Why are cables only made of round shapes? What is the advantage of it? Why aren't there triangular or quadrilateral or pentagon shapes of cables?
Like Jessica Rabbit- because they are drawn that way.
Wire production involves pulling (drawing) the wire through successively smaller dies (often with annealing in between). The dies are most easily made with round holes (they are typically made from very hard materials such as diamond).
Not all wire is round - rounded rectangular wire is sometimes used in inductors and transformers.
When you move away from circular cross sections you run into two significant problems:
First, corners would exert more pressure against adjacent things, and are more likely to result in insulation damage.
Second, for high frequency use the skin current effect would result in higher impedance in a cable with the same cross sectional area but with corners. Further, the greater amount of insulation required to cover that conductor would also change the impedance. While this could be dealt with, we already have a great understanding of the skin effect, impedance, and insulation effect at high frequencies for round conductors that it would be limiting for designers to use alternate configurations without a good reason (such as fitting more copper in a smaller area for efficiency in inductors).
There are many other considerations - wire drawing is already difficult and strenuous, having a circular cross section reduces die contact for a given area of copper. Insulation would increase due to the additional circumference of a triangle or square compared to a circle (again, for the same cross sectional area of copper). When bending or flexing the corners would see more stress, would harden more quickly, and would crack more readily than circular cable.
When discussing cables it becomes even harder to make other cross sections. Imagine taking 10 conductors to make a cable in a triangle. When bent, twisted, or squeezed you'd find the individual wires in the cable moving out of their place and deforming to a more circular shape. How would you keep those wires in place to maintain the desired cable cross section? You could go to a single wire, but then it will be much harder to manage, much less flexible, and more prone to breaking and damage.
From a production point it's easily the most simple shape one can go for, which I would presume is the dominant factor in all of this. It's also mechanically favourable if you consider the strain when you start bending things. And if you think about it, its also is the most efficient way to pack as much cross-sectional area (hence conductive material) in a limited space.
They are not only round. Anyway dies holes are round cause they are easier to make and the most economic. Also round cables are more versatile in some general situations, for example to insert them into corrugated (or not corrugated) pipes, to bend or curve them in little space, for ex. when is needed to connect them to electrical home-office standard outlets etc. So the round section is in general the most used for practical and economic advantages. Anyway the round section is not the only standard. Other shapes are used. For example lastly is spreading the use of adesive flat cables to home or industrial electrical systems: you just attach them to the wall with their adesive and you can putty and paint directly on them without the use of corrugated pipes, without broaching the wall, without plastering. Also flat wires more effectively dissipate the heat produced during their operational phase causes they have a bigger surface exposed to air compared to the section needed for a specific electric load.
For low voltage cables (240 V, 440 V) the shape of the conductor isn't critical.
For high voltage cables (3.3kV and above), the voltage stress needs to be controlled so the insulation doesn't break down.
Voltage stress is strongest at sharp corners. So The easiest way to control this is to make everything round.
Due to the skin effect, alternating current tends to travel on the surface of wire. DC through the whole core. A round wire maximizes the cross-section per unit of material used, so it is the most cost effective at providing a given gauge or capacity. It also has the side-effect of providing the most surface area on the surface of the conductor for a given cross-sectional area of the overall wire.
While the wire making die process itself is largely the reason for it's shape, dies could make it in a variety of shapes, but the circular cross section is also the most cost effective.