Understanding NEC Article 360: Flexible Metallic Tubing (FMT)

Introduction

NEC Article 360 establishes the standards for the installation and use of Flexible Metallic Tubing (FMT), a type of raceway designed for flexibility and protection in specific electrical applications. With its adaptability and resistance to vibration, FMT is a practical choice for wiring in confined spaces or ducts, particularly in air-handling spaces where other raceways may not be suitable.

Scope

This article applies to the use, installation, and construction of FMT and its associated fittings. It addresses both permitted and prohibited uses and provides detailed guidelines for proper installation and safety.

Key Features of FMT

1. Construction:
   • Made from durable metallic tubing.
   • Provides an effective barrier against gases and combustion products.
2. Flexibility:
   • Capable of withstanding vibration and minor movements without compromising integrity.
3. Marking:
   • Clearly marked with essential information, including type, trade size, and compliance standards.

Permitted and Prohibited Uses

Permitted Uses:

FMT may be used in:

1. Dry Locations:
   • Approved for installations where the environment is free from moisture.
2. Concealed Spaces:
   • Suitable for walls, ceilings, and other enclosed areas.
3. Accessible Locations:
   • Ideal for easily reachable installations that allow maintenance.
4. Low-Voltage Systems:
   • Limited to applications not exceeding 1000 volts.

Prohibited Uses:

FMT is not allowed in:

1. Hoistways:
   • Due to potential mechanical stress.
2. Storage Battery Rooms:
   • Where corrosive fumes may degrade the material.
3. Hazardous Locations:
   • Restricted unless explicitly permitted by NEC.
4. Underground Installations:
   • Not suitable for direct burial or encasement in concrete.
5. Physically Damaging Environments:
   • Locations where significant mechanical damage is likely.
6. Long Runs:
   • Lengths exceeding 6 feet are prohibited.

Installation Guidelines

1. Size Requirements:
   • Minimum size: Trade size ½ inch, except trade size ⅜ inch is allowed for certain luminaires or assemblies.
   • Maximum size: Trade size ¾ inch.
2. Conductor Fill:
   • Must adhere to the percentage fill requirements specified in Table 1, Chapter 9 for safe and efficient operation.
3. Bending:
   • For infrequent flexing: Follow Table 360.24(A) for minimum bend radii.
   • For fixed installations: Follow Table 360.24(B) for minimum bend radii.
4. Splices and Taps:
   • Must be made in approved enclosures per 300.15.
5. Grounding:
   • Can serve as an equipment grounding conductor if installed according to 250.118(A)(7).

Applications

1. Air-Handling Spaces:
   • Commonly used in ducts or plenums where its flexibility and resistance to combustion products are advantageous.
2. Residential and Commercial Wiring:
   • Suitable for concealed or dry locations requiring lightweight, flexible raceways.

Compliance Tips

1. Use Listed Components:
   • Ensure FMT and its fittings meet listing requirements for the intended application.
2. Inspect for Damage:
   • Regular checks help identify wear or non-compliance issues.
3. Adhere to Manufacturer Guidelines:
   • Follow specifications for cutting, bending, and securing FMT.

Conclusion

NEC Article 360 provides clear standards for the effective and safe use of Flexible Metallic Tubing. Its versatility, combined with specific use-case restrictions, makes it a valuable option in certain electrical installations. By adhering to these guidelines, electricians can ensure compliance, safety, and reliable performance in systems requiring FMT.