KNX Line Capacity: How Many Devices Can You Put on One KNX Line?

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Table of Contents

1. What Is a KNX Line?

A KNX line is a logical and physical section of a KNX TP installation containing KNX devices connected through twisted-pair bus cable.

A typical line consists of:

  • KNX power supply
  • KNX TP cable
  • Sensors
  • Push buttons
  • Actuators
  • Room controllers
  • Gateways
  • Other KNX TP devices

A simplified architecture looks like:

                 KNX LINE
                     │
              ┌──────┴──────┐
              │ KNX PSU     │
              └──────┬──────┘
                     │
       ───────── KNX TP BUS ─────────
          │       │       │       │
       Sensor  Sensor  Actuator  Gateway

One of the most common questions during KNX design is:

How many KNX devices can be connected to one line?

The answer is more complicated than simply counting devices.

2. The Traditional 64-Device Rule

A conventional KNX TP line is commonly described as supporting up to 64 bus devices under the standard topology arrangement.

However, this number should not be interpreted as:

“Every KNX line can always contain exactly 64 devices.”

The actual design depends on:

  • KNX topology
  • Line segments
  • Couplers
  • Power supply arrangement
  • Bus current
  • Cable installation
  • Device characteristics
  • Applicable KNX specifications

Therefore, 64 is a topology/device-capacity concept, not a substitute for electrical design calculations.

This distinction is particularly important when designing larger projects.

3. Why Device Count Alone Is Not Enough

Consider two KNX lines.

Line A

40 simple push buttons and sensors.

Line B

40 high-consumption devices including:

  • Touch panels
  • Controllers
  • Gateways
  • Displays
  • Complex room controllers

Both have:

40 devices

But their bus current can be very different.

Therefore:

Device count and bus current are two different design constraints.

A line can be below its nominal device-count limit but still require careful power-supply and voltage-drop analysis.

4. KNX Line Capacity Has Multiple Limits

When designing a KNX line, consider at least four major factors:

1. Device capacity

How many devices can be accommodated within the topology?

2. Bus current

How much current do the connected devices consume?

3. Voltage drop

Is adequate voltage available at the remote devices?

4. Physical topology

Does the cable arrangement comply with KNX installation requirements?

Therefore:

KNX Line Capacity ≠ Device Count Alone

A professional design considers all four.

5. The Role of the KNX Power Supply

The power supply determines how much bus current is available to the line.

Common KNX power supplies include:

  • 160 mA
  • 320 mA
  • 640 mA

For example, if a line requires:

410 mA

a 320 mA supply would not be sufficient.

A 640 mA supply may provide adequate nominal capacity, subject to the specific power supply, installation and other design checks.

The important point is:

A 640 mA supply does not automatically mean that the line can support 64 devices.

The devices themselves determine the actual current requirement.

6. Example: 64 Devices on a KNX Line

Suppose a project has:

Device TypeQuantityCurrent
Push buttons2410 mA
Sensors1610 mA
Actuators1210 mA
Room controllers815 mA
Gateways420 mA
Total64700 mA

Although the project has 64 devices, the estimated current is:

700 mA

A 640 mA power supply would therefore not provide sufficient nominal capacity for this assumed load.

This example demonstrates why:

64 devices ≠ 640 mA

and:

10 devices ≠ 100 mA

The actual current must be obtained from the device documentation.

7. KNX Line Segments

A KNX TP line can be expanded using line segments and the appropriate KNX topology components.

The concept can be represented as:

Main Line
    │
    ├── Segment
    │
    ├── Segment
    │
    └── Segment

Each segment has its own electrical considerations.

The use of additional segments can allow larger installations to be structured more effectively, but it also introduces additional engineering considerations involving:

  • Power supply
  • Segment couplers
  • Device count
  • Topology
  • Telegram filtering
  • Commissioning

Therefore, segmentation should be planned during the KNX architecture stage.

8. Why KNX Topology Matters

A KNX line should not simply be treated as an arbitrary length of cable.

The topology affects:

  • Device distribution
  • Cable length
  • Voltage drop
  • Power distribution
  • Coupler placement
  • Fault isolation
  • Commissioning

A good KNX design therefore starts with the building’s physical layout.

For example:

                    KNX LINE
                       │
            ┌──────────┴──────────┐
            │                     │
        Office Area            Meeting Area
            │                     │
       Sensors/Actuators     Sensors/Actuators

Grouping devices according to building zones can simplify both installation and troubleshooting.

9. Line Capacity and Voltage Drop

The previous KNXHUB article covered KNX voltage drop calculation.

This is directly related to line capacity.

As more devices are connected to a line:

More devices → More current

and:

More current → Greater voltage drop

especially in cable sections carrying current for many downstream devices.

