Understanding NEC Article 398: Open Wiring on Insulators

Introduction

NEC Article 398 provides the framework for using open wiring on insulators, a traditional and exposed wiring method. This wiring system is suitable for certain industrial and agricultural applications and specific services where conductors are openly routed on ceramic or noncombustible insulating supports. While its use is limited today, this method remains relevant for specialized installations due to its simplicity and effectiveness.

Scope

This article covers the installation and construction requirements for open wiring on insulators. Such systems are visible and not concealed by building structures, making them suitable for both indoor and outdoor applications, including dry and wet locations, and areas with corrosive vapors.

Permitted and Prohibited Uses

Permitted Uses:

1. Indoor and Outdoor Locations:
   • Open wiring on insulators can be installed in exposed areas, provided they meet environmental and safety standards.
2. Wet or Corrosive Environments:
   • Conductors must have insulation rated for such conditions, as specified in Table 310.4(1).
3. Industrial and Agricultural Applications:
   • Allowed in settings like barns, factories, or other environments where visibility and accessibility of wiring are critical.

Prohibited Uses:

1. Concealed Installations:
   • Wiring must remain visible and cannot be hidden within building structures or finishes.
2. Temporary Power Systems:
   • Prohibited for temporary lighting or power circuits at construction sites.
3. Hazardous Locations:
   • Not suitable for certain environments, including those with high risks of physical damage or fire.

Installation Guidelines

1. Conductor Protection:
   • Conductors must be routed through noncombustible insulating tubes when passing through structural members.
   • A clearance of 2 inches (50 mm) must be maintained between conductors and metallic raceways, piping, or other conductive materials.
2. Drip Loops and Damp Locations:
   • When entering or leaving damp or corrosive environments, conductors must include drip loops and pass through nonabsorbent insulating tubes to prevent moisture ingress.
3. Exposed Work:
   • Conductors in accessible areas must be supported on noncombustible materials like ceramic knobs or cleats.
   • If within 7 feet of the floor, additional protection like guard strips or rigid conduits is required to prevent physical damage.
4. Accessible Attics:
   • Wiring in attics must comply with specific mounting and protection rules to prevent interference or damage.
5. Support Spacing:
   • Conductors smaller than 8 AWG must be supported every 4.5 feet, while larger conductors have greater spacing allowances in industrial installations.

Construction Specifications

1. Conductor Types:
   • Must be selected from Table 310.4(1), ensuring compatibility with the intended environment.
2. Support Hardware:
   • Knobs and cleats must be securely mounted using appropriately sized nails or screws. Cushion washers should be used with nails to minimize damage.

Applications

1. Industrial Buildings:
   • Ideal for open feeders mounted on roof trusses or other visible installations.
2. Agricultural Establishments:
   • Commonly used in barns and similar facilities where visibility and accessibility are critical.
3. Restoration Projects:
   • Employed in historical restorations to maintain authenticity while ensuring electrical safety.

Safety Considerations

1. Clearance Requirements:
   • Maintain appropriate separation between conductors and other objects to prevent arcing or insulation failure.
2. Regular Inspections:
   • Periodically check for damage or wear, especially in high-moisture or corrosive environments.
3. Compliance with Standards:
   • Ensure installations adhere to NEC and local codes for safety and reliability.

Conclusion

NEC Article 398 ensures the safe installation and operation of open wiring on insulators. By adhering to these guidelines, electricians can effectively use this traditional wiring method in specialized settings while maintaining compliance and safety.