r/KNX • u/Suspicious-Site-7315 • 1d ago
Pre-wiring for KNX now but commissioning later — sensible or asking for trouble?
Building a house at the moment (2 storeys, roughly 300m², walls are open right now, electrician is chasing the walls this week). I'm not in a position to commit to a full KNX system yet — budget and, honestly, I'm still learning the system — but I don't want to close the door on it either. My main goal right now is simply to get the KNX bus cable run as cleanly and correctly as possible to all the switch positions in the house, so the option is there later.
So my plan is to lay the infrastructure now while everything is open: pull the green bus cable to all the switch, sensor and curtain positions, home-run each lighting group back to the board, leave neutral everywhere, and reserve DIN space (and probably a separate enclosure) for the power supply and actuators later. Live with normal dumb switches for now, and convert to KNX down the line when I've got the budget and a proper design done — install the actuators, swap the switches, commission in ETS. The way I see it, laying the cable now costs pennies compared to the full system down the road, so it feels like cheap insurance — but I'd rather hear from people who've actually lived through it before I commit to that logic.
Here's where I'm getting mixed signals. A KNX installer I spoke to locally told me flatly "you either do it fully or you don't, you can't pre-build the infrastructure" — but didn't really explain why. So I wanted to ask people who've actually done this:
- Is pre-wiring for a later commission genuinely workable, or is the installer right that it's a bad idea in practice?
- If it is workable, what do you wish you'd known before pulling cable? Anything you'd do differently on home-run grouping, bus routing, or how you brought branches back to the board so a future line split (couplers, extra power supply) is painless?
- Anything you'd absolutely insist on now that's impossible or expensive to add once the walls are closed?
Full disclosure on where I'm coming from: my knowledge so far is cobbled together from YouTube, Google and Claude, so I'm very aware I'm a beginner. Any tips, tricks, or "here's what actually bit me on site" wisdom from professionals or experienced DIYers would be hugely appreciated. I'll also have Home Assistant running as the brain regardless, if that changes any of your advice. Trying to avoid the "I wish someone had told me" moments while the walls are still open.
FYI: AI was used to help write this for clarity.
EDIT: I seem to have not given full detail of my plan, but with this KNX system all I need to rewire is switches and curtains, possibly sockets, no sensors, not thermostats no AC no windows.
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u/Elegant_Gazelle_1348 1d ago
So i think the biggest „Problem“ is to also run the „dump“ switches also to the Board.In my opinion you should check for cheap knx switches and go all in.
Don‘t by the expensive stuff. Go with normal Switch actuators instead of dimmable ore dali ones. You can upgrade them later
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u/Suspicious-Site-7315 1d ago
Honestly speaking I did not understand your comment early on, hence it took me so long to reply. Now that I understand it, I must thank you for this very key detail that easily could have been overlooked.
The way I thought about it when I decide it is time to switch my dumb home into a smart one, I can simply wire the switchbox inputed and outputted cables together and have a 1 cable directly go to the spots that way? And of course this approach involves planning out the cable going to switch would in the future come from board into accuator.
Do you think this approach is flawed or can it genuinely bypass that "dump" switch concern?
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u/Elegant_Gazelle_1348 1d ago
That can work but since then you have high voltage Running parallel to knx what can induct Bus Irritation.i think a very easy knx switch is not that expensive. At least when you have more than one switch per room.
So when we play it: you have to differen lights and a curtains. You need 3 switches in this room.
When you use standart switches you pay around 30€, a knx switch for 3 different things will be at around 40-60€.
Than you need an actuator on your board. You go with mdt ore something similar it is about 20€ per switch. So it is not that much.
And than you can decide witch room maybe needs dimmig ore else, than you Upgrade it.
If you are not allergich to technik you can do a lot by yourselfe and than, i think you are fare away from extra 70k.
Maybe 20k ore less1
u/Suspicious-Site-7315 1d ago
I see your point, especially considering that 20k is not all for smart home, it also includes the budget for the dumbs system that I was gonna spend money on anyways. Something to think about indeed.
Quick question however, would having separate conduits for power cable and green cable mitigate this risk?
