Understanding NEC Article 334 for NM (nonmetallic sheathed cable)

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Understanding Article 334: Nonmetallic-Sheathed Cable (Types NM and NMC) from the 2023 NEC​

IntroductionArticle 334 of the National Electrical Code (NEC) outlines the specifications and requirements for the use, installation, and construction of nonmetallic-sheathed cables, specifically Types NM and NMC. These cables are integral to modern electrical systems, offering a safe and efficient way to route electrical wiring in various types of buildings.

334.1 Scope​

This article focuses on the use, installation, and construction of nonmetallic-sheathed cables (NM and NMC). These cables were first introduced in the 1928 NEC as an alternative to knob-and-tube wiring and open wiring on insulators. The primary benefits included continuous protection provided by the outer sheath, ease of installation, and reduced need for insulating supports.

334.6 Listing Requirements​

Both Type NM and Type NMC cables, along with their associated fittings, must be listed according to ANSI/UL 719 standards. These standards ensure the cables meet performance and safety criteria, including flammability, dielectric voltage withstand, and physical durability.

Part II: Installation​

334.10 Uses Permitted​

Type NM and Type NMC cables are allowed in specific building types, provided certain conditions are met:

1. One- and two-family dwellings, including attached or detached garages and storage buildings.
2. Multi-family dwellings of Types III, IV, and V construction.
3. Other structures of Types III, IV, and V construction, with cables concealed within walls, floors, or ceilings that offer at least a 15-minute fire-resistant rating.
4. Cable trays in structures of Types III, IV, or V construction, where cables are identified for such use.
5. Types I and II construction, provided the cables are installed within raceways.

Building Construction Types Explained:

• Type I (fire resistive) and Type II (noncombustible) require noncombustible structural elements.
• Type III (combination), Type IV (heavy timber), and Type V (wood frame) allow combustible materials for some or all structural elements.

334.12 Uses Not Permitted​

There are specific locations where Type NM and NMC cables cannot be used:

• In any dwelling or structure not listed in 334.10(1), (2), (3), and (5).
• Exposed within dropped or suspended ceiling cavities in non-dwelling units.
• As service-entrance cables.
• In hazardous locations, such as commercial garages, theaters, motion picture studios, storage battery rooms, hoistways, elevators, or escalators.
• Embedded in poured cement, concrete, or aggregate.

334.15 Exposed Work​

For exposed work, nonmetallic-sheathed cables must be installed to follow the surface of building finishes closely and protected from physical damage using approved conduits or tubing. Specific requirements are outlined for installations in unfinished basements, crawl spaces, and when cables pass through floors or parallel to framing members.

334.30 Securing and Supporting​

Nonmetallic-sheathed cables must be supported and secured at intervals not exceeding 4 1/2 feet and within 12 inches of any cable entry into enclosures. Staples, cable ties, straps, or other approved fittings must be used without damaging the cable. Special considerations are given for horizontal runs, unsupported cables, and wiring devices without separate outlet boxes.

Part III: Construction Specifications​

334.100 Construction​

The outer sheath of nonmetallic-sheathed cables must be made of a nonmetallic material.

334.104 Conductors​

Power conductors within these cables should be sizes 14 AWG through 2 AWG for copper or sizes 12 AWG through 2 AWG for aluminum or copper-clad aluminum. Control and signaling conductors must be no smaller than 18 AWG copper.

334.108 Equipment Grounding Conductor​

An additional insulated, covered, or bare equipment grounding conductor is required within the cable.

334.112 Insulation​

Insulated power conductors must be suitable for branch-circuit wiring with a rating of 90°C (194°F). Cables marked NM-B and NMC-B meet these requirements.

334.116 Sheath​

The outer sheath requirements differ for Type NM and Type NMC cables:

• Type NM: The sheath must be flame retardant and moisture resistant.
• Type NMC: The sheath must be flame retardant, moisture resistant, and resistant to fungus and corrosion.

 

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Uses Not Permitted:
5. In theaters and similar locations, except where permitted in 518.4(B)

What use is similar to a theater? I had the idea in my head that assembly uses were a no go for NM/NMC.