The estimation of power losses in ferromagnetic core of series reactor is described in comparison with losses in state of sinusoidal current magnetization. The Karasev's approach (5) was assumed taking into account the so called overloss coefficient An. The estimation was enlarged by application of Author's complex formula deduced from source work (6). The following parameters are needed: lamination thickness d, magnetic permeability µm, frequency f, electrical conductivity γ, specific mass ρ, elliptical hysteresis angle δ. The complex formula effectiveness was checked experimentally during sinusoidal flux excitation of the magnetic laminations (8).