Equitum: 4mm would most def not suffice. Hot Tub needs 16 Amps, 2 x 1.3 kW heaters, plus Lighting, Plus the voltage drop over the run. Worked it all out and i could have gone with 10mm but thought i'd go 16mm so room for expansion later (don't wanna rip up my decking & patio again to rerun...