KNX Wiring Installation Guide for UK Electricians

KNX is the international open standard for home and building electronic systems, defined by BS EN 50090 in Europe and ISO/IEC 14543-3 worldwide. It is the convergence of EIB, BatiBUS and EHS into a single manufacturer-independent specification administered by the KNX Association in Brussels. Over 500 member companies produce KNX-certified devices, and any certified device interoperates on the same bus, commissioned through a single tool called ETS.

In UK projects, KNX is specified on commercial fit-outs, large residential properties, hotels, schools and corporate campuses where lighting, HVAC, shading and energy metering must talk to each other and to a building management system. Unlike a centralised DDC controller, every KNX device contains its own microprocessor and exchanges short telegrams over a common bus. Remove any single device and the rest carries on running.

For a UK electrician, KNX sits in the same territory as DALI lighting control and building management systems — extra-low-voltage control wiring layered on top of mains power, governed by BS 7671 for segregation and protective separation, and commissioned in software once the wiring is complete.

These are the figures that decide whether a KNX design works. They come from the KNX Association Basic Course material and apply to a TP1 twisted-pair line built with standard green KNX bus cable.

The standardised KNX cable is J-Y(St)Y 2x2x0.8 — a green-sheathed, screened twisted-pair instrument cable. YCYM 2x2x0.8 in TP1 design is the equivalent. UK distributors stock it from Hager, Belden, Schneider and ABB, and it is recognised on site by its green sheath.

Only the red/black pair is required for a functioning TP1 bus. The white/yellow pair may be used for an auxiliary, but the safest convention is to cap it and reserve it as a spare.

KNX defines four physical media. In practice, UK projects use TP1 for the structured backbone and IP for inter-line and inter-building routing; RF appears in retrofit and where chasing cables is impractical; PL is effectively obsolete for new build.

KNX is hierarchically structured so that a single project can scale from a small flat (one line, twenty devices) to a corporate campus (several areas, thousands of devices) without changing the protocol. The hierarchy is line, then area, then backbone.

1. Line — the lowest level. A line segment carries 64 devices and is fed by one bus power supply with a choke.
2. Line repeaters — up to three repeaters per line give four segments and a maximum of 255 devices on one logical line. Fault containment and traffic usually argue for separate lines instead.
3. Line couplers — a coupler joins a line to the area line above it and filters telegrams between them, so local traffic stays local.
4. Area — up to 15 lines connected through line couplers to a single area line.
5. Backbone — up to 15 areas, each joined through an area coupler to the backbone line. The backbone is the project-wide spine, TP1 on small projects and KNXnet/IP on anything larger.
6. Project scale — at 64 devices per line that is 15 x 15 x 64 = 14,400 addressable devices, and a project using repeatered lines goes well beyond that.

Each device has a unique physical address in the form Area.Line.Device — for example 1.2.34 — and one or more group addresses, the logical channels that link a switch to the actuator driving a luminaire. Physical addresses are set at commissioning; group addresses are the project designer's logical scheme.

Every KNX TP1 line requires its own KNX bus power supply. It is a DIN-rail module that takes a mains input and produces the monitored 30 V DC SELV bus supply through a choke. The choke is what stops the filter and charging capacitors in the supply short-circuiting the 9600 Hz telegrams, which is why data and DC can share one pair.

For wider containment and resilience patterns see our building management systems guide and the commercial lighting guide.

KNX TP1 is forgiving by industrial protocol standards — no termination resistors, no impedance matching, no master device. What it does have is a short list of hard rules, and breaking them produces installations that pass a continuity test but fail commissioning. That is the worst place for a fault to surface, because it lands on the programme.

1. No closed rings. The bus must be a tree — a line with spurs — never a loop. Joining the head of a line back to the tail produces collisions and unpredictable behaviour.
2. Bus cables from different lines must never be linked together. Test for it by switching off the power supply of the line under test: if the power LED on the line coupler stays lit, there is an inadmissible connection somewhere.
3. Bus cable ends terminate at a device or at a bus connection block — nowhere else. Connection blocks let a device be removed without interrupting the bus and are mechanically protected against mis-mating.
4. Polarity: red/black must be consistent across the whole line. Check every device by pressing its programming button — if the programming LED lights, it is connected the right way round. Press again to return it to normal operation.
5. Keep every device inside the distance limits: 1000 m of cable per segment, 350 m from the supply, 700 m between any two devices.
6. Line couplers filter telegrams by default. Design the group address scheme so that telegrams which need to cross a line do, and local ones do not. Filter table programming is part of commissioning.
7. Mark bus cable ends "KNX TP1" or "BUS", and add the area and line. It makes fault-finding and future maintenance far quicker.
8. Plan out loops at design stage. A large loop area formed by bus cable, mains cable and the equipotential bonding of pipework or metal walls invites lightning-induced overvoltage, and in bad cases makes the installation impossible to program.

