Every KNX TP line requires a dedicated power supply providing 29V DC bus voltage (±0.5V). Undersizing the KNX power supply causes intermittent device failures, telegram corruption, and restart loops on devices entering under-voltage lockout. The KNX standard specifies maximum 640mA (PS640) or 1280mA (PS1280) power supply current per line — but the actual current requirement depends on device count, device types, and cable length. This guide covers the current budget calculation methodology, PS640 vs PS1280 selection criteria, buffer modules for uninterruptible operation, and UPS design for safety-critical KNX functions.

KNX bus current fundamentals

KNX TP operates at 29V DC, generated by a SELV (Safety Extra Low Voltage) power supply conforming to KNX Standard Core 2.3.2. The bus uses the twisted pair both for data communication (Manchester-encoded at 9600 baud) and for powering devices (up to 640mA or 1280mA depending on PSU type). Each KNX device draws a standby bus current: typically 5-25mA for simple actuators (MDT SCN-8V63.01 8-channel actuator: 5mA), 10-20mA for room controllers (Gira 2096: 12mA), 25-75mA for logic controllers or IP routers (MDT SCN-IP100.01 IP Router: 75mA). Sensor/input devices with LED indicators draw more: ABB SU/S 8.20.1 push-button interface with 4 LEDs: 25-40mA total. Note: many KNX devices have separate 230V power supply for their load outputs (actuators, dimmer outputs) — only the KNX bus communication circuit draws from the PS640/PS1280.

Current budget calculation — worked example

Office floor, KNX Line 1 device inventory: 12 × MDT SCN-8V63.01 switch actuator (5mA each = 60mA), 8 × MDT AKD-0816V.01 dimmer actuator (10mA each = 80mA), 6 × MDT SCN-RT55P room thermostat (15mA each = 90mA), 4 × Elsner KNX TH-UP room sensor (8mA each = 32mA), 2 × MDT BE-0800.01 binary input (8mA each = 16mA), 1 × ABB LR/M 1.1 KNX line repeater (if required, 50mA). Cable loss: for a 200m long KNX bus with 0.5mm² YCYM conductor, resistance = 2 × 200m × 0.038Ω/m = 15.2Ω. Voltage drop at 278mA = 15.2 × 0.278 = 4.2V. Bus voltage at farthest device: 29V - 4.2V = 24.8V — still within 21-31V KNX operating range. Total standby current: 278mA. Margin to PS640 limit: 640 - 278 = 362mA available for expansion. Recommendation: PS640 is adequate. If line expansion to 60+ devices is planned: upgrade to PS1280 now.

PS640 vs PS1280 selection

PS640 (KNX standard PSU, e.g., MDT SCN-PS064.01, ABB SV/S 30.640.5.1): output 640mA at 29V DC, suitable for lines with up to ~40 mixed devices depending on device current draw. DIN-rail mount: 4 modules. Price: €80-120. PS1280 (high-current PSU, e.g., MDT SCP-01281.01): output 1280mA at 29V DC, two integrated PS640-equivalent circuits in parallel. Suitable for lines with 60-90 devices or with high-current devices (room controllers with backlights, IP routers). DIN-rail mount: 6 modules. Price: €160-220. Selection rule: calculate line current budget (above). If total > 500mA → PS1280. If total < 450mA and no large expansion planned → PS640. Never connect two PS640 units to one KNX line in parallel without choke coils — without synchronisation, data collision occurs on the combined bus. Two power zones (when line device count > 64 or line length > 350m): use KNX line repeater (ABB LR/M 1.1 or MDT) — this creates two separate electrical segments with one PS640 each, but they share the same logical line (same group addresses, same individual address space).

Buffer modules for uninterruptible KNX bus

In buildings where KNX controls safety-relevant functions — emergency HVAC, smoke extract fans controlled via KNX relay, door access control, fire shutter actuators — KNX bus must remain operational during mains power failure for the required safety system operation time. KNX buffer modules (also called capacitor buffer power supplies) maintain bus voltage for up to 2 hours on a charged capacitor array: Gira 2093 buffer module (1.5Wh capacitor, sufficient for ~10 devices for 2h or full 64-device line for 15 minutes); MDT SCN-USB55S.01 UPS module (external 12V battery connection, indefinite backup). Connection: buffer module placed in parallel with PS640/PS1280 on KNX bus + and - terminals. During normal operation: buffer module charges from bus power. On mains loss: buffer discharges, maintaining 29V ±0.5V. KNX devices detect no interruption — no restart, no telegram loss. Critical for: KNX-controlled fire damper actuators that must stroke to open/closed on fire alarm; stairwell emergency lighting triggering via KNX relay output to DALI emergency test circuit.

UPS integration in KNX panels

For longer backup requirements (hours, not minutes): integrate a standard 230V UPS (APC Smart-UPS, Riello UPS) upstream of the KNX PS640 input. UPS rated: 650VA minimum for a single KNX panel (PS640: 20W, plus actuators' 24V SMPS: 50W typical). UPS output to panel input MCB → PS640 powered continuously during mains failure up to UPS battery run time (typically 30-90 minutes for 650VA). UPS KNX status monitoring: APC Smart-UPS SNMP card (AP9630) → Home Assistant via apc-ups integration → KNX group address write (DPT 1.001, 0 = on battery) → KNX logic activates "power failure" scene: non-essential loads off, security system armed, HVAC to minimum ventilation mode. IEC 60364-4-41: KNX bus is SELV (Safety Extra Low Voltage) circuit — 29V DC. UPS output to PS640 is standard 230V AC mains — maintain 4kV clearance/creepage between SELV (KNX side) and mains in panel layout per IEC 60664-1 for over-voltage category II installations.

Practical power supply installation tips

Wiring: connect PS640/PS1280 output to KNX bus bar with minimum 1.5mm² wire (bus current up to 1.28A on PS1280). KNX bus bar (positive terminal): always use red wire. Negative terminal: always black or blue (KNX colour convention per KNX Standard). Fusing: many DIN-rail panels include a 2A fast-blow fuse on the PS640 output for short-circuit protection — check PS640 datasheet: some have internal fold-back current limiting (no external fuse needed), others require it. Voltage monitoring: some PS1280 models include an LED indicator for bus voltage and a voltage monitoring output relay — wire this relay to a KNX binary input (MDT BE-0800.01) to get a KNX group address alarm if bus voltage drops below 24V. Documentation: record bus current budget in KNX panel schedule. Include: PSU model and current rating, measured bus current (use multimeter clamp on PS640 output wire after commissioning), current headroom for expansion, buffer module model and estimated backup time.