Understanding NEC Article 350: Liquidtight Flexible Metal Conduit (LFMC)

Introduction

NEC Article 350 provides guidelines for the use, installation, and construction of Liquidtight Flexible Metal Conduit (LFMC). This conduit type is designed for environments requiring protection from liquids, oil, and coolants while offering flexibility for challenging installations. LFMC is suitable for a variety of industrial, commercial, and residential applications, particularly in wet or corrosive locations.

Scope

This article addresses the construction standards, permitted and prohibited uses, and installation practices for LFMC. It ensures the safe and effective implementation of LFMC in electrical systems.

Key Features of LFMC

1. Construction:
   • Made of interlocked or corrugated metal strips with a liquidtight, nonmetallic covering.
   • Available in sizes from 3/8 inch to 4 inches (trade size).
2. Versatility:
   • Suitable for exposed and concealed locations.
   • Approved for use in wet, dry, and corrosive environments.
3. Marking:
   • Conduit is marked for its intended use, including temperature ratings and suitability for direct burial.

Permitted and Prohibited Uses

Permitted Uses:

LFMC is allowed for:

1. Wet Locations:
   • When properly marked, LFMC can be used for systems exposed to oil, coolants, and moisture.
2. Direct Burial:
   • If specifically listed for the purpose.
3. Hazardous Locations:
   • Permitted in classified environments as outlined in NEC Chapter 5.
4. High-Temperature Applications:
   • Conductors with higher temperature ratings can be used as long as the LFMC rating is not exceeded.

Prohibited Uses:

LFMC is not suitable:

1. Where subject to severe physical damage.
2. In gasoline or petroleum solvent environments unless marked for that use.

Installation Requirements

1. Securing and Supporting:
   • Secure LFMC within 12 inches of terminations and at intervals not exceeding 4½ feet.
   • Listed cable ties or other fittings are required for proper support in demanding environments.
2. Bending:
   • Conduit bends must maintain the internal diameter to prevent conductor damage.
   • Total bends in a single run must not exceed 360 degrees.
3. Grounding and Bonding:
   • LFMC can serve as an equipment grounding conductor if installed in fixed systems with no movement after installation.
   • For flexible systems, a separate equipment grounding conductor is mandatory.
4. Cutting and Trimming:
   • Ends must be trimmed to avoid sharp edges that could damage conductors or compromise fittings.

Applications

1. Industrial Environments:
   • Used in machinery connections requiring flexibility and liquidtight protection.
2. Outdoor Lighting:
   • Ensures reliable performance in exposed and wet conditions.
3. HVAC Systems:
   • Provides a flexible and durable solution for electrical connections.

Compliance and Safety Tips

1. Use Listed Components:
   • Ensure all fittings and couplings are compatible with LFMC and appropriately listed for the intended application.
2. Inspect Installations:
   • Regular inspections ensure compliance with NEC standards and identify potential wear or damage.
3. Adhere to Marking Specifications:
   • Install LFMC only in environments for which it is marked as suitable.

Conclusion

NEC Article 350 offers essential guidelines for the safe and effective use of LFMC in electrical systems. Its flexibility, liquidtight properties, and robust design make it a preferred choice for environments where protection from moisture, oil, and corrosion is required. Adhering to these standards ensures safety, compliance, and reliability in installations.