Magnetic design with flux density well below the permissible Bmax for the core material. This provides margin for additional iron losses due to harmonics. Composite conductor structure design made up of multiple individually insulated parallel conductors of thin rectangular shape which eliminates the predominant skin effect at higher harmonics resulting in significant copper loss including eddy current loss and overheating. Special coil structure geometries and separated layer of primary and secondory, reduce the magnetic leakage - stray field-and stray eddy current losses significantly. All the above design and structural features reduce the losses due to harmonics significantly. However transformers for higher K factor ratings are designed with built-in over sizing to have additional thermal capacity to handle remaining additional losses due to harmonic currents in non - linear loads. Datsons follow the over sizing levels in the design as discussed later. % over sizing may also be provided as per customeor consultants requirement. During heat-run testing of K-rated transformers,the loading is done to the oversized ratings, which simulate the additional harmonic losses as expected on non-linear loads under real-time application. The derating guidelines of UL1561-ANSI 57.110-curves do not apply to transformers with above designs considerations and structure.