line drop

Line drop is the reduction in voltage that occurs as electrical current travels along a conductor. When current flows through any wire or cable, the resistance of that conductor causes energy to dissipate as heat, and the voltage at the far end is necessarily lower than at the source. This voltage loss accumulates with distance and current magnitude, making line drop a practical constraint in power distribution rather than a theoretical nuisance.

The amount of line drop depends on three factors: the resistance per unit length of the conductor, the total length of the run, and the current flowing through it. A 10 mm² copper cable carrying 100 amps over 50 meters will lose roughly 10 to 15 volts, depending on conductor type and temperature. Aluminum conductors, lighter and cheaper but with higher resistivity than copper, produce greater line drop at the same current and distance. Engineers calculate expected drop during design and choose conductor sizes accordingly, using the formula: drop (volts) = 2 × resistance (ohms per km) × length (km) × current (amps).

Where line drop matters in practice

In long-distance transmission, line drop is managed through high voltage stepping. A 10 MW signal sent at 100 kV over 200 km loses proportionally less power than the same signal at lower voltage; this is why power stations step up voltage immediately after generation. Distribution networks feeding residential areas operate at lower voltages, typically 400 or 230 volts, where line drop becomes more visible. An undersized service cable to a remote outbuilding can cause lighting to dim noticeably when heavy loads start, because the voltage drop under load becomes significant relative to the low starting voltage.

In industrial control circuits and data systems, excessive line drop can prevent equipment from functioning reliably. A motor starter fed through undersized cable may not release its holding coil even after energizing if the voltage drop under coil current leaves insufficient voltage at the coil terminals. Similarly, PLCs and sensors supplied through long cable runs may reset or malfunction if line drop pushes the supply voltage below their minimum operating threshold.

Installers combat line drop by oversizing conductors, shortening runs where possible, running higher voltages in long circuits, or using voltage drop compensators. Building codes and standards typically limit acceptable line drop to 3 percent on branch circuits and 5 percent combined for feeder plus branch, measured at maximum expected load current. Thermal effects also matter: line drop increases as conductors heat up, since resistance rises with temperature, so calculations must account for the hot conductor resistance, not just its value at 20 degrees Celsius.