Introduction
Power supply sizing is one of the most underestimated aspects of KNX system design. In small projects, mistakes often go unnoticed. In medium and large KNX installations, however, incorrect power supply calculation leads to unstable communication, random device failures, difficult commissioning, and long-term maintenance issues.
This guide explains how to calculate KNX power supply requirements correctly, using engineering logic instead of datasheet copying. It focuses on real-world design scenarios, not textbook limits.
Why KNX Power Supply Calculation Matters
A KNX power supply does more than provide voltage. It ensures:
- Stable bus communication
- Reliable telegram transmission
- Proper device startup and recovery
- Long-term system stability
Undersizing may not cause immediate failure, but it creates intermittent faults that are the hardest to diagnose later.
Understanding KNX Bus Power Basics
A KNX TP line operates on SELV DC voltage, typically around 29–30 V DC. All devices connected to the line draw current from the same power supply.
Key points every engineer must remember:
- Power is distributed via the bus cable
- Current is shared across all devices
- Voltage drop increases with cable length and load
The limiting factor is current, not voltage rating.
Typical Current Consumption of KNX Devices
Each KNX device consumes a defined amount of current. While exact values vary by manufacturer, real-world averages are:
- Push buttons / sensors: 5–15 mA
- Binary inputs: 5–10 mA
- Actuators (relay, dimmer): 10–30 mA
- IP routers / interfaces: 20–40 mA
- Touch panels / displays: 30–60 mA
These values must always be taken from the ETS catalog, not assumptions.
Step-by-Step KNX Power Supply Calculation
Step 1: List All Devices on the Line
Create a line-wise device list:
- Count every KNX device
- Include IP routers, couplers, and panels
- Do not exclude “small” devices
Missing even a few devices adds risk.
Step 2: Sum the Current Consumption
Add the current draw of each device based on ETS data.
Example:
- 20 push buttons × 10 mA = 200 mA
- 10 actuators × 20 mA = 200 mA
- 1 IP router × 30 mA = 30 mA
Total calculated load = 430 mA
Step 3: Apply Engineering Safety Margin
Never design at 100% capacity.
Recommended margin:
- 20–30% spare capacity
Using the example:
- 430 mA × 1.25 ≈ 540 mA required
Step 4: Select the Correct Power Supply
Choose the next higher standard size, not the closest.
Common KNX power supply ratings:
- 160 mA
- 320 mA
- 640 mA
- 960 mA
In this case, 640 mA is the correct choice — not 320 mA.
Why “Exact Matching” Is a Design Mistake
Designing a KNX line at 95–100% capacity causes:
- Startup failures during bus recovery
- Voltage drops at line ends
- Telegram retries and delays
- Random offline devices
KNX systems should feel boringly stable. If they don’t, power is often the cause.
Role of the Integrated Choke
KNX power supplies include an integrated choke, which:
- Decouples DC power from communication signals
- Prevents signal distortion
- Maintains telegram integrity
Using a non-KNX power supply or bypassing the choke is not acceptable in professional installations.
Multiple Power Supplies on One Line – Rules
This is a common source of mistakes.
Correct rules:
- Only one active power supply per line
- Additional supplies must be used only with line segmentation
- Parallel power supplies without separation are not allowed
For higher current needs, split the line, not the power supply.
Voltage Drop Considerations
Even with correct current sizing, voltage drop can cause issues.
Factors affecting voltage drop:
- Cable length
- Cable quality
- Total current load
Good practice:
- Keep line lengths well within limits
- Place power supplies centrally where possible
- Avoid “long tail” device layouts
Typical Power Supply Design Mistakes
❌ Using One Power Supply for Too Many Devices
❌ Ignoring future expansion
❌ Copy-pasting previous project values
❌ Underestimating touch panels and IP devices
❌ Assuming “it worked last time”
These mistakes don’t always fail immediately — which makes them dangerous.
Power Supply Planning for Large Projects
In hotels, offices, or campuses:
- Design line-wise power budgets
- Keep spare capacity on every line
- Use IP routing to reduce bus stress
- Plan for future devices from day one
A good power design is invisible. A bad one becomes a service nightmare.
Field Symptoms of Undersized Power Supply
If you observe:
- Devices going offline randomly
- ETS download failures
- Bus voltage fluctuating
- Slow or missed responses
Always check bus current and voltage first before suspecting software.
Conclusion
Correct KNX power supply calculation is not about following minimum values — it’s about engineering margin, stability, and future readiness.
A well-sized power supply:
- Reduces troubleshooting
- Improves reliability
- Protects your reputation as an integrator
In KNX projects, power planning is silent when done right — and loud when done wrong.
