Your electric shower should have a dedicated supply from the consumer unit. The power rating of the shower will determine the size of cable needed and if you are increasing the power say from 7.5KW to 10KW you would most likely need to have the cable replaced to handle the new power requirement. Rooms containing a bath or shower are special areas and as such any work within has to be notified to the local authority. By hiring an electrician who is registered as a Competent Person and Part P approved you can be certain that any new wiring will meet the latest standards and you will not have the hassle and expense of notifying the building control as this will be done for you. You will receive notification from the local authority confirming this as well as an Installation certificate, where the work required makes it necessary. Please contact us to discuss your needs and to get a quotation. Phone Brian on 07733104288 or email :brian@bslelectrical