This guide provides background information and general recommendations of instrumentation, circuitry, calibration and measurement techniques of no-load losses (excluding auxiliary losses), excitation current, and load losses of power and distribution transformers. The test codes, namely, IEEE Stds C57.12.90, C57.12.91, and the test code section of IEEE Std C57.15, provide specifications and requirements for conducting these tests. This guide has been written to provide supplemental information for each test.