Primary wire for marine or undersea cable comprises a conducting core (10), typically a multifilament core of copper, and an insulating sheath comprising an insulating inner layer (12) having a thickness of 0.35 to 1.0 mm, preferably 0.5 to 0.75 mm, and an outer protective layer (14) of polyvinylidene fluoride having a thickness of 0.15 to 0.3 mm, at least the outer layer being radiation crosslinked. The inner and outer layers are preferably crosslinked together using electron beam radiation. The combination of the inner and outer layers of the sheath enables marine and subsea cables and the like to be made with smaller diameters, without loss of capacity or electrical properties and with an increase in overall performance such as temperature range and mechanical properties.