Understanding NEC Article 374: Cellular Metal Floor Raceways

Introduction

NEC Article 374 establishes guidelines for the installation and use of cellular metal floor raceways, which are integrated into the construction of steel-frame buildings. These raceways consist of sheet metal cells that provide a concealed pathway for electrical conductors. They are commonly used in commercial and industrial settings to distribute power efficiently while maintaining a sleek architectural design.

Scope

This article applies to cellular metal floor raceways and associated components, outlining their permitted uses, prohibited applications, and specific installation guidelines. These systems are typically used for wiring in large-scale construction projects, such as office buildings and institutional facilities.

Key Features of Cellular Metal Floor Raceways

1. Material and Design:
   • Made from sheet metal formed into cells within the floor decking.
   • Designed for structural and electrical functionality, with cells extending across the building.
2. Versatility:
   • Cells can be used for multiple systems, including power, lighting, signaling, and communication circuits, with separate headers for different applications.
3. Safety Compliance:
   • Must comply with ANSI/UL 209 for construction, performance, and marking requirements.

Permitted and Prohibited Uses

Permitted Uses:

1. Power and Signal Distribution:
   • Suitable for power, lighting, and communication systems in commercial buildings.
2. Indoor Applications:
   • Designed for installation in steel-framed buildings with specific structural requirements.

Prohibited Uses:

1. Hazardous Locations:
   • Restricted unless explicitly allowed by NEC standards.
2. Commercial Garages:
   • Not allowed except for ceiling outlets or specific below-floor extensions.
3. Corrosive Environments:
   • Prohibited in areas exposed to corrosive vapors or chemicals.

Installation Guidelines

1. Headers and Connections:
   • Connections to raceways must use approved conduits, such as liquidtight flexible metal conduit (LFMC) or rigid metal conduit (RMC).
   • Terminations must provide electrical continuity and prevent water or concrete ingress.
2. Junction Boxes and Inserts:
   • Junction boxes must align with floor grade and be sealed to prevent water intrusion.
   • Inserts must maintain continuity with the raceway and be installed to avoid damage to conductors.
3. Conductor Size and Capacity:
   • Conductors larger than 1/0 AWG require special permission.
   • The combined cross-sectional area of conductors must not exceed 40% of the cell’s interior cross-sectional area.
4. Ampacity Adjustments:
   • Ampacity must be adjusted per NEC 310.15(C) for conductors within raceways.
5. Splices and Taps:
   • Permitted only within header access units or junction boxes.
   • Loop wiring is not considered a splice or tap and must meet specific continuity requirements.
6. Abandoned Outlets:
   • Conductors for discontinued outlets must be removed entirely from the raceway to maintain safety and compliance.

Applications

1. Commercial Buildings:
   • Efficiently distributes power and communication lines in multi-story offices.
2. Institutional Facilities:
   • Used in schools, hospitals, and large-scale public infrastructure.
3. Integrated Architectural Solutions:
   • Conceals wiring while maintaining structural integrity and design.

Compliance Tips

1. Use Listed Products:
   • Ensure all raceway components and fittings are listed and comply with NEC and UL standards.
2. Monitor Fill and Ampacity:
   • Regularly check conductor fill levels and adjust ampacity to prevent overheating.
3. Inspect Seals and Continuity:
   • Verify that all seals, connections, and enclosures are properly installed and maintain electrical continuity.

Conclusion

NEC Article 374 provides a detailed framework for safely integrating cellular metal floor raceways into modern construction. These systems enable efficient power distribution while preserving the structural and aesthetic integrity of steel-frame buildings. Adherence to these guidelines ensures compliance, safety, and long-term functionality.