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National Electrical Code Articles and Information

Based on the 2020 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." Article 647 doesn't, in fact, provide any definitions.
  3. The "General" section of Article 647 will help you better understand the scope. For example, only industrial and commercial equipment are covered. That particular limitation doesn't prevent a conscientious installer from applying Article 647 to a residential job as a means of "best practices" for reducing noise to sensitive equipment. However, doing that is often impractical.
  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 on any branch circuit cannot exceed 1.5% [647.4(D)]. And the combined voltage drop of feeder and branch-circuit conductors can't exceed 2.5%. Tip: Run extra fat "backbone" conductors.
  5. For receptacles, the voltage drop can't exceed 1% [647.4(E)]. This is very important to point out, which is why we have a separate line for it here.
  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, except of the supply transformer [547.6(A); it's a source. You ground at the source because electrons are always trying to get back to their source.
  7. All 15A and 20A receptacles must be GFCI-protected [647.7(A)(1)], though of course this does nothing for reducing electrical noise. Toward that end what you can do is ensure you have tight connections at the receptacle terminals. The push-in type are supposed to eliminate errors in connection tension; if you prefer to loop over the screw to make your connection, make a nice bend using needle-nose pliers.
  8. You have to keep the receptacles separate from other uses such as lighting [647.7(A)(2)]. This requirement does help reduce electrical noise. It also helps to try to keep conductors of different circuits spaced apart from each other, to the extent possible. If you are running power to 120V receptacles, make sure those conductors are well-separated from conductors of 277V or 480V circuits and not run parallel to them in raceway. 
  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.