In this month’s column, I will try to reveal some key requirements that can be found in the National Electric Code when using type NM, Nonmetallic Sheathed cable as a wiring method. The trade name “Romex” was first coined after the introduction of this new type of wiring method back in the early 1900’s. The General Cable Company invented this new wiring method using a flexible cable at its Rome Wire Division in Rome, New York – hence the name “Romex”. It was first listed in the NEC in 1926.
Section 334.2 defines type NM cable as a factory assembly of 2 or more insulated wires enclosed within an overall nonmetallic jacket. These cables are manufactured as type NM and NMC. Type NM will be covered in this article. In past Code cycles, you could find requirements for type NM-A and NM-B. NM-B is the new 90⁰C rated insulation that all manufactures use with this cable assembly. NM-S had a short debut which was Romex with a Cat. 5 cable installed as an integral part of the cable. It was once thought that if a Cat.5 was run to any device it could be used to control it. Now, thanks to Wi-Fi and Bluetooth that thought is obsolete. You cannot find NM-A or NM-S cable in the latest versions of the Code
Section 334.10 Uses Permitted. The NEC allows NM cable to be used in most all one- and two-family dwellings. Multi-family dwellings in types III, IV, and V construction. (See Informative Annex E). Some other structures of types III, IV and V and in some cable tray if listed. Romex may be installed for both exposed and concealed work in normally dry locations and may be fished in air voids of masonry block or tile walls. Check with the local AHJ for additional requirements for permitted uses.
334.12 Uses not permitted. Generally, type NM cable shall not be installed in certain types of occupancies listed in this section and shall not be embedded or installed in shallow chases of concrete poured cement or aggregate. Except for NM-C, it is not to be exposed to corrosive environments, embedded in masonry, concrete, adobe, fill or plaster. It also shall not be installed in wet or damp locations. Often, residential installations require conduits in concrete to feed kitchen islands or other free-standing structures to provide a pathway or sleeve. NM cable is not permitted with this type of installation. NM-C or type UF cable must be used. Another option would be boxes installed at both ends and building wires of the type of insulation rated for wet locations could be used. 300.5(B) determines raceways installed underground as wet locations which includes raceways embedded in concrete at or below grade.
334.15 Exposed Work. This section provides requirements for protection of exposed cables to physical damage. It requires cables passing thru floors to be protected with conduit. (GRC, IMC, EMT or Sch. 80). Exposure to physical damage is not defined in the Code book. It relies on the AHJ to make that determination in the field.
Where installed in basements or crawl spaces, and run at angles to the joists, the NEC permits Romex not smaller than two 6 AWG or three 8 AWG conductors to be secured directly to the joist. Smaller cables must be either installed in bored holes, if permitted, or secured to running boards. Otherwise, cables are permitted to be installed on the side no closer than 1 ¼” from the edge. Article 300 contains additional requirements for all cable installations.
334.23 In Accessible Attics. If run in attics and within proximity of the access to the attic, it shall be protected in accordance with 320.23 (A) or (B) which requires it to be run along the sides of the joists or rafters within 7 feet of a scuttle hole or 6 feet if no permanent stairs or ladder is installed. The pull-down ladders that can be used are NOT considered as permanent stairs or ladders.
334.30 Securing and Supports. NM cable shall be supported at intervals not exceeding 4 ½ feet and within 12 inches of a box. However, 314.17(C) requires it to be secured within 8” if the box is nonmetallic and has no cable clamps. It also allows two cables to enter on opening of a standard NM box. It requires the cable sheath to extend at least ¼” inside the box. Flat cables shall not be stabled on the edge. New language permits 18 inches of looped cable from the box the first cable support. This gives the installer some extra cable to repair any conductor that may have been damaged from drywall installation. This section requires all cables to be supported. The NEC Handbook expands on this by elaborating that, simply draping cables on rafters and joists or ceiling framing members is not in compliance. However, when installed through holes that are bored in framing members that do not exceed 4 ½ feet. This installation is considered to be supported. Not following this rule is a common misintereptation of the Code within the industry when installing Romex.
334.80 Ampacity. This requires that all NM cable shall be rated at not more than 60⁰ C. Even though the cable is assembled with 90⁰ C conductors, after all corrections and adjustments its final ampacity does not exceed the ampacities of the 60⁰ C column in Table 310.16. Where two or more cables are installed in the same opening in wood framing and is to be sealed with insulation, caulk, or foam then the ampacity of the cable must be derated in accordance with 310.15(B)(3)(a) for more than three Current Carrying Conductors. The exception of 310.15(A)(2) shall not apply.
334.104 Equipment Grounding Conductor. This was first required in the 1962 edition of the NEC. It required all branch circuits to have an E.G.C. The first versions were a smaller size E.G.C. and in 1969 the NEC banned all undersized grounds and required them to be the same size of the ungrounded conductors up to 10 AWG.
The 2020 NEC requires box fill calculations to count the first four E.G.C.s as one, then every additional E.G.C. to count as ¼. A simple method would be to divide the number of grounding conductors by 4, then multiply that number by the volume allowance of the largest E.G.C.
Some other interesting facts about Romex is that it is only manufactured from wire size 14 AWG up to 2 AWG copper and 12 AWG up to 2 AWG aluminum or copper-clad aluminum. The voltage rating is up to 600 volts. It is listed under UL standard 719
NM-S was replaced with NM-PCS which can be found with control and signaling conductors not smaller than 18 AWG copper.
All associated fitting and cables must be listed. You might be able to use a two-screw type MC or type AC cable connector with Romex if it is listed for type NM cable. Check with the manufacturer to get information of listed cable clamps provided within certain device boxes or other types of boxes.
There are other requirements as it relates to type NM cable found in Article 334, but I hope this lays a foundation upon which one can use to properly install Romex cable and understand why the Inspectors call out some of these violations on the job sites after reading this edition of the Code Corner.