Shower electrical cable

Hi,
I'm about to swap out the old bathroom suite and replace it all including the rather nasty shower.
The shower is I believe an 8.5KW affair and the power supply appears to be fairly modern (approx 2yrs old ) cable on a 45A breaker.
But, I can't tell what the size of the cable ie. 10mm^2, so I'm not sure if we can have a more powerful shower without risk of overloading the cable. I read somewhere that by measuring the outside diameter of the cable you could determine the size of the conductors in the cable, anyone know of these figures?
Cheers,
Jeff
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go to local wholesaler ask him for 3" of 10 mm sq go home and compare
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