National Electrical Code Top Ten Tips: Article 242 -- Overvoltage Protection
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.
These are the 10 NEC Article 242 items we deem most important, based
on the pervasiveness of confusion and the potential costs of same.
- Article 242 is entirely new with the 2020 NEC. It consists of three Parts: General, Surge-Protective Devices (SPDs) 1000V or less, and SPDs over 1,000V.
- The General requirements are thin, to say the least. There's the statement of scope (summed up in the second sentence above) and a requirement to refer to Table 242.3.
- Table 242.3 lists other Articles that may apply. There are 15 listed.
SPDS rated 1000V or less:
- SPDs may be Type 1, Type 2, Type 3, or Type 4. The definitions are in Article 100, under "Surge-Protective Device (SPD". Types 1 - 3 are differentiated by where they are installed. Type 4 are component SPDs and assemblies. See UL 1449 for more information about these Types.
- Each type of SPD, as you may have guessed, has its particular requirements. See 242.12, 14, 16, and 18.
- SPDs must be inaccessible to unqualified people unless listed for installations in accessible locations [242.22].
SPDs rated for over 1000V:
- SPDs cannot be installed where the SPD rating is less than the maximum continuous phase-to-ground voltage at the power frequency available at the point of application [242.10].
- Where used at a point on a circuit, there must be an SPD for each ungrounded conductor [2424.44].
- You must connect the arrestor to a grounded service conductor, grounding electrode conductor, grounding electrode for the service, or equipment grounding terminal in the service equipment [242.50]. Do you see what's missing here? That's right, you can't just drive a ground rod and connect to that. Why? Because electricity seeks to get back to the source (thus, grounding electrode for the service). Simply driving a ground rod creates a dangerous multi-path of varying impedances to the source.
- You make different kinds of interconnections based on the various conditions listed in 242.54. For example, if you have a multi-grounded neutral primary system there has to be at least one other ground one the ground conductor of the secondary (among other requirements) [242.54(B)(2)].
Check out this grounding