This page last changed on Mar 28, 2009 by 3sv.

Hello all,

On the previous forum there where a lot of professional knx installers. I hope they also read the new forums .

I will install a KNX system in my own home, this will also be my first KNX installation. I would like to use normal UTP cat5 cabling instead of KNX certified cable.

My motivation to use a standard UTP wire is that i already need UTP cable for Ethernet usage, and as so I would need to buy two different cables from which I will have both a bit of surplus.

The second motivation is that there are more extra conductors for future use. I think about an extra DC voltage to supply some hungry devices, or a second separate KNX bus. The third motivation is that I know standard UTP cable, I know where to buy it and I don't think KNX certified cable will be cheaper.

It seems that UTP will give at least the same signal quality. I found a pdf document on http://www.ecoprog.ru/pdf/eib_report.pdf that says on page 3 :

 
Owing to the fact, that the cable of a category 5 is used for the cabling and its parameters considerably
exceed the characteristics recommended for EIB, the limitations set for a total length of
cables in a EIB segment can not be more strict, than in case of a standard cable.

One cable of a horizontal segment of the structured cabling system contains 4 pairs of wires,
which can be used as follows:
Pair #1 - EIB;
Pair #2 - an additional power supply for EIB devices, 24 V, 1 A;
Pair #3 - a backup or an additional power supply for devices of any Intelligent Building application,
24 V, 1 A;
Pair #4 - a backup or an Intelligent Building application providing an electromagnetic compatibility
with EIB, for example, paging system.

But I don't trust it for 100%. Once my cable is placed and I would start getting some signal quality troubles it isn't possible at all to replace all the bus wires across my house.
I would really like to hear from someone who has practical experience in using UTP cable as KNX buswire.

Thanks in advance.
Dries

Dries,

Glad to hear you're taking the plunge. Thankfully I have some experience that will save you a lot of headaches. (Anyone else please do chime in) We had the misfortune of having an Electrician wire up a Distribution Board with Cat5 instead of KNX Twisted Pair.

Long story short, it caused all sorts of units to seem faulty as it wasn't always communicating properly. The main problem is the cable thickness. Even when "doubled up", the CAT5 wasn't making proper contact within the KNX Branch Terminals. This led to many site visits before we pinpointed what was wrong. So, while it seems like an easy out I would definitely recommend against it. (If I were you, I'd actually run, kicking and screaming.)

If sourcing Certified KNX Cable is the problem, I'd recommend looking at http://www.knxshop.co.uk, or else http://www.eibshop.de. (Don't know where you're writing from ... your name would lead me to think the Netherlands, so maybe eibshop rather than knxshop.) They don't seem to sell to the general public, but it's worth a shot. Otherwise, try Eelectron, (http://www.eelectron.com) as I do know they manufacture Certified KNX Cable and should direct you to a retailer in your area.

If you don't care about it being 100% Certified, we've used "Fire Alarm Cable" in the past. Basically all you need a a 4-core sheathed cable, where both pairs are twisted together for noise rejection. The cable should also have a tiny bit of grounding wire in it and each core should be 0.8mm. Here, in South Africa, we call it "Fire Alarm Cable" and it works well. The only problems are that the outer sheath is often thinner and the wrong colour. (Red or White instead of Green, which can lead to confusion on-site with Electricians and Security/Alarm Installers)

Hope this helps.

Jean-Pierre

p.s. This is in direct reference to using Cat5 as a replacement for KNX TP Cable in a TP KNX Installation. This is not in reference to KNX IP, which is a different kettle of fish and, frankly, too new. There just aren't many products taking advantage of it.

If you're thinking of running everything on IP, which doesn't seem to be the case, I'd recommend you read the KNX Journal on the IP Protocol. A lot of it is far too technical for most people on earth, but really it comes down to IP being in addition to and not as a replacement for TP. (Same with RF ... great for retrofits, smaller installs and in a pinch, but a wire's just more reliable IMHO)

Posted by jeanpierrej at Mar 29, 2009 15:39

Thanks for the reply Jean-Pierre. I am actually from Belgium, so you guess based on my name was quite close

You only mentioned that the thickness of the cable would cause a lot of trouble, but not the signal quality itself. For my project I will connect custom made KNX devices without the standard KNX connector terminals. I gonna just solder the cable onto the prints. So in that case it wouldn't be any problem to use CAT5 cable?

And otherwise, it seems that CAT6 is using a bit thicker copper. Is that a better option? Just thinking about the future use of connecting "real" knx devices.

How have you solved the site where there was UTP cable used as KNX TP Cable?  Have you replaced all the cable, or just worked out a solution for making the connection with the devices?

Dries

Posted by 3sv at Apr 04, 2009 17:06

Dries,

don't do it - always imagine that some time in the future someone else has to work on your installation (new owner, electrician or whoever). Everyone knows what a green cable means and which meaning the four wire inside have. If I remember right when I installed KNX in my house three years ago, even the pricing was practically the same for UTP and KNX...

Regards,

Jörg

Posted by jfalkenberg at Apr 10, 2009 17:14

Ok, I found a place where I can buy KNX cable. I will go for the KNX cable.

Thanks for the input.

Dries

Posted by 3sv at Apr 11, 2009 10:52

Hi,

You can find KNX cable at reasonable price here:

http://www.eibmarkt.com/cgi-bin/eibmarkt.storefront/DE/Product/NS0140116?PID=DE_NS0140116

Another argument in favor of KNX cable is that its insulation characteristics (test voltage = 4kV) allow it to be placed next to 230VAC cables. KNX bus is a SELV circuit an thus need an appropriate insulation, especially if the cable is touching 230V wires (something that happens often in the main distribution box). That's not the case with UTP cable.

Br,

Jean-François

Posted by jef2000 at May 15, 2009 23:05
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