Get a pro in for this one as there is much more to it than running in a bit of cable, length of run, load required, volt drop cable type, earthing and supply type. For some domestic supplies ( TNCS I think) its not recommended to extend them outside of the house so alot of the time a phase and neutral is ran from the house BD to the shed DB and them the installation is them earthed via a TT system at the shed.
So short story get a decent spark.