The electrical, cryogenic, and systems design concepts for a dc superconducting power transmission line are discussed. The line consists of a hollow-core flexible coaxial cable, using multifilamentary or tape Nb3Sn superconductor and wrapped tape dielectric, within a rigid cryogenic envelope. Cryogen flows through the core, is expanded in a turbine and returns over the outer cable surface. The concept is applied to power levels of 1 to 10 GW and for a range of line voltages and maximum dielectric stresses. Systems considerations define capacity ratings and optimum operating voltages. Cost estimates for the various components and cost optimizations are presented.