Introduction
An undersized KNX power supply rarely causes an immediate, complete system failure.
Instead, it creates slow, confusing, and intermittent problems that appear weeks or months after commissioning.
This is why power supply issues are among the most misdiagnosed KNX problems worldwide.
In this article, we explain what really happens when a KNX power supply is undersized, how the symptoms evolve, why the system may appear “mostly working”, and how to identify and fix the issue correctly.
First Reality: KNX Power Problems Are Silent
Unlike traditional electrical faults, KNX power issues:
- Do not trip breakers
- Do not show alarms
- Do not always drop the whole system
Instead, the system degrades gradually.
That’s what makes undersized power supplies dangerous.
What “Undersized” Actually Means in KNX
A KNX power supply is undersized when:
- The total current draw of connected devices approaches or exceeds its rated output
- There is insufficient margin for peak load, temperature, or aging
Important:
Being close to the limit is often worse than slightly exceeding it.
Why Systems Work During Commissioning
This confuses many integrators.
During commissioning:
- Not all devices are active
- Logic is incomplete
- Scenes are rarely triggered
- Traffic is low
- Ambient temperature is controlled
As a result, the system appears stable — even if the power supply is already marginal.
What Happens After Handover
Once the building is occupied:
- Motion sensors trigger continuously
- Touch panels stay active
- Feedback objects update constantly
- Logic engines run permanently
- Integration systems go live
Current draw increases without any hardware change.
This is when problems begin.
Early Symptoms of an Undersized Power Supply
These are usually the first warning signs:
- Random ETS download failures
- Some devices occasionally offline
- Delayed responses to commands
- Touch panels rebooting silently
- System behaving differently at different times of day
Because nothing fails consistently, the issue is often blamed on software or networking.
Voltage Drop Is the Real Problem
KNX devices care about voltage at their terminals, not at the power supply.
When current demand rises:
- Voltage drops along the bus
- End-of-line devices suffer first
- Communication becomes unstable
A supply can still output “29 V” at its terminals while devices see far less.
Why Reboots and Resets Happen
When voltage dips below safe limits:
- Devices reset internally
- Communication stacks restart
- The device may reappear after seconds or minutes
From the outside, it looks like:
“The device randomly went offline and came back.”
In reality, the device protected itself.
Why Problems Get Worse Over Time
Undersized supplies age faster.
Contributing factors:
- Constant operation near maximum load
- Heat buildup inside panels
- Reduced efficiency over years
A system that worked “almost fine” for a year may suddenly become unreliable.
Common Situations That Create Undersized Systems
1. No Power Margin
Design based on exact current calculation with zero reserve.
2. Expansion Without Recalculation
Additional keypads, sensors, or IP devices added later.
3. High-Current Devices Ignored
Touch panels, logic modules, and IP devices consume more than basic sensors.
4. One Supply for Too Much Area
Large villas or floors powered from a single source.
Why Replacing Devices Does Not Help
This is a common and expensive mistake.
When power is the issue:
- New devices show the same behaviour
- Offline problems continue
- Client confidence drops
The environment is unstable — not the device.
How to Diagnose an Undersized Power Supply
Step 1: Measure Bus Voltage at Multiple Points
Especially at the farthest devices.
Step 2: Observe Behaviour Under Load
Trigger scenes, motion sensors, logic.
Step 3: Temporarily Reduce Load
Disconnect a few devices and observe stability.
Step 4: Check Power Supply Temperature
Overheating is a strong indicator.
Typical Voltage Patterns in Undersized Systems
- Voltage OK near supply
- Voltage drops significantly at line ends
- Voltage fluctuates during activity
- Devices recover after inactivity
This pattern almost always points to power limitations.
Correct Ways to Fix the Problem
Option 1: Add an Additional Power Supply
Split the line with a coupler.
Option 2: Redesign Line Structure
More lines, fewer devices per line.
Option 3: Move High-Load Devices
Distribute touch panels and IP devices.
Option 4: Replace with Higher-Capacity Supply
Only when topology allows.
Never parallel KNX power supplies on the same line.
Why Temporary Fixes Don’t Last
Common temporary actions:
- Restarting power supplies
- Rebooting devices
- Reducing logic
These may help briefly, but the problem always returns.
Electrical limitations cannot be fixed with software.
Preventive Design Rules (Field-Proven)
✔ Always design with reserve capacity
✔ Assume future expansion
✔ Split floors and zones early
✔ Keep high-current devices distributed
✔ Measure voltage before commissioning sign-off
Power stability is the foundation of KNX reliability.
What Clients Experience vs Reality
Clients say:
“Sometimes it works, sometimes it doesn’t.”
Reality:
The system is constantly operating at the edge of its electrical limits.
Once power is corrected, systems often become boringly stable.
Conclusion
An undersized KNX power supply does not cause dramatic failure — it causes long-term instability, random behaviour, and endless troubleshooting.
Understanding this prevents:
- Unnecessary device replacement
- Blame on programming
- Repeated service visits
- Loss of client trust
In KNX systems, power margin is not a luxury — it is a requirement.
When in doubt, always check the power first.
