Understanding NEC Article 390: Underfloor Raceways

Introduction

NEC Article 390 governs the use and installation of underfloor raceways, a flexible and practical solution for distributing power, lighting, and communication systems beneath the floor surface. These systems are widely used in commercial spaces, retail environments, and office buildings to enable customizable layouts and wiring configurations. By adhering to the provisions of Article 390, professionals ensure safety, compliance, and system longevity.

Scope

This article applies to underfloor raceways installed beneath concrete, wood, or other flooring materials. It provides guidelines for layout, conductor capacity, material requirements, and protection against environmental factors like corrosion and mechanical stress.

Key Features of Underfloor Raceways

1. Material and Design:
   • Constructed from durable materials suited for mechanical protection and environmental resilience.
   • Can accommodate movable partitions and modular office layouts.
2. Applications:
   • Provide power and communication access to desks, tables, and workspaces not adjacent to walls.
   • Useful in environments requiring frequent reconfiguration of electrical layouts.

Permitted and Prohibited Uses

Permitted Uses:

1. Concrete or Flooring Installations:
   • Allowed beneath concrete or other flooring, flush with the surface, and covered with linoleum or similar materials.
2. Office Occupancies:
   • Commonly used in offices where flexible power access is needed.

Prohibited Uses:

1. Corrosive Environments:
   • Not allowed in areas with corrosive vapors unless materials or protective measures are approved.
2. Hazardous Locations:
   • Prohibited unless exceptions specified by 504.20 or 501.10(B)(3) are met.
3. Unapproved Configurations:
   • Systems must not be installed without adherence to specified spacing and coverage rules.

Installation Guidelines

1. Covering Requirements:
   • Raceways less than 4 inches wide require at least 3/4 inch of concrete cover.
   • Wider raceways may need additional spacing and thicker concrete coverage.
2. Conductor Limits:
   • The combined cross-sectional area of conductors must not exceed 40% of the raceway’s interior space.
   • Ampacity adjustments per 310.15(C) apply to prevent overheating.
3. Splices and Taps:
   • Allowed only in junction boxes or trench-type raceways with removable covers.
   • Conductors, splices, and taps must not fill more than 75% of the raceway’s cross-sectional area.
4. Alignment and Support:
   • Raceways must be laid in straight lines with clear alignment between junction boxes.
   • All dead ends must be securely closed.
5. Junction Boxes:
   • Must be leveled with the floor grade and sealed to prevent water or concrete ingress.
6. Markers and Inserts:
   • Markers are required at the ends of straight runs to locate inserts or access points.
   • Inserts must be leveled and securely sealed during installation.

Special Considerations

1. Corrosion Protection:
   • Metal raceways exposed to corrosive conditions must use approved materials or coatings.
2. Connections to Cabinets and Outlets:
   • Connections must be made using approved fittings or methods outlined in Chapter 3 of the NEC.
3. Maintenance and Modifications:
   • Conductors for discontinued outlets must be removed, and raceways must remain free of splices or reinsulated conductors at these points.

Applications

1. Commercial Spaces:
   • Powering modular workstations in offices and retail environments.
2. Large Retail Stores:
   • Flexible wiring for changing layouts and displays.
3. Educational Facilities:
   • Ideal for supplying power and communication systems in lecture halls and classrooms.

Compliance Tips

1. Select Approved Materials:
   • Use raceways and fittings listed for the intended environment and application.
2. Monitor Conductor Fill:
   • Avoid exceeding fill limits to maintain efficiency and safety.
3. Inspect Regularly:
   • Conduct routine inspections to identify wear, damage, or non-compliance issues.

Conclusion

NEC Article 390 provides comprehensive guidance for installing underfloor raceways, ensuring flexibility, safety, and regulatory compliance. By following these guidelines, electricians can deliver reliable and adaptable power and communication solutions for a variety of environments.