Understanding NEC Article 368: Busways

Introduction

NEC Article 368 provides comprehensive guidelines for the use, installation, and construction of busways in electrical systems. Busways serve as compact, efficient, and flexible conduits for distributing electrical power in commercial, industrial, and institutional settings. By clearly defining the rules for their application, this article ensures that busways are installed and maintained in a safe and compliant manner.

Scope

This article applies to service-entrance, feeder, and branch-circuit busways and their associated fittings. It covers general and special-purpose busway designs, including lighting, trolley, and continuous plug-in busways.

Key Features of Busways

1. Types of Busways:
   • Power Distribution Busway: Commonly used as feeders or branch circuits.
   • Lighting Busway: Supplies and supports luminaires, with a current rating up to 50 amperes.
   • Trolley Busway: Allows continuous contact via a slot, often used in industrial settings.
   • Continuous Plug-In Busway: Facilitates plug-in connections along its length, supporting up to 225 amperes.
   • Short-Run Busway: Limited to 30 feet horizontally or 10 feet vertically for feeding switchboards.
2. Design Specifications:
   • Must include provisions for fire, vapor, and moisture barriers.
   • Expansion fittings are required for long runs to accommodate thermal changes.

Permitted and Prohibited Uses

Permitted Uses:

1. Exposed or Concealed:
   • Can be installed openly or behind access panels when properly enclosed.
2. Penetrations Through Walls and Floors:
   • Allowed for unbroken lengths, provided fire and vapor barriers are installed.
3. Vertical Installations:
   • Permitted in high-rise buildings as vertical risers for floor-to-floor power distribution.

Prohibited Uses:

1. Physical Damage:
   • Cannot be installed in locations prone to severe mechanical damage or corrosive vapors.
2. Wet Locations:
   • Prohibited unless specifically identified for wet or damp conditions.
3. Hazardous Locations:
   • Restricted unless explicitly listed for such environments.

Installation Guidelines

1. Support and Securing:
   • Busways must be securely supported at intervals not exceeding 5 feet unless otherwise marked.
2. Overcurrent Protection:
   • Overcurrent devices must match the busway’s current rating to prevent damage from faults.
3. Branch Circuits:
   • Branches from busways can use various approved wiring methods such as armored cable (AC), metal-clad cable (MC), and rigid metal conduit (RMC).
4. Expansion and Thermal Protection:
   • Flexible connections are required in long, straight runs to accommodate expansion due to temperature changes.

Applications

1. Industrial Plants:
   • Powering heavy machinery or production lines with efficient distribution systems.
2. Commercial Buildings:
   • Supplying power to office floors, retail spaces, or distribution centers.
3. Special Environments:
   • Used in high-rise buildings, healthcare facilities, and cleanrooms with specific requirements.

Compliance Tips

1. Fire and Vapor Barriers:
   • Ensure that all wall and floor penetrations meet NEC standards for fire safety and vapor sealing.
2. Inspection and Maintenance:
   • Regularly inspect busways for signs of wear, physical damage, or non-compliance.
3. Adhere to Markings:
   • Follow the manufacturer’s specifications for voltage, current, and installation constraints.

Conclusion

NEC Article 368 provides essential guidelines for the safe and effective use of busways in electrical systems. Their versatility and efficiency make them a critical component in modern power distribution. By adhering to the rules outlined in this article, electricians and engineers can ensure safe, reliable, and compliant installations.