Voltage Drop

Every conductor has resistance and, for AC circuits, reactance. When current flows, these impedances cause the voltage at the load terminals to be lower than the supply voltage. This reduction must be controlled because motors may stall, lighting may dim, and electronic equipment may malfunction if the terminal voltage falls too far below rated values. Most standards limit total voltage drop to 3–5% of nominal supply voltage. The mV/A/m method simplifies calculation: manufacturers publish millivolt-drop-per-ampere-per-metre values for each cable size and type, allowing engineers to compute voltage drop as Vd = (mV/A/m × Ib × L) / 1000 for single-phase circuits. For three-phase balanced loads, use the three-phase mV/A/m values from standard tables — these already account for the phase relationship and no additional factor is needed. Long cable runs, high currents, and small conductors increase voltage drop, often forcing selection of a larger cable than the current-carrying capacity alone would require. Power factor also affects voltage drop in AC circuits — highly inductive loads increase the reactive component of impedance, raising the overall drop.