KNX Power Supply Sizing: PS640 vs PS1280 and Current Budget Calculation
Choosing the wrong KNX power supply is one of the most common causes of intermittent bus faults, device restarts, and communication errors after commissioning. Correct sizing requires summing device current consumption per line, verifying voltage drop at the farthest device, and understanding how topology affects power distribution.
Bus current fundamentals
KNX TP (Twisted Pair) operates at 29V DC SELV using Manchester encoding at 9600 baud. The bus power supply simultaneously provides the 29V rail that powers all bus devices and the data signal carrier. Every device connected to the KNX TP line draws a standby current from this rail. The sum of all device standby currents must not exceed the rated output current of the power supply unit serving that line.
| Device type | Typical standby current | Notes |
|---|---|---|
| Switch/blind actuators (binary output) | 5–25 mA | Lower end for relay types; higher for solid-state outputs with status LEDs |
| Dimmer actuators | 10–20 mA | Includes internal microcontroller and communication circuitry |
| Room controllers / thermostats | 10–20 mA | With display and backlight add 5–10 mA |
| KNX IP routers / line couplers | 50–80 mA | High current — IP routers often require dedicated PS640 per line |
| Push-button sensors with LEDs | 25–40 mA | Status LEDs active; check manufacturer data for per-button LED current |
| Binary inputs | 5–10 mA | Passive voltage-free contact inputs; low bus current |
| CO2 / weather sensors | 10–20 mA | Measurement electronics add current; check datasheet |
Always use manufacturer datasheets: the figures above are representative ranges. Each product has a specified bus current consumption in its datasheet — use those exact values when building the current budget for a project. Estimates based on device type alone can be significantly off for devices with integrated displays or high LED counts.
PS640 vs PS1280: specifications and selection
The two standard KNX bus power supply sizes are the PS640 (640 mA output) and the PS1280 (1280 mA output). Both supply 29V DC SELV and incorporate the bus data signal choke. The selection rule is straightforward: if the calculated total bus current for a line exceeds 500 mA, use a PS1280.
PS640 — 640 mA
- MDT SCN-PS064.01: 4 DIN modules, 29V/640 mA, short-circuit protected
- ABB SV/S 30.640.5.1: 4 DIN modules, diagnostics via KNX object
- Suitable for lines up to approximately 50 devices (depending on type)
- Operating current headroom: use up to 80% rated output = 512 mA
- Select PS640 when calculated current is below 500 mA
PS1280 — 1280 mA
- MDT SCP-01281.01: 6 DIN modules, 29V/1280 mA, current display
- Designed for high-device-count lines or IP-router-heavy configurations
- Supports up to 64 KNX devices on a single line segment
- Operating current headroom: use up to 80% rated output = 1024 mA
- Select PS1280 when calculated current exceeds 500 mA
80% derating rule: never load a PS640 or PS1280 to more than 80% of its rated output current in steady-state operation. The remaining 20% provides headroom for inrush current during device power-up sequences and allows for future device additions without resizing the power supply.
Current budget calculation: worked example
Consider an office floor served by a single KNX line. The devices on this line and their bus current consumption are listed below. All values are from manufacturer datasheets.
Office floor KNX line — current budget
Device type Qty mA/device Total mA -------------------------------------------------------------- Switch actuators (8-channel) 12 5 mA 60 mA Dimmer actuators (4-channel) 8 10 mA 80 mA Room thermostats with display 6 15 mA 90 mA Push-button sensors with LED 4 8 mA 32 mA Binary inputs (8-channel) 2 8 mA 16 mA -------------------------------------------------------------- TOTAL 278 mA PS640 rated output: 640 mA 80% operating limit: 512 mA Calculated load: 278 mA (54% of rated) Headroom: 234 mA (for 18+ additional devices) Verdict: PS640 is adequate for this line.
When PS1280 is required: if this floor also included two KNX IP routers at 60 mA each (120 mA additional), total rises to 398 mA — still within PS640 limits. However, if the project adds an audio zone controller at 80 mA and an additional 20 actuators, the total would exceed 500 mA and a PS1280 becomes necessary.
Voltage drop: cable length and resistance
KNX TP cable carries both power and data signals. As current flows through the cable resistance, the bus voltage drops along the cable. KNX devices require a minimum supply voltage of 21V DC at their terminals. If voltage drop is excessive, devices at the far end of the line may malfunction, lose communication, or fail to restart after a power cycle.
