A system for power supply to subsea installations and plants for production of hydrocarbons, comprising a preferably heat insulated pipeline (10), in which the possibility for internal formation of detrimental hydrates or wax deposits are present, and comprising electrical power supply cables (12, 31) for direct electrical heating of the pipeline (10), and having an electrical connection (36) to the surrounding sea at an end of the supply cable. The power supply cable (12) is further configured, in a second circuit configuration, to provide a three-phase power supply to an electrically powered motor in an subsea installation unit (11) connected to the pipeline (10), and that means (18, 19) are arranged for switching so that the three conductors (28) forming the power supply cables (12) form parallel conductors in a second circuit configuration, wherein the power supply cables (12, 31) supply power for direct heating of the pipeline (10).