• Electrical Cable Sizing Criteria

The sizing of electrical cables is primarily based on the following criteria:

1. Current carrying capability of cable after applied cable derating factors
2. Voltage drop calculation in cable within acceptable limit
3. Cable Short-circuit Rating Capability

• Cable Current Carrying Capability

The current rating of cable should be satisfied:

Continuous Load Current shall be calculated with note below:

• First, incoming feeders to switchgears are sized to deliver the equipment site rated output (viz. GTGs, Transformers, EDG, etc).
• Second, incoming feeders to MCCs are sized based on rating of incoming Circuit Breaker.
• Third, incoming feeders to LV distribution boards are sized based on equipment rated output (UPS Rating, Small Power and Lighting Transformers) and outgoing feeders are sized based on the full load current.

Cable Current Carrying Capability is calculated as follows:

where:

	I	:	Cable Current Carrying Capability at actual site condition
	I0	:	Cable Current Carrying Capability at typical condition i.e. ambient temperature 30oC
	k1	:	Cable Derating factor for variation of surrounding temperature, i.e., ambient temperature (for above-ground installation) or surrounding soil temperature (for underground installation)
	k2	:	Cable Derating factor for variation in ground thermal resistivity of soil (for underground installation)
	k3	:	Cable Derating factor for depth of laying (for underground installation)
	k4	:	Cable Derating factor depending on the cable arrangement, i.e., number of installed cables in a layer, number of layers, cable spacing horizontally and vertically

• Voltage Drop Calculation in cable within acceptable limit

In checking the Voltage Drop in cables, the following criteria should be satisfied:

Unless otherwise specified by project design basic, permissible percent voltage drops in cables should be:

– 2% for main feeder cables

– 5% for motor cables under normal operation conditions and 15% for motor cables during motor starting.

In addition, power factor of motors will be as per motor catalog and a power factor of 0.3 during motor starting.

Moreover, the starting current for LV motors is assumed to be 7 times the motor full load current for DOL starting.

• Cable Short Circuit Rating Capability

The following condition should be satisfied so that cables can carry the short circuit current without any damage to cable integrity.

The basis shall be such that the let-through (I²t) energy of the current limiting devices shall be less than cables thermal withstands (I²t), which is dependent on cable type and cross-section. On other hand, the calculated short circuit at the load end of cable shall consider the fault level of switchgear and cable characteristics (cable resistance, cable reactance and cable length).