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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 504 -- Intrinsically Safe Systems

  1. Article 504 covers the installation of intrinsically safe apparatus in Class I, Class II, and Class III locations (defined in Article 500.5) [504.1].
  2. An intrinsically safe circuit is one that won't ignite due to spark or thermal effect of (flammable or combustible) material in air under prescribed test conditions [504.2].
  3. If it's part of an intrinsically safe installation, you must install it per the control drawing(s) [504.10(A)].
  4. You can use a general enclosure with intrinsically safe apparatus [504.10(B)].
  5. You can use any of the wiring methods suitable for unclassified locations, when installing intrinsically safe apparatus [504.20].
  6. You must separate intrinsically safe conductors from nonintrinsically safe conductors [504.30(A)]. This requirement is the core of Article 504 and its details make up about half of Article 504.
  7. You must connect intrinsically safe apparatus, enclosures, and raceways (if metallic) to the equipment grounding conductor [504.50].
  8. Bond all metallic objects of an intrinsically safe system, per 250.100 [504.60]
  9. Enclosures that contain only intrinsically safe apparatus don't have to be sealed (except as required by 501.15(F)(3)) [504.70].
  10. Intrinsically safe circuits must be identified at terminal and junction locations, in a manner that prevents unintentional interference with the circuits during testing and servicing [504.80].

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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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