Case Study

KNX Wiring and Topology

KNX Cable

There are two aspects to consider when it comes to wiring:

Physical Wiring and Installation Topology

KNX cable is mains rated and due to the arrangement of the internal cores, it can be run directly next to mains cabling. In fact, it is usually much easier to do so as it can be run at the same time by the site electrician. 

When running around a building it can be installed as a daisy chain or star wired and it is possible to create a spur. The only restriction is a loop cannot be created at any point of the bus. It is advised to daisy chain from point to point as it makes fault finding and maintenance easier. It is also common to run the cable around the building or floor in a loop but only terminate one end. This provides a back up if the cable is damaged or if the line needs to be expanded. It is important to label the cable clearly if this method is used. 

A KNX device can be a maximum of 350m from a power supply, which would normally be mounted in a distribution board and the maximum length of bus cable used in a single line is 1000m. 

KNX Line
KNX Line

System Topology

A single KNX line can have up to 256 devices although the actual number depends on the size of the KNX power supply on the line and the total current of the devices. To expand the system beyond a single line, a hierarchy approach is used with each line (up to 15) linking to a main line to create an area. 

In an installation this often takes the form of distribution board on each floor feeding all keypads sensors and control devices with a main line linking each distribution board. 

Alternatively, a line from each floor could go back to a central distribution point where they would each link to a main line. If more than 15 lines are needed then each area (again up to 15) can be linked to a Backbone Area,

Full KNX Topology
Full KNX Topology

Wherever lines are connected, a new line coupler and power supply will be needed, which is why this is normally completed in a distribution board or enclosure.

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