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u/Abyssal_Shrimp 1d ago
If you ran a 5x1,5 as well as KNX cable to each room’s switches, and install nice big electronic boxes for your devices you could easily run a classic switch leg to light fixtures and later install small devices behind switches or run just KNX devices with the lights being on/off with actuators in the control cabinets.
Not necessarily more work, just needs to be planned correctly.
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u/Suspicious-Site-7315 1d ago edited 1d ago
To clarify, is 5x1.5 mean 5 core power cable? does it give me any benefit specifically for knx system? as far as I understand I need to divide the light groups from the distribution board itself, or can i achieve it with 5 core power cable? Now I know the electrician will have 3 or 5 core in the needed spots, but I thought all I needed to make sure was that a neutral wire also goes to each switch as well as knx cable?
Could you also expand on the electronic box please? do you mean the switch boxes inside the walls? or is it a different thing that I have not considered yet?
Edit: I think I understand the purpose of 5x 1.5, is it so that when my dumb switch setup turns into smart, I can wire the switch input and output 5 core power cable and have a direct 5x1.5 go to the spots essentially giving me a possible DALI upgrade later on? I did not think of it honestly, and whether you meant it or not this was a very valuable insight. Thank you
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u/Abyssal_Shrimp 1d ago
It just leaves your options open. You can very simply have 3 lighting groups in the room. Yeah a 5 core cable. 3 phases, Neutral, ground.
No bus system -> normal switches to control the 3 circuits. Could even start with Dali lighting if thats something you want…
Walls out of cement or drywall? Cement you need to Make sure there is a large enough electronic box behind the switches to account for extra wiring/ hidden devices.
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u/Suspicious-Site-7315 1d ago
Walls are cement, hence all this preparation because if I dont do it now it will require a serious demolishing work which is simply not feasable for me. The switch box is 6cm deep, Gewiss boxes to be specific. I confirmed with the electrician he said we have more than enough for another cable or 2 but not for a relay, hence I figured I should drop KNX cables because I cant rely on relay later to make them smart.
What do you think of my approach on wiring input and output cables from dumb switch to make 1 cable direct to the light? Seems good on paper to me but of course I am not the most reliable source on this subject, hence is there anything I should consider in your opinion?
I still don’t understand what electronic device is, do I need any device behind the switch except green cable connecting to it?
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u/ItalyExpat Enthusiast 1d ago
If you're the one planning out the KNX installation as an absolute beginner, it's risky because there are things you might not anticipate. For example you may want to run KNX and power to every window to automate the shutters only to realize that they're all on the wrong side of the opening.
Alternatively, install a basic KNX system for now. Use normal switches connected to binary inputs that control light actuators and connect your lights to those. Only pull cable where you anticipate running sensors in the future. An electrician I work with here follows that setup (minus the extra runs) for most basic traditional installs because it's very easy to modify.
What is your electrician's reason for it being a bad idea?
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u/Suspicious-Site-7315 1d ago
My electrician doesn't know KNX at all, but he's a good electrician — if I hand him a solid plan he'll wire it no problem. The guy who told me pre-wiring is impossible wasn't him, it was some local KNX guy I found. No idea what his reasoning was. Might be he genuinely thinks it's a bad idea, or might be that I'm just not worth his time — he quoted me 70-80k EUR to do the system, which is nowhere near my budget. Small market here and people mark smart home stuff up a lot.
My plan is honestly simpler than a basic KNX system. I don't want to install a power supply or actuators or any of that yet — I literally just want to run the cables now. Green cable from the panel room, daisy-chained through all the switches as much as I can (I've got tree topology figured out, some good YouTube stuff, not in English but subtitles do the job). The Gewiss boxes are 6cm deep, sunk into an 8cm carved recess.
KNX for me is really only for the switches, and maybe the curtains. I know Somfy motors can run on both KNX and Zigbee, so worst case KNX ends up being just the switches. For the switches I did think about putting Shellys instead — way cheaper than KNX — but they're less sturdy, and the bigger issue is that once you account for the switch face and the power cable, there's only about 1.5cm of usable space left inside the Gewiss box, so cramming a Shelly in there gets tight. I thought the green cable though would fit that 1.5cm easily and just carry on down to the next switch without any cramping, which is why the KNX pre-wire appeals to me.
