Recent work has shown that annealing amorphous alloys under an applied magnetic field can enhance the fraction of nanocrystalline phase formed and induce a strong texture. This effect has been attributed to the displacement of thermodynamic equilibrium by the energetic contribution from the applied field. In the present paper, an attempt is made to test the validity of this proposition in the Fe-Si-B(-Nb-Cu) system using equilibrium calculations in the framework of the CALPHAD (CALculation of PHAse Diagrams) methodology and a simple estimate of the energy supplied by the field. However, it is found that the magnetic energy term thus obtained is too small to produce the observed change in nanocrystalline phase fraction. Possible reasons for this discrepancy are discussed.