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National Electrical Code Articles and Information

Based on the 2008 NEC

by Mark Lamendola

National Electrical Code Top Ten Tips: Article 506 -- Zone 20, 21, and 22 Locations

  1. Article 506 covers the requirements for the zone classification system as an alternative to the division classification system covered in Articles 500, 502, and 503 for Zone 20, 21, and 22.
  2. Dust-ignitionproof refers to keeping equipment enclosed in a manner that keeps sparks and flames contained within the enclosure and keeps dust out [506.2].
  3. If you use the zone system, you must generate and maintain specific documentation [506.4]. The documentation requirements are far from casual.
  4. Exactly how you may classify a location depends on the properties of the ignitible fibers or flyings that may be present and on their concentration [506.5].
  5. Summary of Zone 20 definition: Ignitible fibers or flyings are consistently present and are present for long periods [506.5(B)(1)].
  6. Summary of Zone 21 definition: Ignitible fibers or flyings are likely to exist under normal operations, during repair, or during breakdown [506.5(B)(2)].
  7. Summary of Zone 22 definition: Ignitible fibers or flyings are not likely to exist under normal operations [506.5(B)(3)].
  8. Note that we have oversimplified the definitions of Zone 20, 21, and 22 so that you can easily see how they differ. Read the references thoroughly to understand the implications for a given location.
  9. Dual classification and reclassification are permitted [506.6].
  10. Article 506 lists 11 protection techniques. Five of those are new with the 2008 Revision. Two of the new techniques are encapsulation techniques (maD and mbD). The other three are protection by enclosure, pressurization, and intrinsic safety. Only a qualified person can determine the area classification and only a qualified person can determine which mix of protection techniques to apply to a given installation.

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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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