A well-designed KNX control panel is safe to commission, easy to maintain, and thermally stable throughout its 20-year service life. The layout decisions made during panel design — zone separation, device spacing, cable duct sizing, and enclosure selection — directly affect commissioning time and long-term reliability.

Two-Zone Layout Principle

Split every KNX panel into two voltage zones separated by a horizontal cable duct divider:

Zone A (top half): 230VAC and 400VAC three-phase. MCBs, RCDs, contactors, main distribution busbar. All wiring here is line voltage — brown/black/grey phases, blue neutral, green-yellow PE.

Zone B (bottom half): SELV — KNX TP bus (30VDC), 24VDC supplies, DALI (16VDC). KNX actuators, DALI power supply, MDT Modbus gateway, 24VDC PSU. All wiring is extra-low voltage.

Minimum air clearance between zones: 25mm measured between any live conductor in Zone A and any conductor in Zone B. In practice, the cable duct between zones provides physical separation — no conductor should cross between ducts without double insulation or a fully rated 230VAC/SELV insulation barrier.

IEC reference: IEC 60364-4-41 clause 414 (SELV), IEC 60439-1 clause 7.4 (clearances and creepage distances for mixed-voltage enclosures).

Device Spacing on DIN Rail

Standard KNX DIN modules (35mm rail): standard MDT actuator is 4TE (70mm) or 8TE (140mm). Packing density guidance:

Switching actuators up to 6A per channel: zero gap between adjacent modules acceptable (standard DIN rail adjacent mounting). Heat dissipation: 6-channel 6A actuator at 50% load: approximately 1.5W — within thermal budget for adjacent mounting.

Switching actuators 8A per channel (MDT AKK-0800.03): at rated load (8A x 8 channels x 250VAC = 16kW switched): internal dissipation 2.5-3W per actuator. Leave 12.5mm (1 DIN pitch) gap between 8A actuators when multiple units run at full rated load simultaneously.

DALI power supply (Tridonic PC PRO 300mA): internal dissipation 6-8W. Always leave 12.5mm on each side. Never place DALI PSU adjacent to another DALI PSU without gap.

KNX power supply (MDT STC-0640.01, 640mA, 20W): 12.5mm gap on each side. KNX PS is typically the warmest device in Zone B.

Cable Duct Sizing

Horizontal cable duct runs at top of Zone B: 60mm wide x 40mm high for panels up to 8 KNX actuators. 60mm x 60mm for 9-24 actuators. 80mm x 60mm for large panels (25+ actuators with thick motor cables mixed in).

Fill factor: never exceed 40% cross-sectional fill. This provides space for future additions and prevents thermal stress on cable insulation. A 40mm x 60mm duct (2400mm2 cross-section) at 40% fill holds 960mm2 of cable — approximately 12 x 1.5mm2 wires or 7 x 2.5mm2.

KNX TP bus cable (YCYM 2x0.8mm2): run in its own small 25mm duct or alongside 24VDC cables only. Never mix KNX bus cable with 230VAC in the same duct — electromagnetic interference from switching loads will cause KNX communication errors.

Enclosure Selection

Small residential KNX panel (up to 24 DIN modules): Schneider Pragma 24M IP65 (500x500x200mm). Sheet steel, RAL 7035, external mounting. Suitable for utility rooms, plant rooms.

Medium commercial panel (24-60 DIN modules): Schneider Spacial S3DH or Rittal AE 1000x800x250mm. Gland plates at bottom for cable entry. Internal DIN rail kit.

Large panel (60+ modules): Custom-fabricated sheet metal, double-door access, internal lighting (12VDC LED strip), integrated thermostat and fan for forced ventilation when heat load exceeds 30W.

IP rating selection: plant room (dry, no dust ingress risk): IP31 adequate. Outdoor/exposed location: IP65 minimum. Humid plant room (near heat exchangers): IP54.

Thermal Budget Calculation

Estimate total panel heat dissipation before enclosure selection:

Example 24-module panel: KNX PS 20W + 3x switching actuators at half load 1.5W each + 1x DALI PSU 7W + 1x Modbus gateway 3W + KNX bus coupler 1W = approximately 35.5W.

Sealed IP65 enclosure (Schneider Pragma 600x400mm): thermal resistance approximately 1.0-1.2 K/W. Temperature rise = 35.5W x 1.1 = 39 degrees C. At maximum ambient 40 degrees C: internal temperature 79 degrees C — exceeds KNX device operating limit (typically 55 degrees C). Solution: use IP44 ventilated enclosure (thermal resistance 0.2-0.3 K/W) or add ventilation fan with thermostat set at 40 degrees C.

This thermal calculation is mandatory for any sealed panel with total dissipation above 15W. Overheated panels suffer premature relay contact wear and KNX bus communication errors.