EPCOS B66413 EFD 15/8/5 Transformer Ferrite Cores B66413 series EFD 15/8/5 transformer ferrite cores from EPCOS are available in gapped or ungapped varieties. These transformer ferrite cores are E shaped, but feature a lower centre leg making them particularly suited to flat transformer designs. You can select from a choice of N49, N87 or N97 materials.