IEC 61788-12:2013 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. This second edition cancels and replaces the first edition published in 2002. It constitutes a technical revision. The main revision is the addition of two new annexes, "Uncertainty considerations" and "Uncertainty evaluation in the test method of the copper to non-copper volume ratio of Nb₃Sn composite superconducting wires".