This paper presents a case study on power quality and energy usage of a huge multi-purpose building in Hong Kong. The building consists of a variety of space usages, including office areas, lecture rooms & theaters, computer centers, library, laboratories, sport center and restaurants, for everyday use. The study aims to classify various load patterns in the building, and to analyse their relation with the power quality of the low-voltage electrical distribution system. The study is based on metering data collected from a networked intelligent power quality monitoring system. The system is equipped with 265 power quality meters installed throughout the building.