Experience has shown that buildings on average may consume 20% more energy than required for occupant comfort which by one estimate leads to $18 billion wasted annually on energy costs in commercial buildings in the United States. Experience and large scale studies of the benefits of commissioning have shown the effectiveness of these services in improving the energy efficiency of commercial buildings. While commissioning services do help reduce energy consumption and improve performance of buildings, the benefits of the commissioning tend to degrade over time. In order to prolong the benefits of commissioning, a prototype fault detection and diagnostic tool intended to aid in reducing excess energy consumption known as an Automated Building Commissioning Analysis Tool (ABCAT) has been developed and tested. ABCAT is a first principles based whole building level top down tool which does not require the level of expertise and investment associated with detailed component level methods. ABCAT utilizes a calibrated mathematical model to predict energy consumption for given weather conditions. A detailed description of the methodology is presented along with testing results. Results from retrospective and live test case applications are presented where the tool was used to successfully identify significant energy consumption deviations. (C) 2012 Elsevier B.V. All rights reserved.