1. Why KNX Power Supply Sizing Matters
The KNX power supply is one of the most important components in a KNX TP installation. It provides the bus voltage required by KNX devices while also allowing telegram communication over the same twisted-pair cable.
Selecting a power supply simply by counting the number of devices can lead to incorrect designs.
A professional KNX design should consider:
- Number of KNX devices
- Individual device current consumption
- Total bus current
- Startup and operating conditions
- Auxiliary power requirements
- Line topology
- Voltage drop
- Future expansion
- Power-supply diagnostics and redundancy requirements
KNX power supplies are commonly available in 160 mA, 320 mA and 640 mA variants, although specific manufacturers offer other configurations and additional features. The KNX Association product database contains examples of all these ratings.
The objective is not simply to install the largest available power supply. The objective is to select a suitable supply for the actual bus load and system architecture.
2. What Does a KNX Power Supply Actually Do?
A KNX TP power supply performs two fundamental functions:
- Provides electrical power to KNX bus devices
- Separates the DC power supply from KNX bus communication
A typical KNX power supply provides approximately 30 V DC, with an integrated choke for the KNX bus. For example, Schneider Electric’s 640 mA KNX power supply is specified with a 30 V DC output and integrated choke.
The choke is important because KNX communication is superimposed on the bus supply voltage.
A simplified representation is:
230 V AC → KNX Power Supply → KNX Bus
The bus then distributes both:
Power + Communication
to the connected KNX devices.
This is different from a conventional 30 V DC power supply. A generic DC power supply should not simply be connected to a KNX TP bus unless the system architecture specifically provides the required KNX bus supply and decoupling arrangement.
3. How Much Current Does a KNX Device Consume?
Every KNX device connected to a TP line consumes some current from the bus.
Typical examples include:
| Device | Typical current* |
|---|---|
| Push-button / sensor | 5–15 mA |
| Binary input | 5–15 mA |
| Presence detector | 10–20 mA |
| DIN-rail actuator | 5–15 mA |
| DALI Gateway | 10–20 mA |
| KNX IP interface | 10–20 mA |
| Touch panel | 20–50+ mA |
| Complex controller | Manufacturer-specific |
*These are illustrative engineering ranges, not universal KNX device ratings. Always use the manufacturer’s technical documentation for the actual design.
This is particularly important for devices with displays, touchscreens, integrated logic, or additional functionality.
Therefore, a calculation based only on:
Number of devices × assumed current
should be treated as an estimate rather than the final design calculation.
4. The Basic KNX Bus Current Calculation
The fundamental calculation is straightforward:
Iₜₒₜₐₗ = I₁ + I₂ + I₃ + … + Iₙ
Where:
- Iₜₒₜₐₗ = total required bus current
- I₁, I₂, I₃… = current consumption of individual KNX devices
Example
Suppose a KNX line contains:
| Device | Quantity | Current/device | Total |
|---|---|---|---|
| KNX push buttons | 20 | 10 mA | 200 mA |
| Actuators | 12 | 10 mA | 120 mA |
| Presence sensors | 8 | 15 mA | 120 mA |
| DALI gateways | 4 | 15 mA | 60 mA |
| Other devices | 6 | 10 mA | 60 mA |
| Total | 50 | 560 mA |
The estimated bus requirement is:
560 mA
A 320 mA power supply would therefore be insufficient.
A 640 mA supply would provide the required rated current, but the designer should still evaluate the manufacturer’s specifications, voltage-drop conditions and future expansion before finalising the design.
5. Don’t Calculate KNX Power Supply Size Only From Device Count
A common shortcut is:
64 devices = 640 mA power supply
This is not a correct general design rule.
Likewise:
32 devices = 320 mA
is not necessarily correct.
The number of devices and the current consumption are separate parameters.
Two KNX lines can contain the same number of devices but have significantly different current requirements.
For example:
Line A
30 simple sensors × 10 mA
= 300 mA
Line B
30 advanced devices × 20 mA
= 600 mA
Both lines contain 30 devices, but their power requirements are completely different.
This is why the device-specific current consumption from the manufacturer should be used whenever available.
6. Choosing Between 160 mA, 320 mA and 640 mA
KNX power supplies are commonly available in three traditional current classes:
160 mA
Suitable for relatively small KNX lines with low bus consumption.
Typical applications may include:
- Small residential installations
- Small zones
- Small control panels
- Limited sensor/actuator installations
320 mA
Suitable for medium-sized KNX lines.
This can be appropriate when the calculated load is comfortably within the supply capacity and there is sufficient allowance for expansion.
640 mA
The most commonly encountered higher-capacity option for larger KNX TP lines.
For example, the KNX Association database lists 640 mA supplies from manufacturers including Schneider Electric, Eelectron, Ekinex and others.
However, 640 mA should not automatically be interpreted as “64 devices.”
