Please refer to the current edition of the Wiring Regulations, maintained and published by the Institution of Electrical Engineers.

For motors you have to consider the method of starting.If u are using a starter like star delta where applied voltage is reduced, then taking full load current is in order.Direct starting depending upon the load inertia one has to decide which current to consider.If the load is a low inertia load and acceleration time is of the order of 3-4 sec then rated current can be used.If its high inertia load with motor taking longer time to reach the full rated speed then current has to be considered which ensures Voltage drop does not drop below 80%.Most of the motors are designed for acceleration at 80% Voltage.Except for High inertia loads rated current can be taken for Voltage drop calculation as starting current persists for a short time and sizing cable for that may result in over sized cables and extra cost.For high inertia drives normally current values are taken from the graphs indicating Starting Current Vs Time which shows how fast current is dropping.Needs little detailed work and experience.