Understanding NEC Article 455: Phase Converters

Introduction

NEC Article 455 focuses on the installation and use of phase converters, essential devices that enable the operation of three-phase equipment using single-phase power. Phase converters are widely used in areas where three-phase power is unavailable, such as farms, remote industrial sites, and other rural locations. This article outlines the specific requirements for the design, installation, grounding, protection, and maintenance of phase converters.

Scope

NEC Article 455 applies to the installation of both rotary and static phase converters. These devices are designed to:

• Convert single-phase power to three-phase power.
• Supply three-phase motors and other equipment that rely on a three-phase electrical supply.

Types of Phase Converters

1. Rotary Phase Converters:
   • Include an internal rotor that spins to create the third phase.
   • Require proper startup procedures before connecting loads.
2. Static Phase Converters:
   • Contain no moving parts and are simpler in design.
   • Limited to providing three-phase power for starting equipment, with reduced performance during operation.

Key Provisions

Nameplate Markings

Every phase converter must include a nameplate with the following information:

• Manufacturer’s name.
• Input/output voltage ratings.
• Full-load amperes and frequency.
• Minimum and maximum load capacities in kilovolt-amperes (kVA) or horsepower.

Conductors and Grounding

1. Conductor Ampacity:
   • Variable loads require conductors rated at 125% of the converter’s single-phase input full-load current.
   • Fixed loads must consider ampacity at 250% of the full-load current for proper sizing.
2. Grounding:
   • An equipment grounding connection must comply with NEC Article 250 to ensure system safety.

Overcurrent Protection

Phase converters require overcurrent protection based on:

• Variable Loads: Devices rated at 125% of the single-phase input current.
• Fixed Loads: Protection devices sized to the ampacity of the conductors.

Disconnecting Means

Disconnecting devices must:

• Be accessible and within sight of the phase converter.
• Have an ampere rating of at least 115% of the maximum single-phase input full-load current.
• For specific loads, comply with voltage ratio and current requirements.

Manufactured Phase Marking

The generated phase must be marked clearly throughout the system to avoid connecting single-phase loads to this conductor, ensuring operational safety.

Operational Considerations

1. Startup Requirements:
   • Rotary phase converters must be running before applying loads to avoid damage to equipment.
2. Power Interruptions:
   • Systems must automatically disconnect loads during power interruptions to prevent damage or unsafe conditions.
3. Capacitors:
   • Non-integral capacitors used for motor loads must be connected on the line side of the motor’s overload protection device.

Applications

1. Agriculture:
   • Powering irrigation systems and grain elevators in areas with single-phase power.
2. Remote Industries:
   • Supporting machinery in areas without three-phase utility access.
3. Communications:
   • Powering transmitter sites for cellular networks and other communication infrastructure.

Conclusion

NEC Article 455 ensures the safe installation and operation of phase converters by defining clear requirements for conductor sizing, overcurrent protection, grounding, and marking. By adhering to these guidelines, electrical professionals can implement reliable and efficient solutions for converting single-phase power to three-phase systems.