Why we are normally using cylindrical shape conductors?

Example: 

Design a suitable conductor to carry a current of 100A.

Solution:

Let us assume the material is copper

copper has a current carrying capacity of 6A/sq.mm.

We know that the property of current, it only flows through the surface of the conductor.

so 1sq.mm copper has a current carrying capacity of 6A

there fore, 1A current must be carried by 0.1667sq.mm copper conductor

there fore, 100A will carried by a conductor of 16.667sq.mm area.

Design the conductor for 20sq.mm

Let us assume one conductor is cylindrical shape and another one is square prism.

Surface area of cylinder = pi x d x L + pi x (d/2)2