I have been asked to install a sub main supply cable 25mm SWA 4 core cable from a 63A isolator to supply a distribution board. I am however a little confused as 25mm is huge in my opinion and after checking the regs BS7671 a 16mm cable can handle 77A. Also 25mm will not fit into the terminals of the isolator, I can only assume that the person is talking about the total diameter of the cable. Any ideas are welcome as I have not installed a 3 phase system in a while of this size and it's not making sense to me.
For any reasonable length of SWA (or any cable type), the size is more often dictated by the voltage drop limit requirements, rather than the cable's current limit. Without knowing the length of the cable run, and the nature of the circuits it connects to at each end, it's not possible to say if 25mm² is appropriate. You also need to work out the earth fault loop impedance and check this is within spec for whatever fault current protective device is in use.
See the thread "10mm sq Phase CPC" from a week back. In that case, 6mm² would meet the current requirements, but would have too much voltage drop. 10mm² meets voltage drop, but has too high earth fault loop impedance for the type of circuit protective device (unless an RCD is fitted). It would probably need 16mm² to meet earth fault loop impedance without an RCD (although I didn't do that calculation, and wasn't supplied with enough extra info about the installation to do so).
It's certainly possible to get 63A isolators which will take
25mm² conductors. Another alternative would be to crimp on appropriately sized tabs to the cable ends, but getting a suitable isolator would be better (get 100A one if necessary).