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National Electrical Code Articles and Information

Based on the 2017 NEC

by Mark Lamendola

National Electrical Code Top Ten Tips: Article 501 -- Class I Locations

  1. Read Article 500 before reading Article 501. Why? Because Article 500 provides the basis for interpreting and correctly applying Articles 501 - 516. For one thing, you will find the definitions for those Articles in Article 500. So, do not work with Article 501 until you have read and understood Article 500.
  2. Class I locations are those in which flammable gases or vapors are (or may be) present in sufficient quantities to produce explosive or ignitible mixtures [501.1].
  3. Class locations are further broken down into Division 1 (normal operations) and Division 2 (abnormal operations). That is, point #2 above applies in normal or abnormal conditions.
  4. You must use Division 1 wiring methods when combustibles are present under normal operations [501.10(A)].
  5. You must use Division 2 wiring methods when combustibles are present under  abnormal operations [501.10(B)].
  6. Seal requirements for Class I locations [501.15(A)] are highly detailed and far more extensive than those for Class II locations [502.15(B)]. Do not confuse the two.
  7. Any electrical parts that operate at more than 30V can't be exposed, but this drops to 15V under wet conditions. Further, you must apply the appropriate protection technique from 500.7(E), (F), or (G) to these parts [501.25].
  8. The grounding and bonding requirements for Class I locations are in 501.30. If you ground where you should, instead, bond, you will create a difference of potential that violates 501.30 and will pose a threat to people and property. To avoid catastrophic consequences, read the definitions of grounding and bonding in Article 100, and take some time to study Article 250, Part V.
  9. Surge protection is a good idea for Class I locations, for obvious reasons. But it must comply with 501.35, and the rules are different for Division 1 versus Division 2.
  10. Any luminaire used in a Class I, Division 1 location must be identified as a complete assembly for Class I, Division 1 locations [501.130(A)(1)]. Any luminaire used in a Class I, Division 2 location must meet some conditions that the typical person in the field can't possibly conform to with any certainty, or it must conform to the requirements for a Class I, Division 1 location [501.130(B)(1)].


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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