This is a very broad question. There are very many answers depending on the situation, governing standards, codes, etc. Typically busbar capacity is determined by the switchboard construction standards. It can be based on current density or temperature rise. Cable capacities are specified by governing building codes or the cable manufacturer. Circuit breakers are rated by the manufacturer.
ahmadnece - One-step at the time is probable better approach in your learning process.
There are typically two distinct approaches used for designing electrical equipment and for engineering application. In the first case theoretical calculation are usually supported by prototype and production tests in compliance with standards. In the second case, system studies, calculation and engineering judgment are conducted to select the appropriate equipment to withstand the system electrical and mechanical stresses.
Typical calculations are continuous ampacity, short circuit, load flow, insulation coordination, etc.
Some of those factors are ambient temperature, altitude, pollution level, seismic, frequency, voltage, BIL, loading, grounding, etc.
Here are links with basic engineering application:
Cable:
PS: Before getting involved with learning a software, try to understand the technical principle first. Doing the opposite, you be a risk to know how to operate mechanically the software without understanding what you are doing.
