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NEC Quiz: Article 250, Part Eight Answers

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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.

 

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