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National Electrical Code Articles and Information

National Electrical Code Articles and Information


National Electrical Code Top Ten Tips: Article 250 -- Grounding

by Mark Lamendola

Based on the 2017 NEC.

Please note, we do quote from copyrighted material. While the NFPA does allow such quotes, it does so only for the purposes of education regarding the National Electrical Code. This article is not a substitute for the NEC.

These are the 10 Article 250 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

  1. 250.1 helps you overcome a very common problem. Most folks are so overwhelmed by 250 that they immediately get lost when confronted with it. However, it’s divided into 6 logical groupings of information and 250.1 tells you what those are. See also Figure 250.1.

    Looking in the Table of Contents in the front of the NEC is also helpful, as you can get an idea of how Articles break down. You can also get an idea of how they relate to each other, since they tend to be grouped (take a look at the sequence of Article names headings in Chapter 3 for a really good example).

  2. 250.2 clarifies things by defining "Effective round-fault current path," Ground fault," and "Ground-fault current path." These definitions, if understood, are not enough for proper application of grounding. You also need to understand the grounding-related definitions in NEC Article 100. Those are bonding (and variations) and grounding (and variations).

  3. 250.3. Another source of panic and confusion when dealing with 250 is that many other NEC Articles apply. The discussions on this issue during the NEC 2002 revision process was on how to address the concern that the NEC is "too complicated" (as if electricity is simple?) and "all of the related information should be in one place" (which would be fine if every application were identical). NEC Table 250.3 handles this issue quite nicely, by providing an substantial cross-reference.

  4. 250.4. This details the general requirements for grounding and bonding. It begins by distinguishing between, and giving requirements for, five categories of grounding: Electrical system grounding, Grounding of electrical equipment, Bonding of electrical equipment, Bonding of electrically conductive materials and other equipment, and Effective ground-fault current path. It also identifies and gives requirements for four categories of ungrounded systems. Figure 250.4 is a great visual for seeing which Parts of 250 apply to various aspects of grounding.

  5. 250.6 addresses another fundamental concept of grounding. That is, the prevention of "objectionable current flow over the grounding conductors or grounding paths."

  6. 250.24(A) says, "A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service…." Electrons are always trying to get back to the source, not (as many wrongly believe) to ground. The rest of 250.24 details requirements for doing this for different applications.

  7. 250.28. You need a main bonding jumper. "For a grounded system, an unspliced main bonding jumper shall be used to connect the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor of the system within the enclosure for each service disconnect. There are two exceptions to this, but in no case can you use the earth as your bonding jumper—its resistance is simply too many orders of magnitude too high.

  8. 250.34 discusses portable and vehicle-mounted generators. A good reference for understanding why these would differ from stationary systems is IEEE-142.

  9. 250.52 gives the requirements for grounding electrodes. This is a more complex topic than most people think. IEEE-142 gives a thorough theoretical treatise of it. The NEC just gives the minimal requirements for safety.

  10. 250.58 instructs us to use "the same electrode for grounding conductor enclosures and equipment in or on that same building." The concept of "separate ground" is nonsense. Two good sources for more information on this are Soares Book on Groundingand IEEE-142.

We could easily address 10 more "top tips" for Article 250. For example, Section V on Bonding has plenty of good information. However, the purpose of this article is to cover fundamentals in a quick and easy-to-read manner.

To gain a solid understanding of 250, you need to set aside a specific amount of time each week—maybe a 30-minute study session every other night, or maybe fifteen minutes at lunch each day—and tackle one Section at a time. Supplement that by reading Soares Book on Grounding and IEEE-142.

Check out this grounding case history!


How the NEC is arranged

  1. The first four Chapters of the NEC apply to all installations.
  2. Article 90 precedes Chapter One, and establishes the authority of the NEC.
  3. Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
  4. Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
  5. Chapter 8 provides the requirements for communications systems.
  6. Chapter 9 provides tables.
  7. The appendices provide mostly reference information.
  8. Appendix D contains examples that every NEC user should study.

Try your NEC moxy:

  • Do you know the difference between bonding and grounding? Hint: Look in the NEC, Article 100.
  • Does the NEC refer to grounding incorrectly in any of its articles? Yes! So be careful to apply the Article 100 definitions. Don't ground where you should bond.
  • When doing motor load calculations, which Article covers hermetic motors? Answer: While Article 440 covers the application of hermetic motors, it does so only by amending Article 430 because hermetic motors are a special case of motors. For motor load calculations, refer to Article 430.
  • Does the NEC provide a voltage drop requirement? Yes! It does so in a special case, which is Article 648 Sensitive Electronic Equipment. But for general applications, it does not provide a requirement; it merely provides a recommendation in a couple of FPNs.
  • Take our Code Quizzes.

Remember other applicable codes, rules, standards, and references:

  • OSHA's electrical worker safety rules.
  • IEEE standards.
  • NETA standards.
  • NFPA standards.
  • International Codes (if applicable to the installation).
  • State Codes (if the state has them).
  • Local ordinances and permit requirements.
  • Local fire codes.
  • Manufacturer requirements or guidelines.
  • Customer security requirements.
  • Industry standards.
  • Your company's own internal standards, practices, and procedures.
  • Engineering drawing notes.


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