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NEC Quiz: Article 250, Part Eight Answers

by Mark Lamendola

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  1. [250.50]. If you said "separated," you're wrong. All grounding electrodes must be bonded together. And actually, it isn't possible to "separate" electrodes. Read IEEE-142 for a comprehensive explanation. Before doing that, review Kirchoff's Law of Parallel Circuits and look at soil resistivity data.
     

  2. [250.52(A)]. There are actually eight: metal underground water pipe, metal frame of the structure, concrete-encased electrode, ground ring, rod and pipe electrode, other Listed electrodes, and other local metal underground systems or structures.
     

  3. [250.52(B)]. Metal underground piping systems. However, these must be bonded to the grounding electrode system to prevent flashovers.
     

  4. [250.52(B)]. Aluminum.
     

  5. [250.53(B)]. Six feet, assuming 10 foot rods. The reason is shell theory, which basically says that if the rods as the rods get closer than a certain distance they cancel each other out. While the NEC states six feet as a minimum, longer is better. So, for example, if you drive a second rod and you have 35 feet until you reach the end of the wall you are working near it's a good design practice to go 35 feet between rods. The extra copper in the ground, if buried properly, will serve as an additional electrode.

 

 

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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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