This paper deals with a noninvasive calibration system of current transformers in electrical power systems. The proposed system requires precise current sensors located on the primary and the secondary side of the transformer. Current sensors were designed as current transformers with a nanocrystalline magnetic core. The nanocrystalline magnetic materials can reach higher permeability and sufficiently high magnetic flux density values compared to commonly used materials. This means that smaller measurement errors can be obtained. On the other hand, there is a significant reduction of magnetic properties when the magnetic core is cut. The results of experimental measurements of magnetic characteristics, investigation of cutting process influence on magnetic properties and finally the design of both current sensors for the calibration system itself are presented in the paper.