KNX TP Cable — Specification and Routing Rules
Everything a KNX installer needs to know about the physical bus cable: what to buy, how to route it, how long it can be, and how to keep it away from power cables.
The KNX TP cable standard
KNX Twisted Pair (TP) uses a specific cable type: YCYM 2×2×0.8 mm² (also known as J-Y(St)Y 2×2×0.8). This is a 4-wire cable with two twisted pairs, each pair surrounded by an aluminium foil shield, with an overall PVC jacket.
Standard specification
The two pairs have different roles: the red/black pair carries the KNX bus signal (29V DC bus power + data). The yellow/white pair is a spare — it can be used for an auxiliary 12V supply to external sensors, or left unused. Never use the spare pair for a second KNX line.
Permitted topologies
KNX TP supports three physical topologies: tree, star and bus (daisy-chain). All three can be mixed freely on the same line.
Tree topology
One cable from the panel, then branching out to devices. Most common in residential. Easy to trace, requires more cable.
Bus/daisy-chain topology
Cable runs device-to-device in a chain. Uses less cable but harder to isolate faults. Common in corridors.
Star topology
All devices cabled back to a central patch point. Maximum fault isolation. Requires most cable.
⚠️ Ring topology is NOT permitted
KNX TP does not support ring topology. Do not connect the end of a bus line back to its start — this creates signal reflections and bus communication errors.
Line length limits
The KNX standard specifies maximum distances for a single TP line:
1,000 m
Total cable length
all cable on the line combined
700 m
Device-to-device
any two devices on the same line
350 m
Device to power supply
furthest device from the power supply
For larger buildings, use KNX line couplers to connect sub-lines (up to 64 devices, 1000m each) to a backbone line. Up to 15 sub-lines can be connected to one KNX area, and up to 15 areas to a KNX system — giving a theoretical maximum of 57,375 devices in a single KNX installation.
Separation from power cables
KNX TP cable carries SELV (Safety Extra Low Voltage) at 29V DC. Routing it alongside 230V AC mains cables can cause interference and is a safety concern. The required separation depends on installation method:
| Installation method | Minimum separation | Notes |
|---|---|---|
| Open cable tray | 4 cm | Or separate cable trays entirely |
| Same conduit | Not permitted | KNX must be in separate conduit |
| Screened KNX cable | Can share tray | When screen is connected to earth at one end |
| Flush conduit (in wall) | Separate conduits | Or 4 cm separation between conduits |
| Structured cabling trunking | Separate compartment | Most trunking has a divider for SELV |
Devices per line
A single KNX TP line supports a maximum of 64 devices (bus devices + power supply). The power supply is also a device, so effectively 63 bus devices per power supply. Current consumption limits may further restrict the number of devices:
A 640 mA KNX power supply can support devices consuming a total of ~400 mA at the bus (after derating). Most KNX devices consume 10–20 mA. Example: 30 devices × 15 mA average = 450 mA — one 640 mA supply is sufficient. For heavily loaded lines, use two power supplies (separated by a choke) or split devices across multiple lines.
Practical installation tips
- →Label both ends of every KNX cable with the device address or location during installation — much easier than tracing later.
- →Use a cable marker sleeve at the device connection point with the KNX physical address (e.g. 1.1.5) — this makes ETS commissioning significantly faster.
- →Leave 30 cm of slack at each KNX device — KNX TP connectors can fail and need re-termination.
- →Use the shield drain wire at one end only (at the panel/power supply end) — connecting at both ends creates a ground loop.
- →Use green-jacketed KNX-certified cable wherever possible — it's immediately identifiable to future installers and inspectors.
- →Do not use Cat5e as a substitute for KNX cable — while electrically similar, it lacks the KNX colour coding and shield design. It also violates KNX certification.
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