How have 2023 NEC updates changed NM and NMC cable installation rules?
Overview
Nonmetallic (NM) and nonmetallic-sheathed (NMC) cables are widely utilized in residential and commercial wiring owing to their distinct advantages. NM cables are suited for dry, indoor applications, while NMC cables offer versatility for use in dry, moist, damp, and corrosive environments. According to Section 334.10, these cables are permitted in one- and two-family dwellings, as well as their associated storage buildings and garages, and in multi-family dwellings and other structures of Types III, IV, and V construction. In the 2023 version of the NEC, Section 334.10 has been expanded to include the use of NM and NMC cables in detached garages of multi-family dwellings. This update enhances the flexibility and options available for wiring in residential projects involving multi-family dwellings.
Applying the 2023 Code
Nonmetallic (NM) and nonmetallic-sheathed (NMC) cables are extensively employed in both residential and commercial electrical installations due to their versatility and reliability. NM cables consist of insulated conductors and bare ground wires inside nonmetallic sheaths, making them well-suited for dry, indoor applications where moisture exposure is minimal. On the other hand, NMC cables feature a moisture and corrosion-resistant sheath, which enhances their suitability for a broader range of environments, including dry, moist, damp, and corrosive conditions.
Section 334.10 in previous NEC versions outlined the permissible uses of NM and NMC cables. It allowed these cables to be installed in one- and two-family dwellings, including their storage buildings and both attached and detached garages, as well as in multi-family dwellings and other structures classified under Types III, IV, and V construction. These installations required the cables to be concealed within walls, floors, or ceilings that provided a 15-minute fire-resistance rating. The 2023 version of the NEC has expanded Section 334.10 to enhance flexibility further. Now, NM and NMC cables are also permitted in detached garages of multi-family dwellings that fall under Types III, IV, and V construction. This update facilitates greater versatility in the wiring of residential projects involving multi-family dwellings.
What’s New for the 2023 NEC?
The table below presents a comparison between the 2020 NEC and the 2023 NEC regarding Section 334.10.
2020 NEC |
2023 NEC |
| 334.10 Uses Permitted
Type NM and Type NMC cables shall be permitted to be used in the following, except as prohibited in 334.12: (1) One- and two-family dwellings and their attached or detached garages, and their storage buildings. (2) Multi-family dwellings permitted to be of Types III, IV, and V construction. (3) Other structures permitted to be of Types III, IV, and V construction. Cables shall be concealed within walls, floors, or ceilings that provide a thermal barrier of material that has at least a 15-minute finish rating as identified in listings of fire-rated assemblies. (4) Cable trays in structures permitted to be Types III, IV, or V where the cables are identified for the use. (5) Types I and II construction where installed within raceways permitted to be installed in Types I and II construction. (A) Type NM Type NM cable shall be permitted as follows: (1) For both exposed and concealed work in normally dry locations except as prohibited in 334.10(3) (2) To be installed or fished in air voids in masonry block or tile walls (B) Type NMC Type NMC cable shall be permitted as follows: (1) For both exposed and concealed work in dry, moist, damp, or corrosive locations, except as prohibited by 334.10(3) (2) In outside and inside walls of masonry block or tile (3) In a shallow chase in masonry, concrete, or adobe protected against nails or screws by a steel plate at least 1.59 mm (1/16 in.) thick and covered with plaster, adobe, or similar finish |
334.10 Uses Permitted
Type NM and Type NMC cables shall be permitted to be used in the following, except as prohibited in 334.12: (1) One- and two-family dwellings and their attached or detached garages, and their storage buildings. (2) Multi-family dwellings and their detached garages permitted to be of Types III, IV, and V construction. (3) Other structures permitted to be of Types III, IV, and V construction. Cables shall be concealed within walls, floors, or ceilings that provide a thermal barrier of material that has at least a 15-minute finish rating as identified in listings of fire-rated assemblies. (4) Cable trays in structures permitted to be Types III, IV, or V where the cables are identified for the use. (5) Types I and II construction where installed within raceways permitted to be installed in Types I and II construction. (A) Type NM Type NM cable shall be permitted as follows: (1) For both exposed and concealed work in normally dry locations except as prohibited in 334.10(3) (2) To be installed or fished in air voids in masonry block or tile walls (B) Type NMC Type NMC cable shall be permitted as follows: (1) For both exposed and concealed work in dry, wet, damp, or corrosive locations, except as prohibited by 334.10(3) (2) In outside and inside walls of masonry block or tile (3) In a shallow chase in masonry, concrete, or adobe protected against nails or screws by a steel plate at least 1.59 mm (1/16 in.) thick and covered with plaster, adobe, or similar finish |
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