AC Current Draw for SMPS
With 240 W out, the input power will be 240 W plus whatever the supply uses/wastes internally.  You say the supply is 89% efficient at full load.  How efficient is it at the 40% load of 240 W?

To be pessimistic, let's assume 80% efficiency at 240 W.  Then do the math.  (240 W)/80% = 300 W.  To get the current, divide that by the voltage: (300 W)/(230 V) = 1.3 A.

Cable size is a function of the maximum current it must be able to sustain.  Get a wire chart and look it up.