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National Electrical Code Articles and Information

Based on the 2011 NEC

by Mark Lamendola

National Electrical Code Top Ten Tips: Article 647 -- Sensitive Electronic Equipment

Why do we have Article 647? Isn't all electronic equipment sensitive? And hasn't somebody done something about that by now?

These are good questions. Things have changed quite a bit since Article 647 was first written. However, where there's a need to reduce electrical noise for electronic equipment within the scope of Article 647 then the requirements of Article 647 apply.

  1. This article covers the installation and wiring of separately derived systems operating at 120V line to line and 60V to "ground" for "sensitive electronic equipment."
     
  2. Article 647 doesn't provide a definition of "sensitive," so is that up to the user? Actually, it is. The defining issue here is whether you wish to reduce electrical noise or not. If so, then the equipment is "sensitive."
     
  3. The "General" section of Article 647 will help you better understand the scope. For example, only industrial and commercial equipment are covered.
     
  4. It's commonly said that the NEC does not provide a voltage drop requirement, only a fine protection note (FPN). That is true in regard to circuits in general (which is why that FPN appears within Chapters 1 through 4, which apply to all circuits). But Articles in Chapters 5, 6, 7, and 8 are special case Articles. And Article 647 applies to the special case of sensitive electronic equipment. It does provide a voltage drop requirement. Voltage drop cannot exceed 1.5% [647.4(D)].
     
  5. For receptacles, the voltage drop can't exceed 1% [647.4(E)].
     
  6. The discussion of "grounding" in 547.6 can be confusing, especially if you accept the Article 100 definition of "grounding" to be valid. The solution to the confusion is to understand this isn't talking about grounding at all. Well-meaning individuals have driven ground rods to provide "separate" or "isolated" circuits they call "ground" and have consequently created a system of groups loops and shock hazards. You could call part of the "ground" the green wire equipment return circuit (GWERC) and the other part the equipment bonding system, if that helps you keep things straight. But there's no grounding.
     
  7. All 15A and 20A receptacles must be GFCI-protected [647.7(A)(1)], though of course this does nothing for reducing electrical noise.
     
  8. You have to keep the receptacles separate from other uses such as lighting [647.7(A)(2)]. This does help reduce electrical noise.
     
  9. You can use "isolated ground" receptacles [647.7(B)], though this practice is usually implemented incorrectly and typically makes matters worse [IEEE-142].
     
  10. Lighting equipment installed for the purpose of reducing electrical noise must comply with 647.8. Before considering such equipment, look at your power distribution. You may find that using oversized neutrals, putting lights on their own transformers and panels, and specifying low noise ballasts will solve your problem. Don't get hooked on low THD as necessarily being the solution, though it might. The point here is that you need to assess your lighting system in its entirety rather than just installing specially designated lighting that must comply with Article 647 and thinking that's going to be the fix. It might not be, or it might be a fix at a higher cost than you need to spend.

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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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