Understanding NEC Article 235: Branch Circuits, Feeders, and Services Over 1000 Volts AC or 1500 Volts DC
Overview
NEC Article 235 governs the installation, identification, and protection of branch circuits, feeders, and services operating over 1000 volts AC or 1500 volts DC. For instance, these regulations are critical in industrial facilities where high-voltage feeders supply power to multiple large motors, ensuring safe operation and compliance with safety standards. It provides essential safety and operational guidelines for high-voltage systems, ensuring that installations meet the demands of modern infrastructure while maintaining safety for personnel and equipment.
Scope and General Requirements
- Article 235 applies to electrical systems exceeding 1000 volts AC or 1500 volts DC, including branch circuits, feeders, and services.
- It consolidates and builds upon medium-voltage requirements previously found in Articles 210, 215, 225, and 230.
- Adherence to ANSI/IEEE C2-2017, National Electrical Safety Code, is recommended for additional guidance. This code complements NEC Article 235 by providing detailed safety requirements for the design, installation, and maintenance of high-voltage systems in industrial and utility environments.
Branch Circuits
Conductor Identification
- Grounded Conductors: Identified according to NEC 200.6.
- Ungrounded Conductors:
- For AC systems with multiple voltages, identification by color coding, tagging, or other approved means is required.
- For DC systems, positive and negative conductors must be clearly marked using durable means, such as colored insulation or permanent labels.
- Equipment Grounding Conductors: Identified per NEC 250.119.
Voltage Limitations
Branch circuits exceeding 1000 volts AC or 1500 volts DC are permissible for qualified installations where proper supervision and maintenance are ensured.
Load Requirements
Branch circuits must supply loads without exceeding their rated ampacity. Exceeding these limits can lead to overheating, insulation failure, and potential fire hazards, compromising both system integrity and safety. Special provisions apply to multiple-outlet circuits, including restrictions on equipment loads to maintain compliance.
Feeders
Conductor Size and Rating
- Feeders must meet minimum ampacity requirements based on NEC 310.14 and 315.60.
- For supervised installations, conductor sizing may be determined under engineering supervision.
Overcurrent Protection
- Feeders must be equipped with overcurrent protection devices that account for continuous and non-continuous loads.
- Devices must handle short-circuit and fault conditions effectively.
Identification and Documentation
- Ungrounded conductors require identification at all terminations and splice points.
- Documentation of identification methods must be readily available at feeder panelboards or similar equipment.
Services
Disconnecting Means
- Service disconnects must be installed in accessible locations and be capable of simultaneously disconnecting all ungrounded supply conductors.
- For remote or industrial applications, disconnects may use mechanical or electronic remote controls.
Surge Arresters
- Surge protection devices (SPDs) must be installed for overhead service conductors to protect against voltage spikes.
Equipment and Maintenance
- Service equipment, including switchgear, must meet stringent safety and performance standards.
- Systems exceeding 35,000 volts must terminate in compliant switchgear compartments or vaults. Compliance requires that the compartments or vaults provide adequate insulation, ventilation, and protection against arc flash hazards, while meeting all applicable standards for structural integrity and accessibility as specified by NEC and IEEE guidelines.
Clearances and Safety Standards
Overhead and Underground Wiring
- Overhead conductors must maintain specified clearances from roadways, pedestrian areas, and structures to ensure safety.
- Underground installations must follow approved methods such as conduits or direct burial with appropriate depth and protection.
Inspection and Testing
- High-voltage systems must undergo pre-energization tests to ensure proper functionality and safety. These tests typically include insulation resistance testing, high-potential (hi-pot) testing, continuity checks, and grounding system verification to identify potential faults or weaknesses before system energization.
- Test reports must be submitted to the authority having jurisdiction before system energization.
Conclusion
NEC Article 235 provides a robust framework for high-voltage electrical installations, ensuring operational reliability and safety. By addressing identification, conductor sizing, overcurrent protection, and system clearances, this article is indispensable for electricians working with advanced power systems
