Why can't I use a regular twisted pair cable with KNX system components instead of using the certified KNX cable?
KNX certified cable must be used in the KNX system because it's protected against interference or signal interruption, i.e: It can be used along side with the power cable connected to the 220VAC loads.
The regular twisted pair cable can't be used in such way.
Attached is a picture for the KNX cable cross section.
The black and red wires are for the 24 VDC and the data signals.
The yellow and white wires are an extra 24 VDC connections ( i.e: 24 VDC only but no data signals ).