Voltage Drop in Constant-Voltage LED Systems: Calculation and Mitigation

One of the most common field failures in LED strip installations is not a dead driver but a dim tail: the far end of a long run of 24 V strip glows noticeably weaker than the end nearest the driver. The cause is voltage drop in the DC cable. The driver holds its output voltage at the terminals, but the cable is a resistor, and the load current flowing through it consumes part of that voltage before the strip ever sees it.

The physics is Ohm's law applied to a round trip. The resistance of the cable is proportional to its length and inversely proportional to its cross-sectional area. Because the current must travel out along one conductor and back along the other, the effective resistance is double the one-way value. Using the resistivity of copper, about 0.0172 ohm-millimeter-squared per meter, a 2.5 mm² cable offers roughly 0.0069 ohm per meter one way.

A worked example makes the effect concrete. Consider a 24 V system carrying 5 A to a strip 10 m away, wired with 2.5 mm² cable. The round-trip resistance is 2 x 10 x 0.0069, approximately 0.138 ohm. At 5 A, the voltage dropped in the cable is 0.138 x 5, about 0.69 V, or 2.9 percent of the 24 V supply. The strip receives roughly 23.3 V, and the voltage at the strip is not uniform either, because current is drawn along the length of the strip itself.

Voltage drop matters more at low voltages because it represents a larger fraction of the supply voltage. A 2 V drop on a 230 V circuit is negligible; the same 2 V on a 24 V circuit is 8 percent of the supply, enough to cause visible brightness differences and, in extreme cases, color shift at the far end of white strips. The practical guideline used across the industry is to keep total voltage drop below 3 to 5 percent of the nominal output voltage, which for 24 V means no more than roughly 0.7 to 1.2 V.

Several mitigation strategies are available. Increasing the cable cross-section is the most direct fix: doubling the cross-section halves the resistance and therefore halves the voltage drop. Placing the driver closer to the center of the load reduces the maximum run length. Feeding the strip from both ends, a technique called parallel feeding or mid-point injection, halves the current carried by each branch and dramatically reduces drop on long runs. Choosing a higher rail voltage, such as 48 V instead of 24 V, halves the current for the same power, cutting drop by half without changing cable size.

It is important not to confuse the DC output cable with the DALI control bus. The DALI bus is a separate two-wire pair carrying a nominal 16 V signal at up to 250 mA, and the standard limits the voltage drop along the bus to about 2 V. Bus wiring is sized for that limit, not for the load current, and the two circuits should be calculated independently. The output cable sizing is governed by the LED load current; the bus wiring is governed by distance and device count.

Voltage drop calculations should be done before installation, not after a complaint. The calculation requires only three inputs: the total load current, the one-way cable length, and the conductor cross-section, plus the copper resistivity and a target drop limit. For a 24 V constant-voltage DALI-2 installation, applying the 3 to 5 percent rule during design prevents the majority of brightness uniformity problems seen on site.