Definitions, codes and standards for medium-voltage systems

Understand the definitions related to medium-voltage (MV) systems, as well as other voltages

By Taha Mohammed and Robert Magsipoc October 13, 2023
Figure 3: Rocky Gorge Pump Station electrical room layout. Courtesy: Washington Suburban Sanitary Commission

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:

Author Bio: Taha Mohammed, PE, is an electrical engineer at CDM Smith. Robert Magsipoc, PE, is an electrical engineer at CDM Smith.