Voltage drop calculation — YCYM 0.8mm² cable
Formula: V_drop = I_total × R_loop R_loop = 2 × L × r Where: I_total = total bus current (A) L = cable length from PSU to farthest device (m) r = conductor resistance per metre (Ω/m) YCYM 2×2×0.8mm² (standard KNX cable): r = 0.038 Ω/m per conductor R_loop = 2 × L × 0.038 = 0.076 × L Ω Example: 200m cable, 278mA total current R_loop = 0.076 × 200 = 15.2 Ω V_drop = 0.278A × 15.2Ω = 4.2 V Voltage at farthest device = 29V - 4.2V = 24.8V ✓ (≥ 21V) Example: 350m cable (absolute KNX limit), 278mA R_loop = 0.076 × 350 = 26.6 Ω V_drop = 0.278A × 26.6Ω = 7.4 V Voltage at farthest device = 29V - 7.4V = 21.6V ✓ (marginal) Rule: maximum cable length between PSU and any device = 350m
High-current lines need shorter cable runs: if the total bus current is close to the PS640 limit (500 mA), the 350m cable restriction becomes tighter. At 500 mA over 350m: V_drop = 0.5A × 26.6Ω = 13.3V — leaving only 15.7V at the farthest device, which is below the 21V minimum. For high-current lines, keep cable runs to 200m or less, or install a KNX line repeater to create two electrical sections.
KNX line topology and power distribution
KNX TP supports star, tree, and line topologies (but not ring). The topology chosen affects how voltage drop accumulates and how current is distributed across cable branches. In a tree topology — the most common in building installations — multiple branches extend from a central distribution point. To find the worst-case voltage drop, trace the path from the power supply to the farthest device summing all branch lengths along that path.
Tree topology — branch current
In a tree, the current in each branch equals only the sum of device currents on that branch, not the total line current. This means voltage drop per branch is lower than in a single-run line topology with all devices in series. The backbone cable from the PSU to the first junction carries the full current — size and route this segment carefully.
KNX line repeater (ABB LR/M 1.1)
A KNX line repeater splits one logical line into two electrical segments. Each segment has its own PS640, resetting the voltage drop calculation from zero at the repeater. The two segments remain on the same logical KNX line — the repeater is transparent to ETS6. Maximum 3 line repeaters per line (3 segments). Useful for long buildings or when cable runs exceed safe voltage drop limits.
Parallel PS640 prohibition
It may seem logical to connect two PS640 units in parallel to increase current capacity from 640 mA to 1280 mA. This is strictly prohibited by the KNX standard and will cause bus communication errors. Two independent PS640 units operate their Manchester encoding clocks independently — they will be slightly out of synchronisation, creating data collisions on the bus whenever both transmit simultaneously.
Correct and prohibited configurations
PROHIBITED: two PS640 in parallel on same bus
PS640 (A) ──┐
├── KNX bus ──── devices
PS640 (B) ──┘
Result: data collisions, intermittent bus errors
CORRECT: PS1280 (internally synchronised dual supply)
PS1280 ──── KNX bus ──── devices (up to 1280mA total)
CORRECT: two PS640 with line repeater (separate segments)
PS640 (A) ── Segment 1 ── Repeater ── Segment 2 ── PS640 (B)
Same logical line, two separate electrical buses — no collision
Each segment independent: PS640 (A) powers Segment 1 only
Note: the PS1280 is essentially two internally synchronised
PS640 cores in one housing — parallel operation is safe
because clock synchronisation is handled internally.Commissioning measurement
After all devices on a KNX line have been powered up and commissioned in ETS6, measure the actual bus current at the PS640 output. Use a clamp meter on the positive output wire from the PS640 to the bus. This measurement confirms the estimated current budget, identifies any unexpected additional loads, and establishes the baseline for future expansion planning.
Commissioning measurement procedure
1. Power up all devices on the line 2. Wait 60 seconds for all devices to complete restart 3. Clamp meter on positive bus output wire from PS640 4. Record measured current (mA) 5. Compare with calculated budget Headroom calculation: PS640 80% limit = 512 mA Measured current = 278 mA (example) Headroom = 512 - 278 = 234 mA Equivalent additional devices ≈ 234 / 10 = 23 standard actuators If measured current > calculated: - Check for undocumented devices on the line - Verify no short circuit on bus cable (remove devices one by one) - Check for damaged bus cable reducing insulation resistance
Panel documentation requirements
Every KNX panel schedule should include a bus current budget table per line. This table becomes part of the as-built documentation and enables future maintenance engineers to assess expansion capacity without re-surveying the installation.
Bus current budget table — panel schedule format
KNX Line 1.1 — Office Floor West Power supply: MDT SCN-PS064.01 (PS640, 640mA) Panel: DB-L01, DIN rail 3 Device Model Qty mA Total Switch actuator MDT SA-1216 12 5 60 Dimmer actuator MDT AKD-0424 8 10 80 Room thermostat MDT SCN-RT55P 6 15 90 Push-button sensor MDT BE-TA5508 4 8 32 Binary input MDT BE-0800 2 8 16 Calculated total: 278 mA Measured total (post-commissioning): 284 mA PS640 80% limit: 512 mA Remaining headroom: 228 mA Farthest device cable length: 185 m Voltage at farthest device (calculated): 25.4 V
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