Yes, I'm often done that and, as well as reducing the number of conductors in each terminal, it also means that the ring cable remains unbroken.... One solution, and good practice - is to fit the ring cable into the terminals without actually cutting the ring cable. Strip it perfectly to just neatly fit in the terminals and pass straight through the JB. Two more 2.5mm cables can then be easily fitted in the terminals. Earth sleeving cannot then be fitted on the earth wires, but a small concession to make, for a tidier and safer outcome.
The CPC is an issue. The compromise I often adopted is to cut only the CPC, so that it can be sleeved. Since (other than for 1mm² T+E) the CPC is smaller than the live conductors, there should be no problem getting N+1 CPCs into a terminal which can accommodate N live conductors. If one does not do that, given the amount of outer sheath that has to be stripped, there can be an uncomfortable amount of bare CPC floating around ('crammed') within the JB.
However, as TTC has said, this approach (using traditional JBs, with screwed terminals) is no longer allowed for 'inaccessible joints', and the MF JBs invariably are 'one conductor per hole', such that the issue we're discussing does not arise
Kind Regards, John
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