Extending a shower cable by about 20cms

[spantik]

Having changed our CH system we now have the chance to make better use of our old airing cupboard and I want to shelve it BUT the cable to the electric shower (8.5KWH) crosses the airing cupboard door opening about half way up on its way from the shower wall switch on the L (inside the said cupboard) to the shower on the R about 140cms away on a different wall. The cable used to be attached to a 2x4 piece of wood but that's gone now.

I want to extend the cable by about 20cms so that I can route it round the architrave above the door and get it out of the way. It'll be easier for me to do this between the shower and the switch not between the switch and the consumer unit (if that makes sense).

I have a basic knowledge of electricity but nothing flash (I AM a girl - is that any excuse!?) I could wire a plug when that used to be necessary and I change ceiling pendants and renew sockets,for example.

Please don't suggest an alternative. This is what I want to do (safely, of course)!

Ideas and help would be gratefully received. Thanks.

[ericmark]

You clearly know from what you have said that renewing the cable is the way to go. However of course we can joint cable and what we need is either.
A) A box which is always accessible which contains screw connectors.
B) A maintenance free joint.
For A most likely the round Bakelite junction box however these are easy to damage.
For B really bit to big for spring clips so it's a crimped connectors inside a epoxy filled box.
The problem with B is of course you need to buy crimp pliers and the cable joint kit. Likely more expensive and problematic then renewing cable.
Problem with A is joints with such a heavy current if they become slack at all will heat up and in an airing cupboard that could be dangerous.
So it would seem B is the method. There is a variation where epoxy lined shrink sleeve is used instead of a epoxy filled box but either way you will need proper crimp pliers not cheap motor car type and it's a lot of messing.
As I said to start with I am sure you know what should be done. Renew the cable.
Do remember when you alter anything like that anything you do must comply with regulations. So if for example the RCD protection is missing you would need to correct this at the same time.
This then gives a variation on A where you could fit a RCD or second isolator and renew just part of the cable but I would not fit inside the airing cupboard as just too easy to overheat without one being aware.

[spantik]

What a great reply and loads of clear help. Thanks ericmark!