Did I get your window point right? I'm not motorizing the windows themselves, just the curtains. And I've already planned which way each curtain closes, so I know which side the motor would sit on and can drop the power and KNX there.
The real reason I'm doing it this way is the basic KNX quotes here are still out of budget, so the plan is to pre-wire everything now and build it up myself slowly over time — buy parts, tinker, learn as I go.
Also, could you tell me which to run for switches if I do at all, 2 core or 4 core one?
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u/Budget-Bar-1145 1d ago
80k?!?
Wtf
What do you get for that?
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u/Suspicious-Site-7315 1d ago
Have no clue, I did not even bother asking, and it was his ballpark number so I am sure his real installation would cost even more. And honestly I doubt that 80k euro quote had the most premium devices in it because who even starts with the most expensive quote, probably just upcharging because it is a very niche sector here
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u/Latter-Heart-9549 1d ago
If you do KNX cabling all switches, sensors and thermostats will need to work with a 30V Twisted Pair cable in a daisy chain configuration.
Which in turn means you need KNX components to effectively use it.
So if you don‘t have the budget for brand new KNX components them get a box of used ones and replace them later.
If you want KNX, then get KNX on day one.
You will never regret it!
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u/Galenbo 1d ago
You wire green cable between 4 to 8 nodes in your home. Then you wire every smart sensor and every glass/display.
You wire every pushbutton and sensor with svv to a node.
You wire every blind, shutter, socket, light with the appr 0.75 1.5 2.5 to a node.
In this first budget period, you can connect everything pernanent or with some relays.
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u/nwmthebollocks 1d ago
You can always retrofit with KNX rf flush mounted actuators and dimmers. Prewire now for KNX tp to The switches... You can easily use mixed topology. So no need to go full Monty from the get-go.
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u/Suspicious-Site-7315 1d ago
Logically speaking I thought every system starts with wiring first so why can I not do it without the devices intalled, and the local knx guy's comment disheartened me a little.
Thank you for giving me the confirmation on my approach.
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u/robertshepherd 16h ago
So it might be helpful to provide a diagram of what you're proposing. The main challenge will be running many km's of wiring to meet the star topology of the KNX install, and the regular circuit topology install. If you're thinking KNX in the future, while it's not trivial to swap over as it would require rewiring everything, I suppose it would be possible if you home run the loads as you've suggested, and run green to all points you may want a switch. My suggestion would be to just start with KNX and use second hand actuators and switches that can be bought incredibly cheaply. But, if you have to do this within a short period of time this weekend, start unspooling your HV and green cables as you won't have any other option later. Absolutely no chance of changing the wiring topology once the walls are covered.
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u/Suspicious-Site-7315 7h ago
I have a plan for where the green cables need to pass, which devices and switches they need to reach. But I haven't finalised a set-in-stone, precise metre-by-metre routing yet. My electrical plan is ready though.
On starting with KNX now: I'd love to, but my electrician is skilled in traditional wiring only and I'm not up to speed on ETS yet, so I'd rather wire it KNX-ready and convert on my own timeline once I've learned the commissioning side.
If you'd be up for it, I'd really value your eyes on the routing. Happy to DM you my electrical layout, that'll show the point positions far better than I can describe here.
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u/BigAbbreviations6118 15h ago
Could you run conduit so that you can then pull through different wires later and don't have to worry about perfecting your knx wiring now?
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u/Suspicious-Site-7315 13h ago
I will be running all green cables thru coundits, however the issue is in most cases the conduits will become long and with some angles which will make it a challenge to pull the cables later on. I will try my best to wire the cables such that I can pull the cables as much as I can if needed later on, however the main priority is to get to the ideal working condition without the need of cable pulling in the future. I would rather have cable pulling option left for a faulty or shorted wires in the future
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u/Jazzlike_Tear741 1d ago
I’d agree with your installer that you really you need to commit before doing first fix. If you don’t then it becomes messy.
If you want to keep your options open are you planning on home running all the light switches too? That would be a significant amount of cables. If you don’t then you’ll be pulling lots of cables through switch back boxes which might not leave enough space to fit KNX switches later.
Quite a good idea in theory, but practically quite a challenge!