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North American Aviation, Inc., is designing and building the Apollo Command and Service Modules at its Space and Information Systems Division, Downey, Calif., under contract to the National Aeronautics and Space Administration's Manned Spacecraft Center, Houston, Texas. The Apollo Spacecraft Command and Service Module obtains primary electrical energy from fuel cells for flight phases until earth re-entry, and from storage batteries during re-entry and landing. The electrical power distribution system controls, conditions as necessary, and distributes this energy to all spacecraft loads. Constraints, design considerations, and evolution of the electrical power distribution system are described. Load requirements and power characteristics are defined. Fault protection and redundancy to achieve reliability are discussed.