Voltage Drop Engineering for Long LED Cable Runs: Calculations That Prevent Dark Ends

The most common field complaint in LED strip installations is a brightness gradient: the strip is bright at the driver end and dim at the far end. In a properly designed system this should never happen, because voltage drop is one of the few lighting design parameters that can be calculated exactly before a meter of cable is installed. The problem is that many installers size cables by habit rather than calculation, and the consequences are visible wherever a long strip fades into darkness.

Voltage drop is governed by Ohm's law. The fundamental formula for a two-wire circuit is V_drop = 2 × I × L × R, where I is the circuit current in amperes, L is the one-way conductor length in meters, and R is the conductor resistance in ohms per meter. The factor of 2 accounts for current traveling out on the positive conductor and back on the negative. Copper resistance at 20 degrees Celsius is 0.023 ohms/m for 0.75mm², 0.017 for 1.0mm², 0.0115 for 1.5mm², and 0.0069 for 2.5mm². These rise by roughly 15 percent at operating temperature, so a conservative design uses the higher values.

Worked example: a 100W, 24V driver at full load delivers 4.16A. With a 10-meter run of 1.0mm² cable, the drop is 2 × 4.16 × 10 × 0.017 = 1.41V, or 5.9 percent of 24V — above the 5 percent design target. With 1.5mm², the drop is 0.96V, or 4.0 percent — acceptable. With 2.5mm², it is 0.57V, or 2.4 percent — comfortable margin. The same load at 12V would draw 8.33A, and a 10-meter 1.5mm² run would drop 1.92V — 16 percent of 12V, completely unacceptable. This single calculation explains why 24V systems serve longer runs than 12V and why 24V is the practical choice for distributed installations.

The calculation does not end at the cable. LED strips have internal resistance along their copper traces, and voltage drops progressively along the strip's length. A typical 24V strip on a 2-oz PCB has a trace resistance of roughly 0.1 to 0.2 ohms per meter; at a strip current of 0.6A per meter (14.4W/m), a 5-meter strip experiences an internal drop of about 0.3 to 0.6V end to end. Total drop — cable plus strip — should not exceed 5 percent (1.2V on 24V). Beyond this, brightness differences become visible, and at 10 percent the far end is noticeably dimmer and slightly shifted in color temperature.

When one driver feeds loads distributed across a large area, the standard solution is multiple shorter cable runs from a central point rather than one long daisy-chain. Each branch is sized for its own load and length, and the calculation is performed for the worst case. An alternative is power injection: run a larger feeder cable to a midpoint or the far end of a long strip and connect it there, halving the effective strip length and internal drop. Some premium installations use remote sensing, where a dedicated sense wire feeds the load voltage back to the driver so it compensates for cable drop — but this requires remote-sensing capability, which is rare in constant voltage drivers.

The installer's checklist is short. Measure the actual current draw rather than trusting the label. Calculate the drop for the worst-case branch. Add cable drop and strip drop, and verify the total is under 5 percent. If not, increase cable size, shorten the run, move the driver, or split the load. A multimeter reading at the far end after installation confirms the calculation. Ten minutes of calculation prevents years of dim, defective-looking lighting.