National Electrical Code Top Ten Tips: Article 285 -- Transient Voltage Surge Suppressors: TVSS

Based on the 2020 NEC

by Mark Lamendola

Please note, we do quote from copyrighted material. While the NFPA does allow such quotes, it does so only for the purposes of education regarding the National Electrical Code. This article is not a substitute for the NEC.

With the 2020 revision, Articles 280 and 285 were deleted and replaced by Article 242. Below is information that was current as of the 2017 NEC. It is no longer current as of the 2020 NEC.

These are the 10 NEC Article 285 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

  1. Article 285 addresses transient voltage surge suppressors (TVSSs), while NEC Article 280 addresses surge arresters. These are not the same, though they both provide surge protection.

  2. Surge arresters apply to (in most cases) just the supply side (line side) of the meter (280.22). TVSS devices apply to just the load side (285.24). Some devices are listed for use in either location, but typically surge protection devices are designed for use in only one or the other.

  3. TVSS devices (SPDs) reduce potentially damaging short-duration spikes on main power sources in a facility. They are not a substitute for lightning protection--they address voltage levels that lightning protection does not, and therefore are complementary to it.

  4. The general hierarchy is this: lightning protection (service), TVSS (service and feeders), surge arrester (feeder and branch circuits), point of use surge protection (provided by plug-in devices, UPS systems, line conditioners, and so on).

  5.  When you use a TVSS device, you must connect it to each ungrounded conductor. The logic here is pretty obvious, but if you don't find it obvious just thinking about it then draw out the circuit.

  6. Don't get confused over TVSS ratings. More isn't necessarily better. While the NEC doesn't require you to think out the system, reality does. A staged system is almost always the only way to go.

  7. You can put TVSSs indoors or outdoors. It simply is not true that the NEC prohibits locating them outdoors. But, you must make them inaccessible to unqualified persons [285.11].

  8. TVSS has strict wiring requirements, to deal with high frequency. Do not violate these and expect protection.

  9. Connecting your TVSS to a ground rod driven into the dirt is not sufficient. The electricity is always trying to get back to the source--that's why electricity works. You must ground per the applicable requirements of Article 250 [285.28].

  10. TVSS is essentially useless without good grounding and bonding.

Check out this grounding case history!