This part of IEC 61788 describes a test method for determining the copper to non-copper volume ratio of Cu/Nb3Sn wires. The test method given hereunder is applicable to Nb3Sn composite superconducting wires with a cross-sectional area of 0,1 mm2 to 3,0 mm2 and a copper to non-copper volume ratio of 0,1 or more. It does not make any reference to the filament diameter; however, it is not applicable to those superconducting wires with their filament, Sn, Cu-Sn alloy, barrier material and other non-copper portions dispersed in the copper matrix or those with the stabilizer dispersed. Furthermore, the copper to non-copper volume ratio can be determined on specimens before or after the Nb3Sn formation heat treatment process. The Cu/Nb3Sn wire has a monolithic structure with a round or rectangular cross-section. Though uncertainty increases, this method may be applicable to the measurement of the copper to non-copper volume ratio of the Cu/Nb3Sn wires whose cross-section and copper to non-copper volume ratio fall outside the specified ranges. This test method may be applied to other composite superconducting wires after some appropriate modifications.