A new calorimeter has been built with the special purpose to determine the distribution of Tc in industrial superconducting wires. Specific heat measurements have been carried out on a series of multifilamentary Nb3Sn wires, using a long relaxation technique. The advantage of this technique consists in the fact that the measurement is performed in presence of the Cu-Sn matrix, i.e. the filaments are measured under the same stress conditions as under operation, i.e. under the same state of mechanical precompression. In addition, the Tc distribution is obtained for the whole sample volume, ruling out shielding effects. The deconvolution of the data in the region of the superconducting transition was used for getting the precise distribution of Tc, which in turn allows a determination of the Sn distribution across the filaments. These data confirm previous TEM measurements showing a Sn gradient inside of the filaments of bronze route processed Nb3Sn wires. The Tc distribution has been determined in Nb3Sn wires processed by bronze route, internal Sn and powder-in-tube technique. Based on this information, the various processing parameters can be varied to get narrower Tc distributions at transition temperatures closer to 18 K.