Core idea
Cable resistance causes voltage loss and heat. Longer runs, higher current and smaller conductors increase drop.
Practical approach
Confirm units, identify the governing standard or manufacturer data, record assumptions, calculate an initial value, and check whether the result is sensitive to friction, temperature, efficiency, tolerances or material variation.
Use calculators with judgement
Online tools are useful for comparison and first-pass design. Safety-critical work requires suitable factors of safety, verified inputs and review by a competent person.