Power Cable Selection for Lighting and Power Circuits
How to choose the right cable cross-section for every circuit in a KNX panel — current capacity tables, voltage drop, derating, and what changes for LED loads.
Cable types used in KNX panels
NYM-J (H05VV-F equivalent)
Lighting, sockets, fixed appliancesThe standard fixed-installation power cable for European buildings. PVC insulation, PVC jacket. Used for all concealed wiring in walls, ceiling voids and conduit. Available 1.5 mm², 2.5 mm², 4 mm², 6 mm².
NYY-J (underground/outdoor)
Underground, outdoor, damp locationsPVC cable with stronger outer jacket for direct burial or outdoor installation. Used for external circuits, underground feeds to outbuildings.
H07RN-F (rubber flexible)
Flexible connections, outdoor socketsRubber-insulated flexible cable. Used for connections where the cable is moved (appliances, tool connections) or where PVC would crack in cold temperatures.
LIYY / LiYCY (control cable)
Panel wiring, control signalsMulti-core control cable for 24V/230V control signals in panels. Smaller cross-section (0.25–0.75 mm²), multiple cores. Used for KNX actuator inputs, relay coil wiring.
Current capacity table (NYM-J, single circuit)
Current capacity (ampacity) for NYM-J cable in conduit or enclosed in a wall, installation method B2 per IEC 60364-5-52:
| Cross-section | 1-phase (A) | 3-phase (A) | Typical MCB | Max load (1ph) |
|---|---|---|---|---|
| 1.5 mm² | 16 A | 13 A | 10–16 A | 3.7 kW |
| 2.5 mm² | 20 A | 17 A | 16–20 A | 4.6 kW |
| 4 mm² | 25 A | 23 A | 25 A | 5.8 kW |
| 6 mm² | 32 A | 29 A | 32 A | 7.4 kW |
| 10 mm² | 40 A | 36 A | 40 A | 9.2 kW |
| 16 mm² | 52 A | 47 A | 50 A | 12 kW |
Values for 230/400V AC, ambient temperature 30°C, PVC cable, installation method B2 (in conduit in wall). Apply derating for higher temperatures or bundled cables.
Voltage drop calculation
IEC 60364-5-52 limits voltage drop to 3% for lighting and 5% for other circuits from the origin of the installation to the final load. For LED strips and sensitive electronics, 3% is the practical maximum.
Formula for single-phase circuits:
ΔU (V) = 2 × L × I × ρ / A
where L = cable length (m), I = current (A), ρ = resistivity (0.0175 for copper), A = cross-section (mm²)
Multiply by 2 for single-phase (outward + return conductor)
Maximum cable length for 3% voltage drop at 230V (single-phase, full current load):
| Cross-section | at 10A | at 16A | at 25A |
|---|---|---|---|
| 1.5 mm² | 49 m | 31 m | 20 m |
| 2.5 mm² | 81 m | 51 m | 33 m |
| 4 mm² | 130 m | 81 m | 52 m |
| 6 mm² | 194 m | 121 m | 78 m |
| 10 mm² | 323 m | 202 m | 129 m |
Derating for bundled cables
When multiple loaded cables are run together in the same conduit or tray, they share heat and must be derated. Apply the following factors to the single-cable ampacity:
| Number of circuits | Derating factor | Example: 2.5 mm² capacity |
|---|---|---|
| 2 | 0.80 | 16 A |
| 3 | 0.70 | 14 A |
| 4–5 | 0.65 | 13 A |
| 6–8 | 0.57 | 11.4 A |
| 9–12 | 0.50 | 10 A |
| 13–16 | 0.45 | 9 A |
Special considerations for LED circuits
LED drivers and dimmers introduce harmonic currents that don't show up in simple power measurements. A circuit supplying LED drivers may have a true current significantly higher than P/V suggests:
- →Use 1.5 mm² as a minimum for dedicated LED circuits — never share with other loads.
- →For long LED strip runs (10+ metres), always calculate voltage drop — even at 10A, a 30m run on 1.5 mm² drops 4.5% at 12V.
- →For 12V LED strips, prefer 24V strips for long runs: at the same wattage, 24V draws half the current, cutting voltage drop by 75%.
- →LED dimmers (trailing edge, DALI) add harmonic distortion — oversize the cable by one step (use 2.5 mm² where 1.5 mm² would technically suffice).
- →Separate LED driver circuits from high-inrush loads (motors, compressors) to avoid nuisance tripping.
Panel wiring designed to code
Cable sizing, short-circuit protection, voltage drop — all calculated and certified for your project.
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