Understanding NEC Article 330: Metal-Clad Cable (Type MC)

Introduction

NEC Article 330 establishes the requirements for the use, construction, and installation of Metal-Clad (Type MC) cable. This versatile and durable wiring method supports applications across residential, commercial, and industrial environments. With its robust mechanical protection and adaptability, Type MC cable is widely used for power, lighting, control, and signal circuits.

Scope

The provisions of Article 330 apply to Metal-Clad cables used in systems rated up to 2000 volts for general use. The article also outlines requirements for specialized applications, such as medium-voltage MC cable rated up to 35,000 volts and MC-HL cable for hazardous locations.

Key Features of Type MC Cable

1. Construction:
   • Available in smooth, corrugated, or interlocked metallic sheaths.
   • May include a nonmetallic outer jacket for additional protection.
   • Equipped with copper, aluminum, or nickel-coated copper conductors ranging from 18 AWG to larger sizes.
2. Grounding:
   • The sheath may serve as an equipment grounding conductor (EGC) when compliant with 250.118(10).
   • Cables must include a separate bonding conductor for proper grounding.
3. Marking:
   • Each cable is labeled with voltage rating, insulation type, and conductor size for compliance and installation accuracy.

Permitted and Prohibited Uses

Permitted Uses:

Type MC cable can be used for:

1. Services, feeders, and branch circuits.
2. Indoor and outdoor installations, including damp and wet locations.
3. Direct burial where identified for such use.
4. Cable trays, raceways, and aerial applications.
5. Hazardous locations when marked MC-HL, provided the cable meets gas/vapor tightness standards.

Prohibited Uses:

1. Locations where physical damage is likely, unless adequately protected.
2. Environments with destructive corrosive agents, unless equipped with corrosion-resistant sheaths or jackets.

Installation Requirements

1. Securing and Supporting:
   • Secure within 12 inches of terminations and every 6 feet along runs.
   • Vertical installations may allow up to 10-foot intervals for larger cables.
2. Bending Radius:
   • Smooth Sheath: Radius depends on cable diameter (e.g., 10 times diameter for ≤ ¾ inch).
   • Interlocked Armor: Minimum radius of 7 times the external diameter.
3. Through or Parallel Framing Members:
   • Protection is required as per 300.4 to prevent mechanical damage.
4. Exposed Runs:
   • Runs must follow building surfaces closely and avoid areas subject to damage.
5. Hazardous Locations:
   • For MC-HL cable, the installation must include a gas-tight sheath and listed fittings.

Ampacity

The ampacity of Type MC cables is determined by conductor size, insulation type, and installation conditions:

• Cable Tray Installations: Ampacity follows 392.80.
• Bundled or Insulated Runs: Ampacity adjustments are required based on thermal insulation or spacing.

Applications

1. Residential:
   • Ideal for concealed wiring in walls, ceilings, and floors.
2. Commercial:
   • Suitable for power distribution in offices, retail spaces, and public buildings.
3. Industrial:
   • Frequently used in factories and hazardous locations requiring robust and reliable wiring systems.

Best Practices

1. Use Approved Components:
   • Only listed fittings and connectors should be used to ensure compliance and safety.
2. Protect from Corrosion:
   • Apply additional coatings or materials in corrosive environments.
3. Regular Inspections:
   • Inspect installations regularly for wear, damage, or loose terminations.

Conclusion

NEC Article 330 provides the foundational guidelines for Metal-Clad (Type MC) cable installations. By adhering to these standards, electricians and engineers can ensure the safety, reliability, and longevity of electrical systems. Type MC cable offers versatility and durability, making it an essential component in various applications.