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The requirement for a wall switch at each floor for stairway lighting is determined by the number of risers present. According to the National Electrical Code (NEC), a wall switch must be installed for stairway lighting when there are two or more risers. This is aimed at ensuring safe passage for individuals using the stairs, as adequate lighting is crucial in preventing falls and enhancing visibility.

Having a switch at each floor allows for better control and convenience, allowing users to turn the lights on or off as needed when using the stairs. If there were only one riser, it would be less likely that the situation would necessitate that level of control, as the lighting could potentially be managed by a single switch. Thus, the requirement ensures that in spaces where there is an elevation change involving two or more risers, safety and ease of access are prioritized, justifying the installation of a switch at each floor.