The actual device count and topology must still comply with the applicable KNX system limitations and manufacturer’s specifications.
7. Allowance for Future Expansion
A KNX installation is rarely static.
During the design stage, the consultant should consider future additions such as:
- Additional presence sensors
- More push buttons
- Additional actuators
- New HVAC controllers
- DALI gateways
- Touch panels
- Energy meters
- Logic controllers
- Room controllers
For this reason, designing a line to operate continuously at the theoretical maximum current is generally undesirable.
For example, if the calculated requirement is:
520 mA
and the available supply is:
640 mA
the nominal spare capacity is:
640 − 520 = 120 mA
or approximately:
18.75%
This spare capacity may be useful for future expansion, but it should not be treated as a universal mandated reserve. The appropriate engineering margin depends on the project, device characteristics and applicable specifications.
8. KNX Power Supply and Auxiliary 30 V DC Output
Some KNX power supplies provide an additional 30 V DC auxiliary output.
This can be useful for equipment requiring auxiliary power, but the designer must carefully check whether the auxiliary output shares the total current rating with the KNX bus.
For example, the KNX Association database includes supplies where the total current of the KNX and auxiliary outputs cannot exceed the rated current.
Therefore:
640 mA KNX PSU does not necessarily mean 640 mA available for the KNX bus plus another 640 mA available from the auxiliary output.
The manufacturer’s datasheet must be checked.
A simplified example:
640 mA total
If:
- KNX bus = 500 mA
- Auxiliary output = 100 mA
Then:
Total = 600 mA
which is within the nominal 640 mA rating.
But the exact permitted loading must always be checked against the particular power supply.
9. What Happens When the Power Supply Is Undersized?
An undersized power supply can cause several problems.
Possible symptoms include:
- KNX devices repeatedly going offline
- Unstable communication
- Bus resets
- Devices failing to program
- Random device failures
- Communication errors
- Increased diagnostic errors
- Problems appearing only when multiple devices operate simultaneously
This is particularly important during troubleshooting because an installation can appear to work normally during commissioning and then become unreliable after additional devices are added.
A power supply operating close to its limit can also make troubleshooting more difficult because the symptoms may appear intermittent.
10. Power Supply Sizing Is Only One Part of KNX Line Design
A correct KNX design cannot be reduced to:
Device current < Power supply rating
Three related engineering checks should be performed:
1. Bus current
Is the total device current within the power supply capacity?
2. Voltage drop
Does every device receive adequate bus voltage at its actual location?
3. Topology and line limits
Does the proposed line comply with the KNX topology and installation limitations?
These three calculations are closely related.
A 640 mA power supply does not automatically make a poorly designed long bus line acceptable.
KNX Power Supply Design Example
Consider a commercial KNX line:
- 12 actuators
- 25 push buttons
- 10 presence detectors
- 4 room controllers
- 2 DALI gateways
- 1 visualisation interface
Assume the manufacturer’s specified currents produce:
Total calculated load = 430 mA
Available power supplies:
| PSU | Rated Current | Result |
|---|---|---|
| 160 mA | 160 mA | ❌ Insufficient |
| 320 mA | 320 mA | ❌ Insufficient |
| 640 mA | 640 mA | ✅ Capacity available |
The next engineering checks should then consider:
- Voltage drop
- Cable length
- Device distribution
- Topology
- Future expansion
- Auxiliary loads
- Power-supply diagnostics
- Project-specific requirements
Only after these checks should the final PSU selection be made.
KNX Power Supply Sizing Checklist
Before finalising a KNX TP line, check:
Device Load
- List every KNX device
- Record manufacturer-specified current
- Calculate total bus current
- Include devices added during future phases
Power Supply
- Select appropriate rated current
- Check output voltage
- Confirm integrated choke
- Check overload protection
- Check short-circuit protection
- Check reset function
- Check diagnostic capabilities
Auxiliary Output
- Identify any 30 V auxiliary loads
- Check whether auxiliary and bus outputs share current
- Confirm total current limitation
KNX Line
- Check bus voltage
- Check voltage drop
- Check cable lengths
- Check topology
- Check device distribution
- Check expansion requirements
Conclusion
KNX power supply selection should be treated as an engineering calculation, not simply a device-count exercise.
The basic workflow is:
Device list → Manufacturer current → Total bus current → Power-supply selection → Voltage-drop check → Topology check → Future expansion
The commonly available 160 mA, 320 mA and 640 mA power supplies provide different capacity levels, but the correct choice depends on the actual installation. The KNX Association’s current product database demonstrates that manufacturers also provide supplies with additional outputs, diagnostics, emergency-power options and other features, so the rated current is only one part of the selection process.
For KNX designers and integrators, the important principle is simple:
Size the power supply from the actual KNX bus load, then verify voltage drop and topology before finalising the line.
