Understanding NEC Article 348: Flexible Metal Conduit (FMC)

Introduction

NEC Article 348 governs the use, installation, and construction of Flexible Metal Conduit (FMC). FMC is a lightweight and adaptable wiring method used for routing electrical conductors in various settings. Known for its flexibility, it is particularly suited for applications where movement, vibration, or alignment issues are factors. This article establishes safety standards for ensuring FMC installations are effective, durable, and compliant.

Scope

This article applies to the use and installation of FMC and its associated fittings. It covers general use cases, prohibited environments, installation requirements, and grounding and bonding standards.

Key Features of FMC

1. Construction:
   • Made of interlocked or corrugated metal strips.
   • Lightweight, flexible, and easy to install.
2. Sizes:
   • Minimum trade size: 3/8 inch.
   • Maximum trade size: 4 inches.
3. Applications:
   • Common in equipment connections requiring flexibility, such as motors or luminaires.

Permitted and Prohibited Uses

Permitted Uses:

1. General Installations:
   • Exposed and concealed locations.
   • Flexible connections to equipment where movement is necessary.
2. Dry Locations:
   • Suitable for areas not subject to moisture or corrosive substances.
3. Limited Lengths Without EGC:
   • Unlimited lengths are allowed for sizes ½ inch and larger when an equipment grounding conductor (EGC) is installed with the circuit conductors.

Prohibited Uses:

1. Wet Locations:
   • Not allowed in areas with moisture unless specifically protected.
2. Hazardous Locations:
   • Prohibited unless permitted by other articles.
3. Underground Installations:
   • Cannot be used for direct burial or embedded in poured concrete.
4. Physically Damaged Areas:
   • Unsuitable for locations where it may be exposed to severe impacts.

Installation Guidelines

1. Securing and Supporting:
   • FMC must be securely fastened within 12 inches of each box or termination point.
   • Supported at intervals not exceeding 4½ feet for horizontal runs.
2. Bending:
   • Total bends in a run must not exceed 360 degrees between pull points.
   • Bends should maintain the conduit’s internal diameter to avoid damaging conductors.
3. Grounding and Bonding:
   • A separate equipment grounding conductor must be installed if FMC is used in flexible installations.
   • Grounding requirements must comply with 250.118(A)(5).
4. Trimming:
   • Ends must be trimmed to remove sharp edges to prevent conductor damage.

Applications

1. Equipment Connections:
   • Ideal for connecting motors, luminaires, or HVAC systems where flexibility is required.
2. Temporary Installations:
   • Suitable for short-term wiring needs in concealed spaces.
3. Vibration-Prone Environments:
   • Effectively absorbs vibration from equipment like pumps or compressors.

Compliance Tips

1. Use Proper Fittings:
   • Ensure couplings and connectors are listed and appropriate for the specific application.
2. Inspect Installations Regularly:
   • Periodically check for wear, damage, or non-compliance with NEC standards.
3. Avoid Prohibited Uses:
   • Do not install FMC in environments prone to moisture, physical damage, or hazardous materials.

Conclusion

NEC Article 348 ensures the safe and effective use of Flexible Metal Conduit in electrical installations. Its lightweight design and flexibility make FMC a reliable choice for specific applications, particularly those involving movement or vibration. By adhering to these guidelines, electricians can ensure compliance and enhance the safety and longevity of their systems.