Whilst wiring a new extractor fan, I’ve ran out of twin and earth but I have some left over three core and earth from the lighting (1mm^2). The fan doesn't have an over-run and only requires 2+e. It will be on a 6A MCB circuit with a 3-pole isolator with 3A fuse regardless of the cable used.
I don’t fancy buying a new reel of 2+e for such a short length, so besides the fact it’s a bit wasteful, is there any reason I can’t use the 3+e and just leave one core unused?
I can’t think of any reason why it would be a problem as long as the end is properly trimmed so it’s not touching anything.