Electrical Basic Concept
Cable Sizing
1.2 For all cables other than transformer feeders, the eight hour maximum demand shall be the basis for cable capacity calculation.
1.3 Transformer feeders shall have a capacity not less than the transformer fan-cooled rating. When a feeder supplies more than one transformer, its rating shall be at least equal to the summation of the fan-cooled ratings of all secondary selective and spot network substations plus the self-cooled rating of all radial substation transformers supplied by the feeder.
1.4 Lighting feeders feeding lighting panels shall have a capacity not less than the maximum demand of the load.
1.5 Lighting and power wire and cable shall be derated under any of the following conditions:
a. Outdoor ambient temperature of 50 C
b. More than three conductors in a raceway or cable
c. More than one cable in an underground duct bank or trench
d. More than one cable in a cable tray
e. Cables shall be sized to limit voltage drop
1.6 Motor branch circuit conductors shall be sized in accordance with IEC.
1.7 Cable capacity for tray installation shall comply with the IEC including the effects of ambient temperature and solar radiation.
1.8 Feeders rated above 600 volts shall be sized to withstand short circuit thermal stress without damage to the feeders. The maximum short circuit level of the supply, times a 1.25 safety factor, and the clearing time of the feeder protective device shall be used to determine this condition.
1.9 Power cables in underground conduit banks shall be de-rated in accordance with the Neher-McGrath method in order not to exceed the conductor insulation temperature specified by the cable manufacturer. Cable capacity calculation shall take into account the applicable de-rating factors, such as for ambient temperature, ground thermal resistivity, maximum thermal resistivity of selected backfill, depth of laying and grouped installation of cables.
a. For sizing of underground cables, use the following criteria:
Ground temperature1 = 35 deg.C (unless otherwise specified)
Ground thermal resistivity = To be measured
Selected backfill thermal resistivity = 120 Deg. C.cm/W
Load factor = 1.0
Also refer to latest “Metrological and Hydrological Design Information” from Royal Commission/Local Authority for ground temperature to be considered for cable sizing calculations.
1.10 Cable sizing calculations shall include the 20% future cables installed in the top positions of the duct bank or trench. The rating shall be for the worst case location along the route, i.e., exiting the substation, approaching another duct bank, road crossing, etc.
1.11 Medium and high-voltage cables shall be selected to coordinate with the maximum 1 second short circuit rating of the connected equipment.
1.12 Medium-voltage feeders to load-center substations shall be sized for the maximum transformer rating. This size shall include the transformer rating obtainable by forced-air cooling.
1.13 Wire size shall be selected to limit maximum voltage drop for power and high intensity discharge lighting circuits to 2.5 percent for feeder circuits and 2.5 percent for branch circuits.
1.14 Services, feeders and branch circuits for power and lighting installations other than specified above shall be sized in accordance with the requirements of IEC.