The KNX TP1 bus is a SELV system: a safety-isolated source, safe isolation from other networks, basic insulation to earth, and no earthing of the bus itself. That makes it a Band I (extra-low voltage) circuit in BS 7671 terms, while the mains around it is Band II (low voltage). Two regulations then apply, and both have to be satisfied.

Regulation 528.1 (Proximity to electrical services) states that a Band I circuit shall not be contained in the same wiring system as a Band II circuit except where one of the following methods is adopted:

1. (a) every cable or conductor is insulated for the highest voltage present;
2. (b) each conductor of a multicore cable is insulated for the highest voltage present in the cable;
3. (c) the cables are insulated for their system voltage and installed in a separate compartment of a cable ducting or cable trunking system;
4. (d) the cables are installed on a cable tray system where physical separation is provided by a partition;
5. (e) a separate conduit, trunking or ducting system is employed;
6. (f) for a multicore cable, the cores of the Band I circuit are separated from the cores of the Band II circuit by an earthed metal screen of equivalent current-carrying capacity to that of the largest core of a Band II circuit.

The IET On-Site Guide sets out the same conditions at 7.4.1, lettered (a) to (e), and Guidance Note 3 covers the trunking case at 4.8.5. Standard J-Y(St)Y 2x2x0.8 is not insulated for 230 V, so options (a) and (b) are not open to you with normal KNX cable — which leaves separate containment, a separate trunking compartment, or a partitioned tray.

Regulation 414.4.2 then lists the arrangements by which protective separation of the wiring system may be achieved:

1. (a) SELV and PELV circuit conductors enclosed in a non-metallic sheath or insulating enclosure in addition to basic insulation;
2. (b) SELV and PELV circuit conductors separated from conductors of circuits at voltages higher than Band I by an earthed metallic sheath or earthed metallic screen;
3. (c) circuit conductors at voltages higher than Band I may be contained in a multi-conductor cable or other grouping of conductors if the SELV and PELV conductors are insulated for the highest voltage present;
4. (d) the wiring systems of other circuits are in compliance with Regulation 412.2.4.1;
5. (e) physical separation.

The same thinking applies inside enclosures and accessories. The KNX Association requires double or reinforced insulation between mains and bus cables in wall boxes, with unsheathed bus cores never in contact with mains cables. Junctions go in separate boxes, or in a common box with a partition giving 8 mm clearance and creepage distances. Where a bus device shares a box or a common cover plate with mains accessories, the bus device manufacturer must explicitly approve that combination, and the mains parts must stay protected against accidental contact even with the cover removed. At the panel, the bus terminations, power supply, line couplers and IP routers sit in a section partitioned from the mains busbar, on separate glands.

A wired KNX installation does nothing until it is commissioned. Commissioning assigns each device a unique physical address, downloads the application program into each device, and creates the group addresses that link inputs (switches, sensors) to outputs (actuators, dimmers, gateways). This is done in ETS — Engineering Tool Software — produced and licensed by the KNX Association.

ETS is Windows-only in practice and licensed in tiers. The KNX Association publishes device limits for each: Demo is capped at five products per project, Lite at 20 devices, Home at 64 products in a single project, and Professional is the unrestricted tier. UK commercial projects use Professional almost without exception.

1. Create the project — define the building structure (areas, lines, rooms), import device catalogues from the manufacturer ETS databases, and place each physical device in the topology.
2. Assign physical addresses — each device gets a unique Area.Line.Device address, for example 1.2.34. Press the device programming button; ETS writes the assigned address.
3. Assign group addresses — create the logical channels ("Kitchen ceiling lights on/off", "AHU 1 setpoint") and link device objects to them.
4. Configure parameters — set each device's application behaviour: dimming curve, scene memory, timer durations, sensor thresholds, HVAC control values.
5. Download the application — push the configured program into every device over the bus. ETS handles partial and full downloads.
6. Test and document — run group address, scene and fault-injection tests, then generate the documentation pack: wiring lists, group address tables and device parameter dumps.

KNX is specified on UK commercial projects because one bus carries lighting, HVAC, shading and energy metering — all addressable from the same wall panel, sensor or BMS gateway.

For deeper DALI integration see our DALI lighting control wiring guide; for BMS context see building management systems electrical; for ELV drivers under A4:2026 see Section 715 ELV lighting.

KNX is one of several protocols a UK electrician meets on building automation. Knowing where each fits — and where they join — turns a wiring exercise into a coherent system.

The KNX Association certifies both products and people. A device is KNX-certified once it has passed conformance and interoperability testing in an accredited lab. A person becomes a KNX Partner by completing the KNX Basic Course at an accredited training centre and passing the exams, after which they are listed in the KNX Association partner directory.