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When determining the allowable ampacity reduction percentage for raceways containing multiple current-carrying conductors, it's important to understand the guidelines established by the National Electrical Code (NEC). According to the NEC, when a raceway contains more than three conductors, the ampacity of each conductor must be reduced to account for the increased heat that results from the close proximity of multiple conductors.

For a raceway with 10 current-carrying conductors, the NEC indicates a reduction of 50 percent is required for the ampacity of the conductors. This reduction is intended to prevent overheating and ensure the safety and functionality of the electrical system.

Thus, the correct answer aligns with the NEC’s guidelines to ensure electrical safety and proper load management. By applying this percentage reduction when calculating ampacities in situations with multiple conductors, electricians can avoid potential hazards such as insulation failure or fire risks associated with excessive heat build-up in conduit systems.