Design

Medium-Voltage Cable Insulation Ratings

Sept. 23, 2014

Do you know the rules behind the three categories of insulation?

Below Table 310.104(E) in the 2014 National Electrical Code (NEC) are important notes that could be critical in the type of insulation that is specified or purchased for industrial locations. These notes have been part of the Insulated Cable Engineers Association (ICEA) for many years before becoming part of the NEC a few cycles ago.

The notes refer to three acceptable insulation categories for medium-voltage (MV) cable: 100%, 133%, and 173% levels. They all pertain to the thickness of the insulation.

• 100%-rated cables may be used on a system where ground faults are cleared as rapidly as possible via a relay protection scheme. This would probably be the case where the system is solidly grounded, low-resistance grounded, or low-impedance grounded.

• 133% rated cables may be used on systems where ground faults are assured to be cleared within an hour. These are typically high-resistance grounded systems.

Cables in the 173% rated category are permitted in industrial facilities where the conditions of maintenance and supervision ensure that only qualified persons service the installation, the clearing time for the ground fault may exceed 1 hour, where an orderly shutdown is essential for protection of persons and property, and there is adequate assurance that the faulted section will be de-energized in an orderly shutdown.

When specifying MV cable, it’s important to know how much time a ground fault may exist on the circuit before it is cleared as well as the other conditions that may exist. Ideally, MV systems should be designed under engineering supervision, although the NEC doesn’t require it in this case.

About the Author

Eddie Guidry | Senior Fellow

Eddie Guidry is a senior fellow with Fluor Enterprises, Inc., Sugar Land, Texas. He is highly skilled in electrical and control systems for industrial construction, design, and engineering (both domestic and abroad). The majority of Eddie’s 38 years of experience includes upstream and downstream portions of petrochemical plants and refineries. He is also heavily experienced in water and wastewater treatment facilities. Eddie is very active in the development of U.S. national (ANSI) and international codes and standards, and has been a principal member of the National Fire Protection Association (NFPA) National Electrical Code (NEC) committee since 1999 and NFPA’s National Advisory Committee on Electrical Safety Research since 2008. Eddie, who has also developed and conducted many electrical courses and seminars over the years, currently holds the corporate Master Electrician license for Fluor Corporation in the State of Texas and is also an ICC/IAEI certified electrical inspector.