Just wanted to know what size cable I would use on a 8.5KW Shower?

Question

The cable length would be 18 meters.

Cheers

Answer 1

8.5kw will require an 8/3 40 amp

Answer 2

I would need to know the voltage.

In the USA it would be save to figure 8.5 KW  220 volts would be a 40 amp load and would require a #6 copper wire

Answer 3

Giving the length in meters is a pretty big clue that you are not in the USA, where I could refer you to the National Electrical Code to size your conductors. Assuming you are in Europe and using 240 Volts, you would need a 35 Ampere overcurrent Protection Device (at 240 Volts). In the US, that is a "Standard" size, according to NEC Article 240.6; I would purchase a 40 Ampere overcurrent device, as that is the more common size to find in stock at a supply house. I would install # 8 AWG conductors, per NEC Table 310.6 & Article 240.4.

In a residential application, I would not be concerned with there being any substantial Voltage drop in a run of that length.

I don't have a clue how close this comes to being any help to you; but I do hope that it is useful in some way.

The best thing to do is to check your local Codes, and hire a Qualified Person to do the installation.

Good Luck