Understanding NEC Article 470: Resistors and Reactors

Introduction

NEC Article 470 provides guidelines for the installation and operation of resistors and reactors in electrical circuits. These components play essential roles in current limiting, voltage regulation, and system protection. While resistors dissipate energy to control current, reactors use inductance to limit short-circuit currents and manage load conditions. This article focuses on safety, performance, and operational standards for these devices.

Scope

This article applies to the installation of resistors and reactors as separate components in electrical systems. Exceptions include resistors and reactors that are integral parts of other equipment, which are governed by the apparatus-specific provisions.

Key Highlights

Types of Resistors and Reactors

1. Resistors:
   • Common types include wire-wound, ribbon, steel grid, and box resistors.
   • Resistors dissipate heat, requiring appropriate ventilation and placement away from combustible materials.
   • Resistors mounted on switchboards or control panels do not require additional guarding.
2. Reactors:
   • Classified as air-core (no magnetic materials) or iron-core reactors.
   • Designed to limit short-circuit current or balance load conditions.
   • Air-core reactors require careful spacing and bracing to manage mechanical stresses.

General Installation Requirements

1. Reconditioned Equipment:
   • Reconditioned resistors are prohibited.
   • Reconditioned reactors are permitted, ensuring safety and compliance with manufacturer guidelines.
2. Location and Clearance:
   • Resistors and reactors must not be placed where they are exposed to physical damage.
   • A minimum clearance of 12 inches (305 mm) from combustible materials is mandatory unless a thermal barrier is provided.
3. Conductor Insulation:
   • Conductors must support operating temperatures of at least 90°C (194°F).
   • Exceptions exist for motor-starting applications with specific insulation requirements.

High-Voltage Provisions (Over 1000 Volts)

1. Physical Damage Protection:
   • Resistors and reactors must be enclosed or elevated to prevent accidental contact with energized parts.
2. Grounding:
   • Enclosures must connect to the equipment grounding conductor unless designed to operate at other than ground potential.
3. Temperature Control:
   • Metallic enclosures must not pose fire hazards or personnel risks due to temperature rise from circulating currents.
4. Oil-Filled Reactors:
   • Must adhere to additional requirements outlined in NEC Article 450, including fire safety and oil containment measures.

Safety Considerations

1. Thermal Barriers:
   • Prevent heat transfer to nearby combustible materials.
2. Spacing and Bracing:
   • For air-core reactors, follow manufacturer recommendations to ensure mechanical stability.
3. Inspection and Maintenance:
   • Regular inspections are necessary to ensure resistors and reactors operate safely and efficiently.

Applications

1. Industrial Settings:
   • Current-limiting reactors for short-circuit protection in heavy machinery.
2. Power Systems:
   • Voltage regulation and load balancing in utility grids.
3. Motor Circuits:
   • Resistors for motor starting and speed control applications.

Conclusion

NEC Article 470 ensures the safe and efficient installation of resistors and reactors in electrical systems. By addressing grounding, insulation, spacing, and maintenance, these guidelines provide a robust framework for professionals managing critical electrical components.