The performance of coils wound with cables consisting of strands of superconductor and copper can be predicted from a knowledge of the minimum current at which propagation of a normal region in the superconductor can occur. Experiments are described in which the property has been measured on a variety of cable designs, and the performance of a large coil has been successfully predicted from short specimen measurements.The design can be varied to optimize either the overall current density or the cost, and it is possible to take advantage of the improved `short sample' critical current of heat-treated superconductors.Current sharing between the superconductor and the copper after transition can take the form of total or partial transfer of current to the copper, depending on the proportion of copper present in the composite conductor.