In the world of the growing energy needs it becomes more and more important to know and minimize the losses of inductive components. For EMC applications low iron losses can cause not just low efficiency but overheating too. Nanocrystalline cores, especially the ones with lower permeabilities can have phase shift very close to the ideal 90°. In this condition measuring power loss with flux metric method can be affected with large errors because of the current probe delay. In certain cases calorimetric measurement can be more accurate. I performed power loss mearuement with flux metric method by using a Brockhaus MPG200 instrument. For the calorimetric method I measured the temperature increase of a core dwelled into transformer oil. The losses were calculated by comparing each sample to a reference. I also investigated the losses of oval shaped, low permeability and cut cores. For common mode chokes copper losses are more relevant, which can be measured easily. Calculating the overtemperature caused is more difficult. I compared the existing empirical formulas to measured data, and made a more accurate calculation.