National Electrical Code Top Ten Tips: Article 310 -- Conductors for General Wiring

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.

These are the ten Article 310 items we deem most important, based on the pervasiveness of confusion and the potential costs of same. Article 310 hugely changed from the 2008 revision (the original writing of this article) to the 2017 revision. The 2020 revision introduced relatively small changes.

Article 310 is arguably the most misunderstood and misapplied area of the National Electrical Code. The same claim can be made for Article 250 (grounding) and Article 430 (motors). Our intent here is to help clear up some of the mystery, but a true understanding requires dedicated study. Do keep in mind ampacity is the amperage capacity of a conductor—the lower the ampacity, the larger the conductor must be to handle a given current (though this is in a step fashion, as conductors come in standard sizes).

  1. What do those conductor jacket letters, such as RHH or THHN mean, and what are the application provisions? You will find the answer in Table 310.4(A). It's a big, multi-page table.
  2. Conductors must be identified in specific ways, depending upon whether they are grounded [310.6(A)], equipment grounding conductors [310.6(B)], or ungrounded conductors [310.6(C)].
  3. 310.10 provides an overview of permitted uses. For example, it lists which types of insulated conductors can be used in dry locations and provides yet another list for wet locations. In the 2008 revision, this information was in 310.8. In the 2020 revision, 310.8 is all about marking.
     
  4. 310.10(G) provides the requirements for conductors in parallel. The gist of these requirements is that parallel conductors need to be identical: same material, same length, same wire gage, same ampacity adjustments, etc.
     
  5. Once place people get confused in the ampacity tables is they don't select the correct temperature column. Read and understand 310.14(A)(3), 310.15(A) and 310.15(B)before attempting to use these tables.
     
  6. See and apply Table 310.15(B)(1) and Table 310.15(B)(2) for the ambient temperature correction factors when determining the adequacy of your conductor insulation for the application, before selecting the temperature column when using the ampacity tables.
     
  7. Table 310.18 applies to situations where you have three or fewer current-carrying conductors in a single wireway. This is typical for services and feeders, but not very typical for branch circuits. You must select from the column that shows the cable (identified by the insulating material letter designation) you intend to use, and choose between copper and aluminum.
  8. Table 310.17 and Table 310.19 apply to situations where you have single-insulated conductors (that doesn’t mean a single conductor—it means the conductor isn’t insulated twice, as would be the case if it’s in an insulated sheath with other conductors) in free air. This is typical for branch circuits. You must select from the column that shows the cable (identified by the insulating material letter designation) you intend to use, and choose between copper and aluminum.
  9. Table 310.20 applies to conductors on a messenger.
     
     
  10. Table 310.21 provides the allowable ampacities of bare or covered conductors in free air..