I am struggling a bit to visualise this - is the 7 core especially long so volt drop is a worry or is the concern heating ?

If you have 3 loaded cores and 4 cool ones, the multicore cable rating can be pushed relative to the '7 loaded cores' case, but how much is a question for the cable makers. Are you pulling more than about 20A phase ? for 2,5mm as part of a larger group, that is perhaps where thermal effects may start  to crimp the style but it will depend how the cable is installed and how it can cool itself.

Mike

Many thanks for the reply. This stems from the fact that there is a very limited amount of space for the cable installation and running two cables as opposed to one is far more convenient. I've checked the regulations and although not the prefered way this is an acceptable way. The voltage of the equipment is 600 volts and the calculations at 2.5mm prove to be acceptable for volt drop. Where it falls down is the short circuit calculation, I realise that using an appropriate MCCB is a mitigated but I'm curious, because I don't know and its not something I've ever come across, If using a multicore cable with only three cores used for power, and the remained for 24 volt switching and indication, does this allow the seven core to be calculated as a three or does it still require the seven core for the calculation of short circuit current?

Many thanks for the reply. This stems from the fact that there is a very limited amount of space for the cable installation and running two cables as opposed to one is far more convenient. I've checked the regulations and although not the prefered way this is an acceptable way. The voltage of the equipment is 600 volts and the calculations at 2.5mm prove to be acceptable for volt drop. Where it falls down is the short circuit calculation, I realise that using an appropriate MCCB is a mitigated but I'm curious, because I don't know and its not something I've ever come across, If using a multicore cable with only three cores used for power, and the remained for 24 volt switching and indication, does this allow the seven core to be calculated as a three or does it still require the seven core for the calculation of short circuit current?