External Conduit for shower cable

You personally can do anything you want. Although it won’t be mechanically protected or even installed properly to BS 7671 17th edition amendment 3 (2015). If you are installing it then we professionals needn’t worry about what you can and can’t do where electricity is concerned. Edit... if you want to know why it is not compliant, get a registered electrician in and he will tell you, only I think you will have to pay for that service.

No mechanical protection, Use an Electrician.. don't burn your house down :) Why change the question, the answers are still the same . Don't do it. You don't need 16mm for a shower in a domestic property! Secondly, why are you using 16mm? what shower is it? would you mess with gas? you cant smell a loose connection, nor can you smell an undersided cable or a non-earthed electrical appliance.. why risk it ?

Installing a circuit to supply an electric shower is not a job that can be done on a DIY basis. The fact that you are using 16mm twin and earth to supply an electric shower in a domestic dwelling and running this cable externally (why???) is evidence that you don't really know what you're doing. How did you determine that this was the correct size of cable? Have you checked to ensure that the existing earthing and bonding arrangements are adequate, and that the existing installation is capable of carrying the additional load that will be placed upon it? Does your existing consumer unit have RCD protection? These are all questions that need to be answered in the affirmative before the circuit is installed. And after it is installed do you know how to carry-out an insulation resistance test, CPC continuity test, earth-loop test and RCD trip-time test? If not, then you are strongly advised to stop right and now and hire a professional electrician.