Free tool
Cable Size & Voltage Drop Calculator
Pick the right cable cross section in square millimetres for a given current and run length, and check the voltage drop stays inside the three percent limit.
Two separate things decide the cable size
The first is current carrying capacity, which stops the cable overheating. The second is voltage drop, which decides whether the equipment at the far end actually receives a usable voltage. On short runs the first one governs. On long runs the second one does, and it is the one people forget.
The usual limit is three percent for lighting and five percent overall. Beyond that, motors run hot and draw more current, lamps dim, and electronics start behaving unpredictably in ways that look like faults in the equipment rather than in the cable feeding it.
Low voltage DC is the brutal case. The same wattage at 12 V draws roughly eighteen times the current it would at 220 V, so a solar or battery run that looks short on paper still needs surprisingly heavy cable. The calculator flags both limits separately so you can see which one is binding.