timeclock in house,boiler in garage

[collie]

Hi guys,
I am needing advice here. Currently wiring my own house and as the header says the timeclock is in a room in the house. Boiler and pump are located approx 45m away(cable run distance) in the garage. I had planned on running from the wiring control center to them in 5 core 0.75 flex but have been advised that its only good for 3/4 meter runs? Anything more and it should be 2x T+E or 5 core Hytuff? Can't see any info in the BRB to help me understand why.
thanks for your help
Eugene

 

[vinny electric]

1.5 5 core mymj

 

[Geoff76]

2.5 sq t+e vplt drop any lower cable size