KNX Bus Cable Planning Before Renovation
Not sure yet if you want KNX right away? Fine — but it's far cheaper to pull the bus cable NOW, during renovation, than to reopen a finished wall for a few extra metres later.
How to plan it in 4 steps
Sketch a room-by-room point list
Walk through the house and note where you might someday want a sensor, switch, or actuator — even if you're installing a normal switch there for now. Rough is fine.
Choose certified bus cable
KNX TP1 needs certified green cable, typically J-Y(St)Y 2x2x0.8mm — one pair carries power and data, the other stays spare for later.
Leave a spare tail at every point
At each marked point, leave a length of cable outside the box and label it — much easier than fishing a new run through a finished wall.
Photograph the route before plastering
Before the wall closes, photograph every cable run with a tape measure in frame. These photos are your only proof of where the cable goes once the render is on.
What's worth knowing about the cable and topology
KNX topology is line/tree
Devices branch off the main bus like branches off a tree — but never form a closed loop. Say this to your electrician at the cable-pulling stage, not after.
Certified cable, not just any wire
KNX TP1 is the twisted-pair medium defined by ISO/IEC 14543-3 and EN 50090 — not whatever 4-core cable happens to be on the shelf.
Physical separation from 230V
The KNX cable is an SELV/PELV low-voltage circuit and must run physically separated from mains cables along the whole route — not just bundled in the same conduit.
No termination resistors needed
Unlike some other bus systems, KNX TP doesn't need termination resistors. Don't let anyone 'close the loop for safety' by analogy with structured cabling.
The second pair is your reserve
The green KNX cable has two pairs: red/black carries power and data, white/yellow stays free — useful for a second independent line, or just left for later.
Couplers let it grow without rebuilding
Line and area couplers let the system expand from a few rooms to a whole building, filtering traffic between segments — the topology doesn't need to be rebuilt from scratch.
What to prepare before talking to your electrician
A short checklist to bring to the site or send ahead.
- {'t': 'Room-by-room point list', 'd': 'Which rooms and spots might want a sensor or actuator someday — a rough list beats no list.'}
- {'t': 'Cable specification', 'd': "Certified green KNX TP1 cable, e.g. J-Y(St)Y 2x2x0.8mm — so it doesn't get confused with an ordinary signal cable."}
- {'t': 'Questions for your electrician', 'd': 'Will the cable be physically separated from 230V along the whole route? Will the topology stay line/tree, not a loop?'}
- {'t': 'A labeling system', 'd': 'A number or tag for every cable tail, matched to a point on your room list.'}
- {'t': 'Route photos before plastering', 'd': 'Photos showing the cable path in the wall, ideally with a tape measure in shot — keep them with your renovation paperwork.'}
Frequently asked questions
Do I need to install KNX right away if I pull the cable?
No. You can pull the cable now and connect devices later — even a year or five down the line. What matters is that the cable is certified and correctly routed, not that the system is live from day one.
How much cable do I actually need?
That depends on your floor plan — we can't give you an exact figure without one. Rule of thumb: a spare tail at every point beats running short after the wall is closed.
Can I use the spare pair for something else?
The cable has two pairs — one for the bus, one spare. Leave it unused for later, or use it for a second independent KNX line, but not for a regular 230V circuit.
What happens if the electrician wires it as a loop?
KNX bus topology needs to stay a line or tree — a ring isn't supported and can cause problems later. Say so clearly before the cable disappears behind the plaster.
Does it need its own separate conduit from the 230V wiring?
Not necessarily its own conduit, but it does need physical separation along the whole run — not just tied together with the power cable with nothing between them.
Don't plan the cable alone
The knowledge base is free, but a specific plan deserves a second opinion. Check what's in the shop to get started with KNX, or reach out to a partner who can help with the install.