as corner-grounded delta systems, ungrounded systems, or high-impedance grounded neutral systems, but they are typically limited to three-phase industrial applications and are not addressed in this textbook.

Author's Comment: To help keep the subject of Grounding versus Bonding straight in this textbook, we have chosen to provide the following color-code borders to each of the graphics:

  • Dark Orange Border - Grounding to the earth, typically for lightning. Internationally this is called "Earthing."
  • Green Border - Relates to the concept of bonding to provide low-impedance path for fault current to the power supply. Often times the NEC uses the term Grounding, when in fact they intend that the metal parts be bonded to the power supply neutral.
  • Yellow Border - Indicates an improper installation where neutral return current flows on the metal parts of the electrical installation in violation of 250.6.
  • No Color Border - Indicates that the graphic is important, but that it's not directly related to grounding (orange border) or bonding (green border).

Part I. General

  • Additional text helps the Code user understand that the purpose of the effective ground-fault current path is to help clear a ground-fault by facilitating the operation of the overcurrent device.
  • Change clarifies that swift operation of an overcurrent protection device in the presence of a ground-fault depends upon the existence of an effective ground-fault current path from the point of the fault to the power supply neutral.
  • The last sentence was revised to make it clear that the earth cannot be used as the effective ground-fault current path. It will not facilitate the opening of the circuit protection device from a ground fault.
  • The last sentence was revised to make it clear that the earth isn't to be used as the effective ground-fault current path needed to facilitate the opening of the circuit protection device in the event of a ground fault.
  • Revised text clarifies that terminals for grounding conductors or bonding jumpers must be attached to enclosures in a suitable method to ensure an effective ground-fault path.

Part II. System Grounding (Bonding)

  • To avoid confusion when dealing with bonding jumpers, the requirement for the system bonding jumper was added to this section.
  • Text clarifies the method of sizing equipment bonding jumpers when the separately derived system conductors are run in parallel. And the text relating to the "common" grounding electrode conductor for multiple separately derived systems was changed to require this conductor not to be sized smaller than 3/0 AWG.
  • A new sentence was added to the exception to clarify that a grounding electrode isn't required at a remote building or structure supplied by a single multiwire branch circuit.
  • Revision clarifies when portable and vehicle-mounted generators are not required to be grounded to a grounding electrode.

Part III. Grounding Electrode System and Grounding Electrode Conductor

  • The words "if available" have been replaced with "are present." The effect is that a concrete-encased electrode (Ufer) is always required for new construction, because it is present. However, a new exception adds clarity that a concrete-encased electrode [250.52(A)(3)] isn't required for existing buildings or structures.
  • The language in the 2002 NEC was vague and subject to wide interpretation as it described when the metal frame of a building or structure could serve as a grounding electrode. The new text establishes the requirement for the structural metal electrode. In addition, listed galvanized rods of the 1/2* in. diameter are now permitted.
  • New FPN reminds the Code user that gas piping that is likely to become energized must be "bonded" in accordance with the 250.104(B).
  • Revision to help the Code user understand that an equipment grounding (bonding) conductor is intentionally constructed to assist in clearing ground faults, and that the earth never will serve this purpose.
  • New text permits the grounding electrode conductors and bonding jumpers to terminate to a busbar as an alternative method.
  • The revisions clarify when the raceway for the grounding electrode conductor is required to be bonded at each raceway/cable termination between the service equipment and the grounding electrode. And additional text clarifies how to size the bonding jumper for the grounding electrode conductor raceway.

Part V. Bonding

  • The text was revised to clarify the bonding requirements of metal raceways that enter a hazardous (classified) location. An additional sentence was added to require the metal raceway to be bonded, whether or not an equipment grounding (bonding) conductor is installed in the metal raceway.
  • The words "that may become energized" were replaced with "is likely to become energized" to editorially clarify when the metal piping system, including gas piping, is required to be bonded to an effective ground-fault current path to remove dangerous touch voltage from a ground fault.
  • The words "that may become energized" were replaced with "is likely to become energized" to editorially clarify when structural metal is required to be bonded to an effective ground-fault current path to remove dangerous touch voltage from a ground fault.
  • The structural metal member bonding requirements that were located in 250.30(A)(3)(d) and 250.104(A)(1) were combined and relocated to 250.104(D) to improve NEC usability.

Part VI. Equipment Grounding (Bonding) and Equipment Grounding (Bonding) Conductors

  • New text specifies when an equipment (bonding) conductor is required for flexible metal conduit and liquidtight flexible metal conduit.
  • New sentence to make it clear that conductors that have an outer finish that is green or green with one or more yellow stripes cannot be used for ungrounded or grounded (neutral) conductors.
  • New exception removes the identification of some equipment grounding (bonding) conductors in conduit bodies.
  • New rule describes how the equipment grounding (bonding) conductors for feeder taps are to be sized.

Part VII. Methods of Equipment Grounding (Bonding)

  • The section on grounding (bonding) of ranges, ovens, and clothes dryers was rewritten to make it easier to understand and apply.
  • New sentence clarifies how to ensure direct metal-to-metal contact between the metal mounting yoke of a receptacle and the metal box when a bonding jumper isn't installed to the receptacle.

Mike Holt's Comment: If you desire more information about any of the above changes, be sure to order my Changes book and / r library (Video/DVD).

2005 NEC Changes, Part 1, Articles 90-314, DVD

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