Light load harmonics can be measured using the Total Demand Distortion (TDDi) equation. The calculation uses rating current as the base factor. It is a fixed factor for both light load and heavy load operations, which means that at light loads the TDDi is smaller when the harmonic current is small, and larger for heavy loads as harmonic current increases. When the load increases to nearly 100%, the TDDi measure will be close to the THDi measure. Current at 100% load.