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National Electrical Code Top Ten Tips: Article 424 -- Fixed Electric Space Heating Equipment

by Mark Lamendola

Based on the 2020 NEC

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.

These are the 10 NEC Article 424 items we deem most important, based on the pervasiveness of confusion and the potential costs of same. Fixed electric space heaters differ from portable ones in multiple ways. For example, portable ones are usually designed to sit on the floor while fixed ones are usually designed to be affixed to the wall or suspended from overhead. Portable ones are usually designed to plug into a 120V, 15A receptacle. Fixed ones are nearly always hard-wired. In commercial and industrial settings, their branch circuits are often a single phase of a 3-phase system just as lighting loads often are.

  1. Even though you use heat intermittently, any fixed heater must be considered a continuous load for purposes of branch circuit sizing [424.2(B)]. That brings the 125% rule into play.
  2. If there's not enough space to allow for combustible materials, you can't install the heater [424.13]. How do you know what that space is? Different heater designs will have different space limits, and this limit will be stated in multiple places including the installation instructions. Note that if a combustible is something like a fuel container, the fumes from that not just the container, must be outside the space limit. Also, combustibles are often portable; this means just because you don't see combustibles inside the space limit today that doesn't mean they won't be there tomorrow. The NEC does not explicitly require it, but use some method to identify the "no combustibles" zone. For example, paint the floor red and/or permanently mount permanent signs stating "No combustible within 10 feet of this sign) or some such.
  3. You must provide a disconnecting means within sight of the heater [424.19]. Install it in such a way that the operator does not have to stand directly in front of it to interrupt current.
  4. Thermostatic controllers can also serve as the disconnecting means, provided they comply with 424.20.
  5. All switches and breakers used as a disconnecting means must be of the indicating type [414.21].
  6. Don't assume overcurrent protection automatically meets code. Carefully examine 424.22 for each installation. This takes up a bit more than half a page. Note the difference between the branch circuit OCPD [424.22(A)] and a supplementary OCPD [424.22(C)].
  7. If you install space heating cables, note that the NEC devotes Part V of Article 424 to this equipment.
  8. If you install duct heaters, note that the NEC devotes Part VI of Article 424 to this equipment.
  9. If you install resistance-type boilers, note that the NEC devotes Part VII of Article 424 to this equipment.
  10. If you install electrode-type boilers, note that the NEC devotes Part VIII of Article 424 to this equipment.