Understanding NEC Article 378: Nonmetallic Wireways

Introduction

NEC Article 378 provides comprehensive guidelines for the construction, use, and installation of nonmetallic wireways. These wireways, typically made of plastic or other nonmetallic materials, offer a lightweight, corrosion-resistant alternative to metallic wireways, making them ideal for specific applications. Their design allows easy access for wiring, repairs, and modifications, contributing to their popularity in exposed and controlled environments.

Scope

This article applies to nonmetallic wireways and their fittings. It specifies their permitted uses, prohibited applications, and construction standards, ensuring compliance with safety and performance requirements. Nonmetallic wireways are often used in industrial and commercial environments, particularly in corrosive or wet conditions when properly listed for such use.

Key Features of Nonmetallic Wireways

1. Construction and Materials:
   • Manufactured from durable, nonmetallic materials resistant to corrosion and moisture.
   • Lightweight and easy to install.
2. Flexibility:
   • Designed for use in diverse environments, including wet or corrosive locations, provided they are appropriately listed.
3. Accessibility:
   • Removable covers allow easy access for adding, removing, or modifying conductors.

Permitted and Prohibited Uses

Permitted Uses:

1. Exposed Work:
   • Ideal for installations where wireways are visible and accessible.
2. Corrosive and Wet Locations:
   • Allowed when identified and listed for these conditions.
3. Wall Penetrations:
   • Permitted for transverse wall passages if the section remains unbroken, with access to conductors maintained on both sides.

Prohibited Uses:

1. Physical Damage:
   • Not suitable for locations prone to physical damage.
2. Hazardous Locations:
   • Prohibited unless explicitly allowed by other NEC articles.
3. Sunlight Exposure:
   • Not permitted unless listed and marked for UV resistance.
4. High-Temperature Areas:
   • Cannot be used where ambient temperatures exceed their rated limits.

Installation Guidelines

1. Conductor Size and Capacity:
   • Conductors must not exceed the size for which the wireway is designed.
   • The cross-sectional area of conductors must not exceed 20% of the wireway’s interior cross-sectional area.
2. Supporting and Securing:
   • Horizontal runs must be supported at intervals not exceeding 3 feet.
   • Vertical runs require supports every 4 feet, with a maximum of one joint between supports.
3. Splices and Taps:
   • Allowed within the wireway but must be accessible.
   • Total fill, including splices, must not exceed 75% of the wireway’s cross-sectional area.
4. Expansion Fittings:
   • Required for thermal expansion and contraction when length changes exceed 0.25 inches in straight runs.
5. Dead Ends:
   • All unused ends must be securely closed with listed fittings.
6. Grounding:
   • A separate grounding conductor must be installed when equipment grounding is required, except where the grounded conductor serves this purpose as allowed by 250.142.

Applications

1. Industrial Environments:
   • Suitable for wiring in factories and warehouses where corrosion resistance is critical.
2. Commercial Buildings:
   • Used in areas requiring accessible wiring pathways.
3. Specialty Installations:
   • Common in areas where easy modification or frequent reconfiguration of wiring is needed.

Compliance Tips

1. Adhere to Listing Requirements:
   • Use only listed wireways and fittings for specific environmental conditions.
2. Monitor Fill and Spacing:
   • Avoid overfilling and ensure proper spacing to maintain compliance and safety.
3. Regular Inspections:
   • Periodically inspect installations for damage, wear, or non-compliance with NEC standards.

Conclusion

NEC Article 378 outlines clear standards for the safe and effective use of nonmetallic wireways. By adhering to these guidelines, electricians and engineers can ensure reliable performance, compliance, and safety in a wide range of electrical installations.