The insulation of the cable must be able to withstand the voltage stresses experienced during normal and abnormal operating conditions. Therefore the selection of the cable insulation should be made on the basis of the applicable phase-to-phase voltage and the general system category which are classified as either 100%, 133%, or 173% insulation levels.
These insulation levels are discussed as follows:
1. 100% level: Cables in this category may be applied where the system is provided with relay protection which normally clears ground faults within 1 min. This category is usually referred to as the grounded systems.
2. 133% level: Cables in this category may be applied where the system is provided with relay protection which normally clears ground faults within 1 h. This category is usually referred to as the low resistance grounded, or ungrounded systems.
3. 173% level: Cables in this category may be applied where the time needed to de-energize the ground fault is indefinite. This level is recommended for ungrounded and for resonant grounded systems.
The current capacity that the cable needs to carry is determined by the load it serves. The NEC is very specific in terms of sizing conductors for systems operating below 600 V.
The current-carrying ability of cable is based upon an operating ambient temperature. When cables are installed in multiple duct banks, it is essential to derate the cable current capacity in order not to exceed its thermal rating.
In cases where cables may be load cycled, the currentcarrying capacity may be calculated by the following formula:
Ieq = Ei^2t/T
Ieq is the equivalent current-carrying capacity
I is the constant current for a particular time period
t is the time period of constant current
T is the total time of duty cycle
E is the voltage of the cable