As a cable route gets longer, its resistance increases. Increasing the conductor cross-sectional area is the usual way to control voltage drop while carrying the same load current.
Start with the design current
Establish the current the circuit is expected to carry in normal service. If only power is known, calculate current using the correct DC, single-phase or three-phase relationship and an appropriate power factor where relevant.
Compare standard cable sizes
Calculate the drop for a proposed cable and move to the next standard size until the chosen target is satisfied. Avoid selecting a non-standard theoretical area that is not available for the intended cable type.
Apply installation factors
After the voltage-drop check, verify the cable's tabulated current rating and apply correction factors for ambient temperature, thermal insulation, grouping and the installation method.
This article provides general information only. Electrical installation design and verification should be completed by a competent person using the current regulations and manufacturer data.