The track bus wires cannot be directly connected to the back of the PCP unless I mount the PCP directly on one of the baseboards, even then the wire size would probably be too large for the connector supplied with the PCP. I was planning on a detachable control panel (it's a portable layout) which is no problem with the EzyBus which only needs two pairs in a CAT5e cable.
However, as you point out above, the PowerCab is connected through an RJ cable which I have just measured at 6.5 mm wide. There are 6 wires in it and therefore they cannot be bigger than 1.0 mm so why would I need anything heavier to connect the PCP to the track bus?
My track bus will only ever carry the load for one or two 009 locos, no sound or lights, so I was hoping to get away with a smaller wire size.