What factors do we need into consideration while drafting the cable routing plan?

to cut things short you should mainly consider the following :

1-Cable grouping factor (how many cables in the same duct and the heat they produce due to the passing current )

2-the duct filling percentage to allow the cables to be pulled ,

3-the different service levels ( a cable carrying high current shouldn't be passing in the same duct or on the same cable tray level with a cable carrying instrumentation signals or a control cable to avoid induction of currents that could lead to major problems .

4-the binding ratio of the cables and the cable thickness .