Amongst the hundreds of other jobs involved I’m trying to get my head around this.   When we extended the house, I had the foresight to ask the electrician to put a 6mm2 cable on its own MCB and connect it to a weatherproof box on the outside of the house. It’s approx 7m cable length - through wall cavity/service gap/ceiling, it’s not running fully in insulation.    Externally the new supply cable will need to run approx 25m, probably shorter. Size of that SWA cable to be determ