LED Strip Wiring: Cable Cross-Section and Voltage Drop
The most common wiring mistakes in LED installations — and how to calculate the right cable size for any run length.
Why 12V LED strips lose brightness at the far end
LED strips are current-driven devices. At 12V, a 14.4 W/m strip draws 1.2 A per metre. On a 5-metre run, that's 6A through the entire cable length. Even with 1.5 mm² cable, a 10-metre cable run (5m out + 5m back) produces a voltage drop of approximately 0.7V — dropping the LED supply from 12.0V to 11.3V. This causes noticeable dimming at the far end compared to the near end.
⚠️ 12V vs 24V: the fundamental difference
At the same wattage, a 24V strip draws half the current of a 12V strip. Half the current means one-quarter of the voltage drop. This is why 24V LED strips are strongly preferred for runs over 3 metres.
Voltage drop formula for LED strips
For DC circuits (LED strips):
ΔU (V) = 2 × L (m) × I (A) × 0.0175 / A (mm²)
L = cable length one-way, I = strip current, A = conductor cross-section
Factor 2 for return conductor; 0.0175 = copper resistivity (Ω·mm²/m)
Maximum acceptable voltage drop: 5% (0.6V at 12V, 1.2V at 24V)
Maximum run lengths by cable size
At 5% voltage drop limit, assuming standard 14.4 W/m LED strip:
12V LED strip (1.2 A/m current draw):
| Cable size | 1 m strip (1.2A) | 3 m strip (3.6A) | 5 m strip (6A) |
|---|---|---|---|
| 0.75 mm² | 4.4 m | 1.5 m | 0.9 m |
| 1.0 mm² | 5.9 m | 2.0 m | 1.2 m |
| 1.5 mm² | 8.9 m | 3.0 m | 1.8 m |
| 2.5 mm² | 14.8 m | 4.9 m | 3.0 m |
24V LED strip (0.6 A/m current draw):
| Cable size | 1 m strip (0.6A) | 5 m strip (3A) | 10 m strip (6A) |
|---|---|---|---|
| 0.75 mm² | 35 m | 7.0 m | 3.5 m |
| 1.0 mm² | 47 m | 9.4 m | 4.7 m |
| 1.5 mm² | 71 m | 14 m | 7.1 m |
| 2.5 mm² | 118 m | 24 m | 12 m |
Cable length = one-way distance from power supply to end of strip. Assumes 14.4 W/m strip at full brightness.
Injection methods for long runs
End injection
Up to 3m (12V) or 7m (24V)Power supply at one end, current flows the full length of the strip. Simplest wiring, but maximum voltage drop. Only suitable for short runs.
Centre injection
3–10m (12V) or 7–20m (24V)Power supply in the middle, two equal halves fed in opposite directions. Cuts effective run length in half — reduces voltage drop by 75%. Best approach for medium runs.
Parallel injection
Long runs, high-quality installationsMultiple power supply cables run in parallel from the power supply to multiple points along the strip. Most uniform brightness, most cable. Used for architectural feature lighting.
Multiple power supplies
Runs over 10m (12V) or 20m (24V)Separate power supply for each section. Eliminates voltage drop entirely per section. Requires careful power supply synchronisation for smooth dimming.
Wiring LED strips via KNX DALI dimmer
In KNX panels, LED strips are typically controlled via DALI-2 constant-voltage drivers (e.g. Lunatone DALI CV dimmer) or via PWM output modules. The wiring path:
230V AC mains → LED power supply (24V DC DALI, or 24V DC constant voltage)
DALI bus cable (2-wire, polarity-free) → from panel DALI gateway to LED driver
24V DC output from driver → LED strip via correctly sized cable
KNX/DALI gateway in panel → receives KNX scene command → sends DALI command → driver dims
Use DALI constant-voltage (CV) drivers rather than constant-current (CC) drivers for LED strips — strips require a fixed voltage, not fixed current. DALI CV drivers are available in 12V and 24V output with DALI-2 certification.
LED lighting panel with correct cable design?
DALI-2 drivers, correct voltage sizing, uniform brightness — included in every PanelCraft lighting panel.
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