Wiring an electric shower in 2.5mm may sound surprising, but this is what exactly happened. I was called to a job because a lady wasn’t happy about the way an electric shower was wired up. She said that the shower only worked for a short period and she could smell burning. After a brief investigation I found that the shower had been wired up to adjacent bed room socket using 2.5mm cable. 

IEE Wiring Regulation is a very boring book, but in essence it is a book that governs the correct, safe and legal way that every domestic, commercial and industrial electrical job should be done in the UK.

Your consumer unit or fuse box should have the correct size equipotential bonding cables. (Earth cable for Gas and water) and be adequate for the loading of the new electric shower and its already existing circuits, and in the interest of safety a 30mA RCD ( Residual current device) should be incorporated. If the above conditions are not met, then a new small consumer unit might be needed for the new electric shower.

It is essential that the correct cable size is used for the permissible circuit length.  Its necessary to satisfy the disconnection time and thermal constraints which means that for a given combination of current demand, cable route, voltage drop and cable size, there will be a permissible correct circuit length for a given size cable.  IEE Wiring Regulation book will have all these values and a qualified Part P electrician will be able to help here. A 45amp double pole isolating switch should be incorporated in the circuit and should be clearly identified and position in an accessible location close to the shower.  The cold water supply can be taken from a cold water storage tank provided the is minimum head high of 10meters.

IEE Wiring Regulation states that for every complete installation of a new electric shower a “Electrical Installation Certificate” should be completed. The information on this test certificate will verify that all the necessary electrical safety things have been competed and that the electric shower is safe to use. The “Electrical Installation Certificate” will be signed by the qualified registered Part P electrician and a sheet will be given to the consumer and the local build control department will be notified.