Definitions, codes and standards for medium-voltage systems

Throughout the United States, several different voltages are used to power residential areas, commercial buildings and industrial facilities. These voltages generally range from 120 volts (V) for lighting and appliances that are low-voltage (LV), up to 69,000 V or 69 kilovolts (kV) for campuswide distribution that is medium-voltage (MV). When voltages exceed 69 kV, the system is considered high-voltage (HV), which is more commonly used for transmission lines.

ANSI Standard C84.1: Electric Power Systems Voltage Ratings (60 Hz) defines the system voltage classes as follows:

• LV: 1,000 V or less.

• MV: Greater than 1,000 V and less than 100 kV.

• HV: Equal to or greater than 100 kV and equal to or less than 230 kV.

• Extra-high voltage: Greater than 230 kV and less than 1,000 kV.

• Ultra-high voltage: Equal to or greater than 1,000 kV.

Similarly, NFPA 70: National Electrical Code (NEC) also defines MV equipment as 1,000 V and higher. Before the 2014 edition of the NEC, MV was applied to equipment rated 600 V and higher. This article discusses design considerations for MV electrical systems in industrial applications.

For MV electrical systems, the following codes, standards and references apply:

• ANSI
• NEC 2023 edition
  • NEC Article 110 Part III: Requirements for electrical installations over 1,000 V, nominal
    • NEC Article 110.34: Working space
  • NEC Article 240 Part IX: Overcurrent protection over 1,000 V, nominal
  • NEC Article 315.60 Tables: Ampacities of conductors 2,001 V to 35 kV
  • NEC Article 450: Vault and fire requirements for transformers and transformer vaults over 35 kV
• NFPA 70B: Recommended Practice for Electrical Equipment Maintenance.
• NFPA 70E: Standard for Electrical Safety in the Workplace.
• Institute of Electrical and Electronics Engineers (IEEE).
• International Building Code (IBC).
• International Electrical Testing Association (NETA) MTS: Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems.
• International Fire Code (IFC).
• International Mechanical Code (IMC).
• National Electrical Manufacturers Association (NEMA).

Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our WTWH Media editorial team and getting the recognition you and your company deserve. Click here to start this process.