Understanding NEC Article 460: Capacitors

Introduction

NEC Article 460 outlines the installation and operational requirements for capacitors in electrical systems. These components are crucial in managing reactive power, improving power factor, and optimizing system efficiency. Capacitors are widely used in residential, commercial, and industrial settings, particularly for power factor correction and voltage regulation.

Scope

Article 460 applies to capacitors installed in electrical circuits, excluding:

• Surge Capacitors: Incorporated within other equipment.
• Components: Capacitors that conform to apparatus-specific standards.

This article establishes rules for capacitor installation, grounding, overcurrent protection, and marking.

Key Provisions

Enclosing and Guarding

1. Flammable Liquids:
   • Capacitors with more than 11 liters (3 gallons) of flammable liquid must be enclosed in vaults or outdoor fenced enclosures compliant with Article 110.
2. Accidental Contact:
   • Capacitors accessible to unauthorized persons must be enclosed or guarded to prevent accidental contact with energized parts.

Discharge of Stored Energy

1. Time of Discharge:
   • Capacitors must reduce residual voltage to 50 volts or less within 1 minute of disconnection.
2. Means of Discharge:
   • A discharge circuit must be permanently connected or automatically engaged when voltage is removed.

Conductor Ampacity

1. Minimum Requirements:
   • Circuit conductors must have an ampacity of at least 135% of the capacitor’s rated current.
   • Conductors connecting capacitors to motor circuits must be at least one-third the motor conductor ampacity and no less than 135% of the capacitor’s rated current.

Overcurrent Protection

1. Protective Devices:
   • Overcurrent devices must protect each ungrounded conductor in capacitor circuits.
   • A separate overcurrent device is not required if the capacitor is downstream from a motor overload device.

Disconnecting Means

1. Simultaneous Operation:
   • Disconnecting means must open all ungrounded conductors simultaneously.
2. Requirements:
   • Must handle 135% of the capacitor’s rated current.

Marking

Capacitors must include a nameplate displaying:

• Manufacturer details.
• Voltage, frequency, kilovar, or ampere rating.
• Number of phases.
• Liquid type and volume, if applicable.

Applications

1. Industrial Settings:
   • Capacitors improve power factor and reduce energy losses in manufacturing plants.
2. Commercial Use:
   • Used in large office buildings to stabilize voltage and enhance efficiency.
3. Renewable Energy Systems:
   • Capacitors manage reactive power in solar and wind power installations.

Safety Considerations

1. Grounding:
   • Capacitor cases must connect to the equipment grounding conductor unless designed for operation at other than ground potential.
2. Harmonic Resonance:
   • Systems with harmonic-producing loads must conduct engineering studies to avoid harmonic resonance, which can cause overcurrent and overvoltage conditions.

Conclusion

NEC Article 460 provides essential guidelines for installing and operating capacitors safely and efficiently. These standards ensure the protection of personnel and equipment while optimizing system performance.