Code Change Summary: Section 310.12 clarifies correction and adjustment factor application and now limits the use of Table 310.12(A) to conductors or cables rated 75°C or greater.

SME commentary: SME commentary: The 2026 NEC^® introduced important refinements to Section 310.12, which governs the sizing of single-phase dwelling service and feeder conductors. The first change involves the relocation of language from 310.12(D) into the parent text of 310.12. Although largely editorial, this code change clarifies that correction and adjustment factors apply to all service and feeder conductors, not just grounded conductors.

In the 2023 NEC^®, the provision allowing correction and adjustment factors to be applied to the ampacity associated with the conductor’s temperature rating appeared only in subdivision (D), which addressed grounded conductors. This placement created confusion, since the allowance was always intended to apply broadly to all service and feeder conductors. In the 2026 NEC^®, the rule has been relocated into the main body of 310.12, ensuring that both grounded and ungrounded conductors are covered under the same requirements for applying the factors in 310.15(B) and 310.15(C).

The more substantive revisions occur in 310.12(A) and 310.12(B). Both sections now include nearly identical language clarifying two critical points: how correction and adjustment factors are applied, and the minimum insulation temperature rating required to use Table 310.12(A). The new text specifies that Table 310.12(A) is permitted only when conductors or cables have insulation rated 75°C (167°F) or greater. This closes the previous gap in clarity about whether 60°C rated conductors could qualify. Because Table 310.12(A) allows a reduced ampacity of not less than 83 percent of the service or feeder rating, it is now explicit that this reduced sizing cannot be applied to 60°C conductors.

This clarification has significant design and installation implications. For example, a dwelling service rated at 200 amperes may be sized at 83 percent of that value using Table 310.12(A), but only if the selected conductors are rated 75°C or higher. Installations using 60°C rated conductors must instead be sized from the standard ampacity tables without applying the reduced factor. This ensures that conductor insulation is suitable for the thermal conditions when using the table, reducing the risk of overheating or premature insulation failure.

In practical terms, electricians, designers, and inspectors must now verify the insulation ratings of conductors before applying Table 310.12(A). The NEC^® has eliminated ambiguity by stating that 60°C rated conductors are not eligible for the reduced sizing from the table, whether in service or feeder applications. By relocating the correction and adjustment factor rule and adding the explicit 75°C requirement, the 2026 NEC^® strengthens clarity, prevents misapplication, and promotes safe, consistent practices for dwelling service and feeder installations.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2026 NEC^® edition of NFPA 70.

2023 Code Language:

310.12 Single-Phase Dwelling Services and Feeders…

310.12(D) Grounded Conductors. Grounded conductors shall be permitted to be sized smaller than the ungrounded conductors, if the requirements of 220.61 and 230.42 for service conductors or the requirements of 215.2 and 220.61 for feeder conductors are met.

Where correction or adjustment factors are required by 310.15(B) or (C), they shall be permitted to be applied to the ampacity associated with the temperature rating of the conductor.

310.12(A) Services. For a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating. If no adjustment or correction factors are required, Table 310.12(A) shall be permitted to be applied.

2026 Code Language:

310.12 Single-Phase Dwelling Services and Feeders. For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.12(A) through 310.12(D).

For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of two ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permitted to be sized in accordance with 310.12(A) through 310.12(C).

Where correction or adjustment factors are required by 310.15(B) or 310.15(C), they shall be permitted to be applied to the ampacity associated with the temperature rating of the conductor.

310.12(A) Services. For a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating. 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.

310.12(B) Feeders. For a feeder rated 100 amperes through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating. 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.