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To provide power to electrical power distribution systems cost effectively, it is necessary to understand the efficiency of large turbine generators. Stator cores, which account for 10%–15% of total generator losses, are supported by a either a dovetail key bar or a square key bar. The dovetail key bar, mainly used in large turbine generators, has a shallow depth of penetration into the stator core; consequently, the need for research related to core loss has been limited in the case of dovetail key bars. By contrast, the square key bar, which is attracting attention owing to its ease of assembly, deeply penetrates the stator core, and thus has a greater effect on core loss than the dovetail key bar. However, core losses due to the key bar design have not been examined quantitatively in the literature. This paper analyzes the core-loss changes due to the key bar. Using electromagnetic finite-element analysis, we calculate the differences in core loss between the two aforenoted key bar types, which are commonly used in large turbine generators. In addition, we propose points for consideration when designing key bars for large-capacity generators.