The possibility to create the reliable and solid magnet system of FCC project working with reasonable margin is defined by the development of the internal tin Nb3Sn wires with combination of contradicting properties including the non-Cu critical current density larger than 3000 A/mm2 (12 T&4.2 K) altogether with low effective diameters of the Nb3Sn filaments (down to 20 µm) and high RRR of stabilizing copper. This challenging set of parameters has to be met in a commercially produced wires with strong limitation on the cost of their production. The analysis of the design features of such internal tin Nb3Sn wires has been done and some considerations on the possibilities to attain the required set of parameters has been given. Some of the proposed designs of the internal tin strands with one common diffusion barrier has been verified by the production in industrial conditions. The effective separation of the groups of the filaments inside the each of the sub elements assembled inside the diffusion barrier has been shown. The formation of Nb3Sn microstructure in the tightly packed bundles of Nb filaments during the reaction heat treatment has been experimentally investigated. The possibility to form the well separated Nb3Sn macro filaments with predominantly defined dimension of cross section around 20 µm has been confirmed. The microstructure of the Nb3Sn macro filaments was shown to be non-uniform and consisted of the zones of columnar structure and the peripheral zones of large equiaxed grains. The correlation of Nb3Sn macro filaments microstructure with heat treatment parameters is discussed. The ways of further optimization of Internal tin Nb3Sn strands with common diffusion barrier for attaining of the FCC complete set of parameters are outlined.