Medium-Voltage Cable Insulation Ratings

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.

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