Not so long ago I was wiring up an underfloor heating system. The controls for the heating were all downstairs, along with domestic hot water controls. The zone valves, along with the water tank and gas boiler were all in the loft. To complicate things, there was also a solar thermal input and a 'warm loop' system. I had a twin n earth cable feeding the system and supplying a large junction box with its cpc. I also had two 5 core flex going up to the JB in the loft. Therefore, I had two green/yellow cores which were not required as a cpc. I did need to use one of these as a live conductor and oversleeved it at each end accordingly. Isn't this an example of when an unused cpc is legitimately used as a live conductor?