1Start from the real project: Central or decentralized 24V supply?

When planning a 24V lighting system, you rarely start with the formula. The first questions are usually practical: Where does the 24V supply begin? Where are the power supply, driver or controller located? How long will the 24V cables be? Why can individual spots at the end of a run appear dimmer?

Voltage drop does not happen because the LED spot is defective. It happens because every cable has resistance. When current flows, part of the voltage is lost along the cable path. For planning, the actual length and load on the 24V side behind the power supply, driver or controller channel matter most.

2Circuit grouping and wire size: Reduce load first, then size the cable

Do not choose a central or decentralized supply layout first and then try to make the circuits fit. Start with real spot positions, lighting groups, room zones and expected cable lengths. Then decide whether central 24V distribution, room-based or semi-decentralized planning, or a load-near position for the 24V power supply, dimmer or controller fits better. Voltage drop is one factor; access, heat, protection, documentation and cost matter too.

The strongest electrical lever is usually not a thicker cable. It is clean load distribution. Less current on one 24V run or controller channel means less voltage loss on that path. Topology also matters: star wiring or several short branches usually keep spot input voltages closer together than one long line with many tap-off points. If a continuous line is used, return both the beginning and the end of the line to the power supply or a defined feed point. This is not series wiring; the spots remain electrically parallel on 24V DC.

1. Clarify spot positions and zones before selecting the final equipment layout.
2. Reduce load per circuit by using fewer spots or lower total wattage per run.
3. Evaluate wiring topology: star wiring, several short branches or a looped line with both ends returned to the feed point.
4. Shorten cable length through different zoning or closer equipment placement.
5. Increase wire size when length or current requires it.
6. Check the voltage at the most unfavourable spot by calculation or later by measurement.

The formula ΔU = 2 × L × I × ρ / A is still the useful final check. For pre-planning, ask the simpler question first: is too much current flowing through too much 24V cable? For the standards background, IEC 60364-5-52 covers the selection and erection of low-voltage wiring systems and includes voltage-drop limits in Clause 525. National implementation and project details still need to be checked by the electrician or planner.

3Controller position: Central control or load-near zone control

The next planning point is the position of the DALI or KNX controller. In many projects, the controller is first imagined inside the control cabinet. That is clean and service-friendly, but it can create longer 24V load cables to the spots. Alternatively, the controller can be placed closer to a lighting zone so that the 24V cables between controller output and LED spots remain shorter.

The controller position is not only a control question. It is also a voltage-drop question, because the critical current flows on the 24V side between the controller channel and the luminaires. For Tunable White systems, the DALI Alliance identifies DT8(Tc) as the DALI colour-control type for correlated colour temperature control under IEC 62386 Part 209. That helps define the control method; it does not remove the need to size the 24V load cables correctly.

Cabinet: 24V inputDIN-3SU DT8 controller for central 24V distribution and clear channel structure. Zone-near: 24V inputCompact DT8 controller for decentralized lighting groups and shorter 24V load cables. Zone supply: 230V inputIntegrated 100W power supply with DALI DT8 control for 24V Tunable White loads.

A 24V circuit with TILLUME LED spots has room between its lower and upper input boundaries. Still, 25V is a boundary value, not a preferred continuous operating target. For long-term installations, keep the highest spot voltage below that limit with reserve, for example closer to about 24.5V instead of permanently near 25V.

4Position the supply equipment: Central, decentralized or zone-based

The 24V DC power supply defines where the 24V supply begins. So the practical check is simple: how long is the path from the power supply, dimmer or controller channel to the most unfavourable spot, how high is the load and what wire size is used?

A central solution is not wrong. It is often the cleanest option when cable lengths stay short and load per circuit is limited. A decentralized or zone-based supply becomes useful with long distances, several floors, larger rooms or high loads per zone. In real projects, decentralized does not mean one driver per spot. It more often means one supply per room, zone or lighting group.

5Spot selection: Check minimum input voltage and voltage-drop reserve

Only after circuit grouping, controller position and power supply position have been considered is spot selection technically complete. In a 24V circuit, wattage alone is not enough. You also need to know what input voltage actually arrives at the end of the cable and whether that value is still inside the permitted operating range.

Between the 24V nominal system value and the 22V boundary at the spot, there is up to 2V of voltage-drop reserve. Across the full 22V to 25V input range, the theoretical window between the highest and lowest spot voltage is up to 3V. Do not spend that whole window in normal operation. For long-term installations, keep the highest spot voltage below the upper boundary with reserve, for example around 24.5V or lower.

TILLUME 24V DC power supplies can typically be adjusted at the output in the range of 23V to 27V. This can help during commissioning to compensate for small cable losses. It does not replace correct cable planning and should not make spots run permanently near the upper boundary. The practical check is not: "Can I simply turn the power supply up?" It is: "Are all spots within the permitted input-voltage range of the selected series, and is there enough reserve for continuous operation?"

If the most distant spot is below 22V, the cable design is critical. Adjust the layout by using fewer spots per circuit, a larger wire size, shorter cable runs, a different controller position or a different power supply position. If a near spot is permanently too close to the upper input boundary, the supply voltage is set too high or the layout needs to be reviewed.

See the follow-up practical article for room layouts, spot counts, cable lengths and voltage-drop calculations with TILLUME 24V LED spots, 24V DC power supplies and DALI DT8 controllers: 24V LED Spot Configuration: Examples for Power Supply, DT8 Controller and Voltage Drop.

The difference from simple 24V LED modules lies in the internal driver circuit. Simple modules without internal regulation become darker directly as voltage falls. TILLUME LED spots keep LED current stable inside their specified input-voltage range. Voltage drop still exists, but the spot can be planned against a clear operating window.

6Layout decision: Evaluate central, semi-decentralized and decentralized supply cleanly

If you want to control voltage drop in 24V LED systems, do not decide blindly for centralized, semi-decentralized or decentralized supply. Every layout has clear advantages and real disadvantages. The right decision only appears when circuit grouping, wiring topology, wire size, controller position, 24V power supply or dimmer position and spot operating range are evaluated together.

• Central supply remains useful when cable paths are short or sufficiently sized, channel structure is clear and cabinet service access is a priority. It becomes critical when many spots, high load and long 24V cables meet on the same run.
• Semi-decentralized or room-based supply is often the realistic middle ground: not everything sits in the main cabinet, but not every device has to sit directly at each spot. Rooms, floors or lighting groups become their own planned zones.
• Decentralized supply reduces 24V supply and load cables but needs more conscious planning of access, heat, protection and documentation. In DALI projects this may also involve a 230V input device or driver near the load, while the concrete 24V side to the TILLUME spot still has to be checked.
• Controller position complements power supply planning; it does not replace it. Controller channels, load per channel, service access and wiring topology must be planned together.
• Wiring topology should be chosen deliberately: star wiring, several short branches or a continuous line returned to the feed point lead to different voltage distributions.
• Wire size and grouping should be based on the total current of the run or controller channel, not on one individual spot. Split many spots across several circuits or channels instead of forcing everything onto one run.
• Power supply reserve should be planned at about 20% for stable long-term operation, but it does not replace correct wiring, grouping or voltage-drop calculation.
• Input voltage range must be checked at both the most distant and nearest spot. At the farthest spot, at least 22V should arrive; the upper boundary should not be used as the preferred continuous operating point. In borderline cases, coordinate the layout with TILLUME.

7FAQ

8What to read next?

9Decision Hub: What Should You Read Next?

A good 24V LED spot project is not decided by voltage drop alone. Use the next articles in this guide series to connect cable planning with buying decisions, light quality, Tunable White strategy, dimming and system integration.