Why the Longest Cable Run Should Decide the Conductor Size
A cable can pass every current-carrying check on paper and still starve the equipment at the far end. The reason is voltage drop, and it is the check that gets skipped most often. A current rating answers one question: how much current a conductor can carry before it overheats. It says nothing about how much voltage is lost pushing that current along the run. On a short circuit inside a flat the loss is trivial. On a 120 metre feeder to a borewell pump or a distant site panel, it decides whether the load works properly at all.
Every conductor has resistance. The longer the route and the higher the current, the more voltage is spent as heat in the cable itself, leaving less at the load. A motor supplied several percent below its rated voltage draws more current to hold its torque, runs hotter and gives up insulation life. Lighting dims and flickers. Drives and electronic controls trip on undervoltage or reset without warning. None of this shows up on an ammeter at the board, because the current is within rating. The fault is the voltage that never arrived.
What the standards allow
Indian practice follows IS 732 and IEC 60364-5-52. Both keep the total drop from the origin of the installation to any load point within roughly 3 per cent for lighting and about 5 percent for power. Sensitive electronic and drive loads are usually held tighter still. These are not arbitrary numbers. They are the margin that keeps equipment inside the voltage band it was designed for after the worst-case run.
The drop for a given circuit is easy to estimate from the datasheet. Each cable is quoted a millivolt-per-ampere-per-metre figure, written mV/A/m. Multiply it by the design current and the route length, divide by 1000, and the answer is the volts lost. A larger conductor has a smaller mV/A/m, so the loss falls as the cross-section rises. The APAR voltage-drop reference table carries these values for APAR LV cables.
Three ways to bring the drop back inside the limit
The first and most common fix is a bigger conductor. Going one or two sizes up lowers the resistance and the loss in proportion, at the cost of copper or aluminium and a larger gland. The second is to shorten the electrical route, by moving the sub-board closer to the load rather than running one long radial from the main panel. The third, where the design allows it, is to distribute at a higher voltage or in three phase, so the same power moves as less current and the drop shrinks.
How APAR recommends sizing it
Size on two separate checks and take the larger conductor. First run the current-carrying check, and derate it for the real ambient temperature and for grouping where cables sit together. Then run the voltage-drop check for the actual length of the run. On short circuits the current rating wins and the cable is small. On long feeders the voltage-drop check almost always calls for the bigger conductor, and that is the size to order. The kW-to-cable-size chart is a quick starting point for the ampacity side before the voltage-drop figure is applied.
For LV feeders and distribution, APAR supplies LV XLPE power cables in copper and aluminium up to 1000 sq mm. Where the run is long but the route is congested and there is no room for a larger cable, high-ampacity 105°C XLPE cables carry more current at the same cross-section by running at 105°C, which can hold the ampacity while a heavier conductor is used to control the drop. The rule does not change: check both, and let the longest run decide.
