• Alternative cooling concept for superconducting cables in liquid hydrogen: indirect configuration by placing the cable in a helium-filled protective pipe to avoid material incompatibilities. • Thermal coupling by natural convection in stagnant helium as a contact gas: no circulation system for helium. • Experimental validation of a specific correlation for heat transfer by natural convection in enclosed spaces [ 1 ] for fluids with Prandtl numbers < 1 at cryogenic temperatures. This contribution presents conceptual designs for a hybrid power transfer line (PTL) with liquid hydrogen (LH 2 ) and high temperature superconductors (HTS). The synergetic combination of chemical and electrical energy transfer provides a high overall efficiency and large power density. To avoid critical material compatibility with hydrogen and improve safety aspects, one option is to place the HTS cable within the same cryostat but separated from the LH 2 flow by a protective pipe. A key challenge of this indirect cooling is the heat transfer capability from the cable to the hydrogen. To improve the thermal contact, the protective pipe is filled with helium as a contact gas. Evaluating the indirect cooling needs modelling of heat transfer by natural convection, which relies on empirical correlations. In this work, an experiment is designed to validate the correlations at cryogenic temperatures. The presented test set-up is of reduced geometric complexity and experiments are executed with liquid nitrogen (LN 2 ) at 77 K. The measurement series includes different cable sample topologies and the overall results show a good accordance with a chosen empiric description of natural convection in enclosed space. The outcome enables further quantitative investigations of the indirect cooling concept for hybrid PTLs. Preliminary results predict a sufficient thermal coupling between the HTS cable and stagnant helium.