Understanding NEC Article 358: Electrical Metallic Tubing (EMT)

Introduction

NEC Article 358 provides comprehensive guidelines for the use, installation, and construction of Electrical Metallic Tubing (EMT). Known for its durability and cost-efficiency, EMT is widely used in electrical installations across residential, commercial, and industrial settings. This article ensures compliance with safety standards and promotes reliable and long-lasting installations.

Scope

This article governs the installation, permitted uses, and construction standards for EMT. It outlines best practices for securely fastening EMT and using approved fittings, ensuring the tubing’s integrity and safety.

Key Features of EMT

1. Material:
   • Constructed from galvanized steel, aluminum, or stainless steel with protective coatings.
   • Lightweight and easy to handle, making it a preferred choice for exposed or concealed installations.
2. Marking:
   • Marked every 10 feet with essential information, including material type and compliance standards.
3. Size Range:
   • Available in sizes from ½ inch to 6 inches in diameter.

Permitted and Prohibited Uses

Permitted Uses:

1. Exposed and Concealed Locations:
   • Allowed for installations in walls, ceilings, or other concealed spaces.
2. Corrosive Environments:
   • Approved for use in corrosive conditions when protected with suitable coatings or materials.
3. Wet Locations:
   • Suitable when used with corrosion-resistant materials for supports and fittings.
4. Concrete and Earth Contact:
   • Allowed when installed with fittings identified for direct burial or corrosive conditions.

Prohibited Uses:

1. Severe Physical Damage:
   • Not suitable for environments with severe mechanical impacts.
2. Equipment Support:
   • Cannot be used to support luminaires or other equipment unless specified.

Installation Guidelines

1. Securing and Supporting:
   • EMT must be securely fastened every 10 feet and within 3 feet of each outlet or junction box.
   • Horizontal runs can be supported through framing members spaced no more than 10 feet apart.
2. Bending:
   • Bends must not reduce the tubing’s internal diameter. Total bends in a run should not exceed 360 degrees.
3. Reaming and Threading:
   • Cut ends must be reamed to remove sharp edges. EMT must not be threaded except for specific factory-threaded couplings.
4. Couplings and Connectors:
   • Must be tightened securely and suitable for the environment (e.g., concrete-tight or raintight).
5. Splices and Taps:
   • Must be made in approved enclosures, ensuring compliance with 300.15.

Applications

1. Commercial Buildings:
   • Commonly used in office buildings for power and lighting circuits.
2. Industrial Plants:
   • Suitable for nonhazardous areas requiring durable conduit systems.
3. Outdoor Installations:
   • Approved for certain wet or corrosive environments when properly protected.

Compliance Tips

1. Avoid Dissimilar Metals:
   • Prevent galvanic action by avoiding contact between incompatible metals.
2. Inspect and Maintain:
   • Periodic inspections ensure compliance and identify potential wear or damage.
3. Follow Manufacturer Guidelines:
   • Adhere to recommendations for bending, securing, and connecting EMT.

Conclusion

NEC Article 358 provides clear standards for the safe and effective use of EMT in electrical installations. Its versatility, affordability, and durability make it a reliable option for various applications. By adhering to these guidelines, electricians can ensure safety, compliance, and long-term performance.