Abstract: Three-phase four-wire distribution systems are very common and widely used in commercial and industrial installations and therefore power systems harmonics is an area that merits a great deal of attention. Advancement in semiconductor devices has prompted increase in the use of non-linear loads which are the main causes of harmonic distortion in three-phase, four-wire distribution systems. The harmonic distortion is the change in the waveform of the voltage from the ideal sinusoidal waveform. It is caused by the wide use of non-linear loads that draw current in abrupt pulses rather than in a smooth sinusoidal manner