Therefore, increasing the number of devices can affect both:

  • Electrical capacity
  • Voltage distribution

This is one reason why line design should be completed before finalising the device schedule.

10. How to Plan a KNX Line for a Large Project

Suppose a building has:

120 KNX devices

Instead of simply putting all 120 devices into one design, divide them logically.

For example:

Line 1 – Ground Floor

35 devices

Line 2 – First Floor

42 devices

Line 3 – Second Floor

43 devices

Then connect the lines through the appropriate KNX topology.

This provides advantages such as:

  • Easier fault isolation
  • Better physical organisation
  • More manageable commissioning
  • Logical ETS structure
  • Easier maintenance
  • Better expansion planning

The exact architecture should be determined from the project requirements and KNX topology rules.

11. When Should You Create Another KNX Line?

Consider creating additional lines when:

The device count approaches the design limit

Splitting the installation can simplify architecture.

Bus current becomes high

A second line can distribute the electrical load.

The building has distinct physical zones

For example:

  • Ground floor
  • First floor
  • Office block
  • Hotel rooms
  • Plant room

Fault isolation is important

Separate lines can simplify troubleshooting.

Future expansion is expected

Additional capacity can be planned into the architecture.

Cable distribution becomes complicated

Logical line separation can simplify installation.

12. KNX Line vs Area

For larger KNX projects, the architecture can extend beyond individual lines.

A simplified hierarchy is:

KNX Installation
       │
       ├── Area
       │     ├── Line
       │     ├── Line
       │     └── Line
       │
       └── Area
             ├── Line
             ├── Line
             └── Line

For modern installations, KNX IP can also be used as part of the backbone architecture.

This allows larger systems to be organised into logical areas and lines rather than creating one extremely large physical bus.

13. KNX Line Capacity and ETS

ETS provides the logical project structure used to configure the KNX installation.

A well-designed project should have a clear relationship between:

Building → Area → Line → Device

For example:

Area 1
 ├── Line 1.1 – Ground Floor
 ├── Line 1.2 – First Floor
 └── Line 1.3 – HVAC

Area 2
 ├── Line 2.1 – Office Block
 ├── Line 2.2 – Meeting Rooms
 └── Line 2.3 – Service Area

This makes commissioning and maintenance considerably easier than an arbitrary device arrangement.

14. Common KNX Line Capacity Mistakes

Mistake 1: Assuming 64 devices is always the target

A line does not need to be filled to its nominal capacity.

Mistake 2: Selecting the PSU based only on device count

Calculate actual bus current.

Mistake 3: Ignoring voltage drop

A line may have acceptable device count but still require voltage-drop analysis.

Mistake 4: Adding devices without reviewing the PSU

Future expansion can change the electrical loading.

Mistake 5: Poor physical distribution

A poorly distributed line can make troubleshooting and voltage analysis harder.

Mistake 6: Treating every building floor as one mandatory line

The correct architecture depends on project requirements, topology and system design.

15. Practical KNX Line Design Workflow

For a professional KNX project, use this sequence:

Step 1 — Create the device schedule

List every KNX device.

Step 2 — Record current consumption

Use the manufacturer’s specifications.

Step 3 — Calculate total bus current

Add the current of all devices.

Step 4 — Select the power supply

Choose an appropriate KNX PSU.

Step 5 — Check voltage drop

Verify the worst-case device location.

Step 6 — Design the topology

Determine:

  • Lines
  • Areas
  • Couplers
  • IP backbone
  • Segment structure

Step 7 — Allocate devices

Distribute devices logically.

Step 8 — Plan expansion

Leave appropriate engineering capacity.

Step 9 — Commission in ETS

Program and verify devices.

Step 10 — Document the final installation

Record:

  • Device addresses
  • Line architecture
  • PSU details
  • Cable routes
  • Measurements
  • ETS project backup

16. A Simple KNX Line Capacity Checklist

Before approving a KNX line:

  • Device count checked
  • Device current checked
  • Total bus current calculated
  • Power supply selected
  • Voltage drop calculated
  • Cable length checked
  • Topology verified
  • Line/area structure defined
  • Future expansion considered
  • ETS structure prepared
  • Commissioning documentation planned

Conclusion

KNX line capacity should never be determined by device count alone.

The commonly referenced 64-device figure is only one part of KNX TP line design. A professional design also considers:

Device count + bus current + power supply + voltage drop + topology + future expansion

For small installations, this may be relatively straightforward.

For commercial buildings, hotels, offices and large residential projects, proper line planning becomes much more important.

The best approach is to design the KNX architecture before the installation begins and verify the electrical characteristics during commissioning.

A well-designed KNX line is not simply a line with fewer than 64 devices—it is a line with the right topology, power capacity, voltage distribution and logical structure.

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