Understanding NEC Article 355: Reinforced Thermosetting Resin Conduit (RTRC)

Introduction

NEC Article 355 provides guidelines for the use, installation, and construction of Reinforced Thermosetting Resin Conduit (RTRC). Known for its lightweight and durable properties, RTRC offers excellent resistance to corrosion, chemicals, and moisture. This conduit is widely used in industrial, commercial, and utility applications, especially in environments where traditional metal conduits may degrade.

Scope

This article addresses the requirements for installing and using RTRC, covering general construction standards, permitted and prohibited applications, and specific installation practices. RTRC is suitable for both aboveground and underground installations when conditions meet specified guidelines.

Key Features of RTRC

1. Material:
   • Manufactured from nonmetallic thermosetting resins.
   • Resistant to moisture, chemicals, and heat distortion.
2. Marking:
   • Clearly marked every 10 feet with type, temperature rating, and other specifications.
3. Sizes:
   • Trade sizes range from ½ inch to 6 inches.

Permitted and Prohibited Uses

Permitted Uses:

1. Concealed Locations:
   • Allowed in walls, floors, and ceilings.
2. Corrosive Environments:
   • Approved for severe corrosive areas when protected with materials resistant to specific chemicals.
3. Wet and Damp Locations:
   • Permitted in areas like dairies or laundries, provided all components prevent water ingress.
4. Underground Applications:
   • Suitable for direct burial or encasement in concrete as per 300.5 and 305.15.
5. Aboveground Exposed Use:
   • Allowed if specifically identified for physical damage resistance.

Prohibited Uses:

1. Hazardous Locations:
   • Not allowed in classified hazardous environments unless permitted by specific NEC sections.
2. High Ambient Temperatures:
   • Restricted in environments exceeding 50°C unless listed otherwise.
3. Supporting Luminaires:
   • Cannot support luminaires or other equipment.
4. Air Handling Spaces:
   • Prohibited in ducts, plenums, or other air-handling spaces where smoke or combustion products may be a concern.

Installation Guidelines

1. Securing and Supporting:
   • Securely fastened within 3 feet of termination points.
   • Supported at intervals according to conduit size (e.g., 3 feet for ½ inch conduits, 8 feet for 6-inch conduits).
2. Bending and Trimming:
   • Bends must maintain the internal diameter and be made using approved equipment.
   • Ends must be trimmed to avoid damaging conductors.
3. Grounding:
   • Requires a separate equipment grounding conductor unless specific NEC exceptions apply.
4. Expansion Fittings:
   • Required for temperature-induced length changes of ¼ inch or more over a straight run.

Applications

1. Chemical Plants:
   • Ideal for areas exposed to corrosive chemicals.
2. Utility Systems:
   • Suitable for underground power and data lines.
3. Outdoor Installations:
   • Common in exposed locations where environmental resistance is critical.

Compliance Tips

1. Use Appropriate Materials:
   • Ensure RTRC is listed and rated for its intended application.
2. Inspect Connections:
   • Joints, couplings, and terminations should be secure and compliant with NEC standards.
3. Prevent Physical Damage:
   • Use RTRC marked as “XW” for areas subject to physical impact.

Conclusion

NEC Article 355 provides comprehensive standards for the safe and effective use of RTRC. Its versatility, durability, and resistance to environmental hazards make it an excellent choice for various applications. By following these guidelines, electricians can ensure compliance, safety, and longevity in their installations.