About Cable Sizing-The following items shall be considered:

Cable Sizing
The following items shall be considered:
a) Short circuit capacity
b) Continuous current carrying capacity
c) Voltage drops in normal conditions & transient conditions.
d) Any special operating conditions stated in electrical design criteria and/or other project specifications
The largest size of cables as determined from a, b, c, and d shall be used.

a) Short circuit capacity
Maximum short circuit current shall be considered for determining cable short circuit capacity. The fault clearing time under short circuit conditions of power cables connected to circuit breakers shall be calculated by the following:
1) The backup relay operating time at the maximum fault level
2) 0.05 second to cover variation of settings
3) The circuit breaker operating time
Where power cables are protected by fuses, its cut off time shall be considered.
b) Continuous current carrying capacity (I0)
Continuous current carrying capacity depends on the maximum permissible continuous conductor temperature and various types of cable installation. When applying that temperature, cable terminations and associated equipment shall have the ability to withstand the temperature without damage and to dissipate the heat due to cable temperature.
c) Voltage drop
The overall voltage drop in power cables will be limited to a maximum 5% at full load and will not exceed 15% during motor starting. This voltage drop is from the main/source to the end consumer.
The nominal system voltage, the maximum impedance of each component and the full load current of each cable are used for voltage drop calculations.