Here's a question for you. What NEC Articles are the most important for proper surge protection? If you answered, “Art. 250, 280, and 285,” then you're correct. But do you know where these apply, and do you know what other areas of the NEC also have bearing on this issue?

The answer to the second part of that question is “Chapters 1-4, and Art. 90.” In Chapter 1, for example, people tend not to pay much attention to Art. 100. This single fact explains the existence of many power quality problems. You understand why, when you read the Art. 100 definitions of grounding and bonding. Mix these up, and your surge protection devices act more like surge recirculation devices.

Now, where do these three Articles apply?

Because of the energy levels involved, Art. 280 generally applies to wiring outside the facility. An example is a spark gap arrester on the service drop. Article 285 nearly always applies to wiring inside the facility. One example is an SPD unit on an internal power distribution panel.

For the purposes of surge protection, think of Art. 250 as having two components:

  • Grounding, covered by Parts II and III. These parts are analogous to Art. 280 in some ways, and generally do not apply to the load side of the service connection. If you have ground rods on the load side, then you probably have NEC violations that increase equipment failure and decrease safety.
  • Bonding, covered by Parts IV, V, VI, and VII. The NEC is confusing in this regard, because the titles of all but Part V refer to grounding, but they mean bonding.