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 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 Section 528 for segregation, and commissioned in software once the wiring is complete.

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

TP1 is electrically simple. The bus is a two-wire pair carrying 30V DC supply and 9600 bit/s differential telegrams on the same conductors. A 21V minimum must be maintained at every device terminal. Polarity is enforced by the red/black KNX terminal — reverse polarity will not damage a certified device but will prevent it communicating.

TP1 is class 2 / SELV in BS 7671 terms — extra-low voltage from a safety-isolated source. That classification is what underpins the Section 528 segregation rules covered later in this guide.

The standardised KNX cable is J-Y(St)Y 2x2x0.8 — a green-sheathed, screened twisted-pair instrument cable. UK distributors stock it from Hager, Belden, Schneider and ABB. It is recognised by its bright green sheath, the de facto colour code for KNX bus cable in UK installations.

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 is hierarchically structured so that a single project can scale from a small flat (one line, twenty devices) to a corporate campus (multiple areas, thousands of devices) without changing the underlying protocol. The hierarchy is line, area, backbone — and the limits are fixed by the KNX standard, not by manufacturer convention.

1. Line — the lowest level. Up to 64 devices on a single TP1 segment, fed by one bus power supply (with a choke). 1000 m maximum cable length. If more than 64 devices are required on the same logical line, additional line repeaters or line couplers extend the count.
2. Line couplers / line repeaters — a line coupler joins two lines and filters telegrams between them. Up to three line repeaters per line allow up to 256 devices on one logical line (4 segments × 64), though performance and fault containment usually argue for separate lines instead.
3. Area — up to 12 lines connected via line couplers to a single area line. An area carries the inter-line traffic for its 12 child lines.
4. Backbone — up to 15 areas, each joined via area couplers to the backbone line. The backbone is the project-wide spine, typically implemented on TP1 in small projects and on KNXnet/IP for anything larger.
5. Project maximum — 15 areas × 12 lines × 64 devices = 11,520 individually addressable devices on a single KNX installation, before line repeaters are even considered.

Each device on the project 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, say, a light switch to the dimming actuator that drives a luminaire). Physical addresses are set at commissioning; group addresses are the project designer's logical scheme.

Every KNX TP1 line requires a dedicated KNX bus power supply (BPS). The BPS is a DIN-rail module that takes a mains input (230V AC, sometimes 110V) and produces a regulated 30V DC bus output through an integrated 100 mH choke. The choke is what allows data and DC supply to share the pair without the data being short-circuited by the supply.

For wider redundancy 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 — but it has hard rules. Breaking them produces installations that pass continuity testing but fail commissioning, and that is the worst place for a fault to surface because it adds days to programme.

1. No closed rings. The bus must be a tree topology — line with spurs — never a loop. Joining the head of a line back to the tail produces signal collisions.
2. Maximum line length: 1000 m of cable per line (segment), summed across the main run and all spurs.
3. Maximum device-to-BPS distance: 350 m. Beyond this the bus voltage drops below the 21V device threshold.
4. Maximum device-to-device distance: 700 m. Even across two devices on opposite ends of a long line, the differential signal degrades beyond this.
5. Maximum 64 devices per line segment before adding a line coupler or line repeater.
6. Line couplers filter telegrams by default — design the group address scheme so that telegrams which need to cross a line do, and telegrams that should stay local do not. Filter table programming is part of commissioning.
7. KNX bus cable spurs may tee off the main run. Spurs are not impedance-matched — keep them short where possible and within the overall 1000 m line budget.
8. Polarity: red/black must be consistent across the entire line. A polarity flip at one device terminal is the single most common KNX commissioning fault.

KNX TP1 is a SELV circuit derived from a safety-isolated source (the bus power supply). As a SELV circuit installed alongside LV mains, it falls under BS 7671:2018+A4:2026 Section 528 — segregation between circuits at different voltage bands.

BS 7671 Reg 528.1 (reflected in GN3 Reg 4.8.5 and OSG Reg 7.4.1) classifies KNX TP1 as a Band I (extra-low voltage) circuit and mains as Band II (low voltage). Band I circuits must not share a wiring enclosure with Band II circuits unless one of the following conditions is satisfied:

1. Physical separation — the KNX bus cable runs on a separate tray, in a separate conduit, or in a separate compartment of multi-compartment trunking from the LV mains conductors.
2. Partitioned trunking — a single trunking is permitted provided an internal partition maintains separation between the Band I bus and the Band II conductors (Reg 528.1 / OSG 7.4.1 condition (c)).
3. Cable rated for highest voltage present — if the bus cable shares trunking or conduit with LV mains without partition, every cable in the enclosure must have insulation rated for the highest voltage present (300/500V minimum, often 600/1000V). Standard J-Y(St)Y 2x2x0.8 is NOT rated for this — a different KNX-rated multicore would be required (OSG 7.4.1 condition (a)).
4. Earthed metal screen multicore — for a multicore cable containing both Band I and Band II conductors, an earthed metal screen of equivalent current-carrying capacity to the largest Band II circuit conductor separates the bands (OSG 7.4.1 condition (e)). The screen must be earthed and sized to carry prospective fault current.
5. Mechanical protection at crossings — where KNX bus must cross LV mains (e.g. in a panel), maintain separation or use a sleeve to prevent contact.

At the panel, the same principles apply — KNX bus terminations, the BPS, line couplers and IP routers sit in a dedicated low-voltage section with a partition from the mains busbar, and on separate glands. For wider containment design see our building management systems electrical guide.

A wired KNX installation does nothing until it is commissioned. Commissioning assigns each device a unique physical address, downloads the application programme 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, licensed in tiers (Demo, Lite, Home, Professional). UK commercial projects almost always use ETS Professional, with licences bound to the engineer's KNX Association ID and the project file.

1. Create the project — define the building structure (areas, lines, rooms), import device catalogues from the manufacturer ETS database, and place each physical device in the topology.
2. Assign physical addresses — each device gets a unique Area.Line.Device address (e.g. 1.2.34). Push the device programming button; ETS writes the assigned address.
3. Assign group addresses — create the logical communication channels (e.g. "Kitchen ceiling lights on/off", "AHU 1 setpoint") and link device objects to them.
4. Configure parameters — set each device's application-specific behaviour (dimming curve, scene memory, timer durations, sensor thresholds, HVAC PID values).
5. Download the application — push the configured programme 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, 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 four protocols a UK electrician routinely encounters on building automation. Understanding where each fits — and where they meet — turns a wiring exercise into a coherent system.

The KNX Association certifies both products and people. A device is KNX-certified if it has passed conformance and interoperability testing in an accredited lab. A person becomes a KNX Partner by completing the five-day KNX Basic Course at an accredited training centre and is then listed in the KNX Association partner directory.