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.
- This article covers the installation and wiring of separately derived
systems operating at 120V line to line and 60V to "ground" for "sensitive
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- You can use "isolated ground" receptacles [647.7(B)], though this practice is usually implemented incorrectly and typically makes matters worse [IEEE-142].
- 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.