Understanding NEC Article 356: Liquidtight Flexible Nonmetallic Conduit (LFNC)

Introduction

NEC Article 356 outlines the requirements for Liquidtight Flexible Nonmetallic Conduit (LFNC), a versatile conduit type used to protect electrical conductors in environments prone to moisture, vapors, or corrosive agents. LFNC is valued for its flexibility, lightweight construction, and liquid-tight properties, making it ideal for installations requiring adaptability and protection.

Scope

This article defines the construction standards, permitted and prohibited uses, and installation guidelines for LFNC. It ensures that the conduit meets safety and durability requirements in various environments, including wet locations, corrosive areas, and direct burial applications.

Key Features of LFNC

1. Material:
   • Made from nonmetallic, moisture-resistant materials.
   • Lightweight and flexible, facilitating ease of installation.
2. Types:
   • LFNC-A: Flexible with smooth inner and outer surfaces.
   • LFNC-B: Corrugated inner surface and smooth outer surface.
   • LFNC-C: Both inner and outer surfaces are corrugated.
3. Marking:
   • Marked every 2 feet with type, size, and specific application suitability, including ratings for wet locations or direct burial.

Permitted and Prohibited Uses

Permitted Uses:

LFNC is suitable for:

1. Wet Locations:
   • Approved for outdoor use when marked as suitable for the purpose.
2. Direct Burial:
   • Allowed where listed for burial in earth or encasement in concrete.
3. Flexibility Needs:
   • Used where flexibility is essential for installation, operation, or maintenance.
4. Corrosive Environments:
   • Resistant to chemicals and suitable for areas with severe corrosive influences.
5. Encasement in Concrete:
   • Installed where listed for such use.
6. Outdoor Installations:
   • Suitable for areas exposed to weather, provided the conduit is listed and marked for the purpose.

Prohibited Uses:

LFNC cannot be used:

1. Where Subject to Physical Damage:
   • In locations where severe impact or abrasion is expected.
2. In Excessive Temperatures:
   • Not suitable for temperatures exceeding its listed rating.
3. Hazardous Locations:
   • Prohibited in classified hazardous areas unless specified by NEC standards.

Installation Guidelines

1. Bending:
   • Manual bends must avoid reducing the internal diameter or damaging the conduit.
   • Total bends in a single run must not exceed 360 degrees.
2. Securing and Supporting:
   • Must be securely fastened at intervals not exceeding 3 feet and within 12 inches of terminations.
   • Horizontal runs may rely on framing members for support if spaced appropriately.
3. Conduit Ends:
   • All cut ends must be trimmed to remove sharp edges to protect conductors during installation.
4. Grounding Requirements:
   • Requires a separate grounding conductor unless specific NEC exceptions apply.
5. Splices and Taps:
   • Must be made in junction boxes or other approved enclosures per 300.15.

Applications

1. Moisture-Prone Areas:
   • Suitable for laundries, dairies, and outdoor equipment connections.
2. Corrosive Locations:
   • Ideal for industrial environments with exposure to chemical vapors or liquids.
3. Temporary Installations:
   • Used for short-term wiring solutions requiring flexibility and moisture protection.

Compliance Tips

1. Use Listed Components:
   • Ensure LFNC and fittings meet the listing requirements for the specific application.
2. Prevent Overloading:
   • Follow the allowable percentage fill specified in Table 1, Chapter 9.
3. Inspect Periodically:
   • Regular inspections help ensure the conduit remains compliant and free of damage.

Conclusion

NEC Article 356 establishes clear standards for the safe use of LFNC in electrical installations. With its flexibility, durability, and resistance to moisture and chemicals, LFNC is a reliable choice for specific applications. Adhering to these guidelines ensures compliance, safety, and long-term performance in electrical systems.