IEC 62317-12:2016(E) specifies the dimensions that are of importance for mechanical interchangeability for a preferred range of ring-cores, also called toroid cores, and the effective parameter values to be used in calculations involving them. The selection of core sizes for this document is based on the philosophy of including those sizes which are industrial standards, meaning that they are in broad-based use within industry. See IEC 62317-1 for more detail concerning the philosophy of selecting core sizes to be included. This first edition cancels and replaces the first edition of IEC TR 61604 published in 1997. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to IEC TR 61604: a) amendment of Clause 5 concerning the relationship between standard of European, Japanese and U.S.A. sizes; b) addition of Subclause 5.3 concerning coating.