Voltage Drop and Conductor Sizing for 24V DC Systems

On a 230V mains circuit, losing a volt or two in the wiring is barely worth mentioning. On a 24V DC bus, the same loss is a serious percentage of the supply voltage, and it grows quickly with distance. Voltage drop is the quiet cause of many LED problems: dim or flickering output at the far end of a run, modules that drop out of regulation, and color differences between fixtures fed from the same driver. Understanding and calculating it is basic, and it belongs in every installation plan.

The calculation is Ohm's law applied to a round trip. The voltage drop in volts equals 2 times the one-way conductor length in meters times the current in amperes times the conductor resistance in ohms per meter. The factor of 2 exists because current travels out on one conductor and returns on the other. Conductor resistance comes from the manufacturer's tables for the chosen wire size, and it is the only number in the formula that the installer controls.

Worked examples make the scale of the problem clear. A 1.5mm squared copper cable has a resistance of roughly 12.1 milliohms per meter. At 3A over 10 meters, the drop is 2 times 10 times 3 times 0.0121, about 0.73V, or 3% of a 24V bus. Doubling the distance to 20 meters doubles the loss to 1.45V, over 6%. A 0.75mm squared cable at the same current and distance loses more than 12%, which is enough to push a 24V module out of its input window entirely. The percentage, not the absolute volt, is what matters on a low-voltage system.

The symptoms of excessive drop are easy to misdiagnose as driver faults. When the voltage at the far module falls below its regulator's minimum input, the regulator drops out and the module flickers or dims. Because modules closer to the driver still see full voltage, the failure pattern is distance-dependent, which is a strong diagnostic clue. In constant-current systems the driver compensates automatically, which is one reason professional installers favor them for long runs; a constant-voltage driver cannot compensate, so the wiring must be sized correctly from the start.

Good practice follows a few rules. Size the conductors so that total drop stays below 3% of the bus voltage, and prefer 2% for critical installations. Keep the driver close to the load; a long low-voltage run is almost always worse than a longer mains run with the driver near the luminaires. Where loads are distributed, feed the bus at multiple points or use a thicker trunk with short taps rather than a single thin run. Terminate and torque connections properly, because a loose connection adds resistance, heat, and a fire risk that no cable size can fix. In practice, many installers run a thicker trunk cable and tap short branches, which keeps the bulk of the drop on the shortest possible path and simplifies future additions.

The checklist before commissioning is short. Measure the actual voltage at the driver's output terminals, then at the farthest load, with the system at full current. The difference is the real drop, and it should be under 3%. If it is not, shorten the run, increase the conductor size, or feed at additional points. Voltage drop is not glamorous, but on a 24V system it is the difference between a lighting installation that performs as designed and one that generates service calls for years.