Total Harmonic Distortion

Total harmonic distortion (THD) is how much a voltage waveform deviates from its fundamental sinusoidal form. Non-linear loads such as electrical motors, transistors, and variable frequency drives can draw current that is not perfectly sinusoidal, creating disturbances and distorting the voltage waveforms. Unwanted distortion can cause excessive heating and core loss in motors and increased power usage. Distortions in the power supply are especially damaging for manufacturing or research facilities where calibration is critical to the equipment’s performance accuracy. It is paramount for these facilities to receive clean power for equipment efficiency.

Calculation of total harmonic distortion

The THD endpoints return a ratio of the amplitude of higher order harmonics to the amplitude of the fundamental sinusoidal waveform. This ratio is calculated using the following formula:



RMS(higher order voltage amplitudes) ÷ RMS(fundamental voltage amplitude)


High distortion ratio

A value higher than 0.03 across all panels in your building may indicate a bad power quality from your utility provider.

If found at the panel level, this can indicate a load imbalance, a bad transformer, or even faulty equipment. Sharp changes in demand or supply can cause distortion in harmonics, and can often be traced to electrical equipment such as variable frequency drives or other motor speed controllers.

If the harmonic distortion cannot be addressed fundamentally, it is recommended to invest in power conditioners with active power filters (APF) to prevent damage to equipment that may be sensitive to dirty voltage.