IEC 63093-15:2025 specifies the dimensions that are of importance for mechanical interchangeability for a preferred range of U-cores made of ferrite, the essential dimensions of coil formers to be used with them, as well as the effective parameter values to be used in calculations involving them and gives guidelines on allowable limits of surface irregularities applicable to U-cores. The selection of core sizes for this document is based on the philosophy of including those sizes which are industrial standards, either by inclusion in national standards, or by broadbased use in industry. This document is a specification useful in the negotiations between ferrite core manufacturers and customers about surface irregularities. The use of “derived” standards which give more detailed specifications of component parts while still permitting conformance with this document is discussed in Annex A.