Understanding NEC Article 334: Nonmetallic-Sheathed Cable (Types NM and NMC)

Introduction

NEC Article 334 establishes the requirements for the use, construction, and installation of nonmetallic-sheathed cable, commonly referred to as Types NM (Nonmetallic) and NMC (Nonmetallic Corrosion-Resistant). First introduced in the 1928 NEC, these cable types were developed as a safer and more efficient alternative to knob-and-tube wiring. They provide mechanical protection, flexibility, and ease of installation, making them ideal for residential and certain commercial applications.

Scope

Article 334 covers the use and construction specifications for Types NM and NMC cables. It addresses their applications, limitations, and installation requirements in various environments.

Key Features

1. Construction:
   • Composed of solid or stranded copper or aluminum conductors.
   • Insulated with flame-retardant and moisture-resistant materials.
   • Includes a bare or insulated grounding conductor.
   • Outer sheath provides mechanical protection and additional resistance to environmental factors.
2. Voltage Ratings:
   • Rated up to 600 volts for power and control circuits.

Permitted and Prohibited Uses

Permitted Uses:

1. Type NM Cable:
   • Suitable for dry locations.
   • May be used in walls, floors, or ceilings that provide a 15-minute fire-rated thermal barrier.
   • Ideal for one- and two-family dwellings, multifamily units, and attached garages.
2. Type NMC Cable:
   • Suitable for damp, wet, and corrosive locations.
   • Can be embedded in plaster, concrete, or masonry when protected with steel plates.
   • Commonly used in agricultural buildings where resistance to chemicals like ammonia is required.

Prohibited Uses:

1. Both Types NM and NMC cables are not permitted:
   • In ducts, plenums, or air-handling spaces.
   • As service-entrance cables.
   • In hazardous locations unless specifically permitted.
2. Type NM cable is restricted from use in wet or corrosive locations.

Installation Guidelines

1. Securing and Supporting:
   • Must be secured within 12 inches of a box or fitting and every 4.5 feet along its run.
   • Unsupported installations are only permitted when fishing through concealed spaces.
2. Protection:
   • Must be protected from physical damage using rigid conduit, EMT, or similar raceways where exposed.
   • Cable passing through framing must be protected with bushings or grommets.
3. Bending Radius:
   • Bends must have a radius of at least five times the cable’s diameter to prevent damage.
4. Ampacity:
   • Limited to 60°C (140°F) for conductor ampacity.
   • Adjustment factors apply when bundled or in contact with insulation.

Applications

1. Residential Buildings:
   • Widely used in homes for lighting, outlets, and small appliance circuits.
2. Agricultural Facilities:
   • Type NMC is used in environments with corrosive chemicals or high moisture levels.
3. Multifamily Dwellings:
   • Installed in concealed spaces, such as walls and ceilings.

Safety and Compliance Tips

1. Use Approved Components:
   • Ensure cables, connectors, and fittings are listed for their intended use.
2. Follow Local Codes:
   • Adhere to additional requirements set by local building and electrical codes.
3. Inspect Regularly:
   • Check for wear, damage, or improper installations, especially in accessible areas.

Conclusion

NEC Article 334 provides essential guidelines for the safe and efficient use of Nonmetallic-Sheathed Cables (Types NM and NMC). These cables are a reliable solution for residential and certain commercial applications, offering ease of installation, cost efficiency, and safety. Understanding their proper use and limitations ensures compliance with electrical codes and enhances the longevity of electrical systems.