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The current carrying capacity of different design internal tin strands at high magnetic fields was investigated. Two high critical current density internal tin Nb3Sn strands with different level of mechanical properties have been designed. The increase of mechanical strength has been attained by replacing part of the stabilizing copper by strengthening elements made of “in situ” nanocomposite Cu-Nb rods. The analysis of the mechanical properties of the strands in virgin state and after the reaction heat treatment has been done. The microstructure of strands components including Nb3Sn filaments has been investigated. The results of current carrying capacity measurements of 0.6 mm diameter strands in magnetic fields up to 20 T (4.2 K; 0.1 μV/cm) and hysteresis losses measurements (± 3 T cycles) are presented. It was shown that for the strand with enhanced mechanical strength the non-Cu critical current densities attained were 2580 A/mm2 and 235 A/mm2 at 12 T and 20 T, respectively, and for the strand without strengthening elements the non-Cu critical current densities were equal to 2250 A/mm2 and 500 A/mm2 at 12 T and 20 T, respectively. The correlation between the microstructure and superconducting properties of the different strands designs are discussed.