Practically Speaking: A Classic Example of Intent Versus the Literal Meaning

When reading Sec. 310.12(B) of the 2026 Code, it is imperative to know the intent behind the revisions made to the last sentence. In order to figure that out, we will need to look at the public input (PI) submissions for that revision. PI No. 447 was submitted with the intent to prohibit using Table 310.12(A) for the sizing of NM cables. However, I’m not so sure the literal wording in the Code quite gets there. Let’s take a deeper look.

The person who submitted PI 447 wanted to add the words “for conductors not limited to the 60C ampacity” to the last sentence of Sec. 310.12(A). Since Sec. 334.12(3) prohibits NM cables from being used as service-entrance cable, and Sec. 230.43 does not recognize using NM cable as a wiring method for services, any revisions to Sec. 310.12(A) would not affect NM cable. Revisions to Sec. 310.12(B), however, absolutely could change the requirements for sizing feeders when using NM cable. CMP-06 made changes to Sec. 310.12(B) with First Revision No. 7953 based on the wording submitted for PI 447. The last sentence of Sec. 310.12(B) now states, “If no adjustment or correction factors are required, Table 310.12(A) shall be permitted to be applied for conductors or cables rated 75°C (167°F) or greater.” That same wording was also added to Sec. 310.12(A) via First Revision No. 7943. This language is quite different than what was suggested in PI 447 — and that is where it gets tricky.

According to Sec. 334.112, NM cable is constructed with conductors rated 90°C. The product standard for NM cable, ANSI/UL 719, Standard for Nonmetallic-Sheathed Cables, states that both NM cable and NMC cable carry the suffix letter “-B” to indicate the use of conductors with 90°C insulation. Knowing this and then reading Sec. 310.12(B), one might conclude that Table 310.12(A) could in fact be used for sizing NM cable feeders, since NM cable is constructed with conductors having insulation rated 90°C. However, that is not the intent. While Sec. 334.80 limits the ampacity of NM cable to that of a 60°C conductor, the conductors are in fact rated 90°C. The copper power conductors in NM cables range in size from as small as 16 AWG to as large as 2 AWG. Using NM cables for dwelling unit feeders may be perfectly fine, but when determining which size to use, I recommend not using Table 310.12(A) based on the intent of PI 447. Instead, use Table 310.16 for sizing those NM cable feeders.