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National Electrical Code Articles and Information

NEC Quiz: Article 393 Answers

by Mark Lamendola

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  1. This isn't a wiring method in the sense we typically think of one. It's the low-voltage suspended ceiling power distribution system [393.1], and such a system replaces wiring methods. It does away with a lot of ad hoc design and greatly simplifies installation. The result is a neater, more reliable installation at a lower cost. At the heart of it is a "box" that serves as a sort of hub for distributing power to utilization equipment supplied by a Class 2 power supply. See my article on these systems in the May, 2014 edition of EC&M Magazine,

  2. First of all, it has to be permanently connected [393.10]. It can supply listed utilization equipment operating at up to 30VAC or 60VDC in dry locations indoors. It can be used in residential, commercial, and industrial applications. It can be used in other spaces used for environmental air (see 300.22(C)).

  3. You can't use it in damp or wet locations, where subject to corrosive fumes or vapors, where subject to physical damage, in concealed locations, in hazardous locations, as part of a fire-rated assembly (unless specifically listed as part of the assembly), or for lighting in general (or critical patient) care areas.

  4. Use the mounting hardware that comes with it or the mounting fasteners specified in the instructions [393.30]. You can secure it to the mounting surface of the building by hanging wires, screws, or bolts in accordance with the installation instructions.
  5. Use listed insulating devices when making connections to busbar grid rails, cable,s and conductors. The connections must remain accessible (see Article 100 definition of accessible). You can use any of the four connector types specified in 393.40(A).



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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