Hi Stefan,
It is not as easy as you think.....If we knew how long the run of cable was, we could give you a bit of an idea though, but there is a lot lot more to it than that...
Other problem is, if you have to ask this sort of question, you have not got the knowledge to do this safely, it will also require building regulations approval too. Apart from this you will not have the equipment to do the tests you will need to do during the design and installation, or to test the finished job for safety either...
What exactly are you going to connect the cable to at the house end, what you going to use for earthing, what sort of earthing system has your house got, what about any bonding requirements?
Please do not think we are being awkward, but it is a bit like trying to anaesthetise someone. Very easy to do, much more difficult to do safely....
But hey, well done for asking advice and not just trying to guess what you need. At least you are trying to do things properly, which is more than can be said for an awful lot of people..
Give us some more information, and we might be able to point you in the right direction at least.
john...