In combustible surfaces, boxes and other fixtures shall extend to what location?

The correct answer is that boxes and other fixtures must extend to the finished surface when they are installed on combustible surfaces. This requirement is based on the NEC's emphasis on safety and preventing fire hazards. By having the boxes extend to the finished surface, it ensures that the wiring within the box is adequately protected and minimizes the risk of igniting combustible materials that may be nearby. This practice provides a clear demarcation that helps maintain the integrity of the electrical installation, allows for better access and support of the devices being attached, and ultimately enhances the overall safety of the electrical system in relation to combustible surfaces.

Other options do not align with NEC standards. For instance, extending 10 inches beyond the surface or only a minimum of 1/2 inch may not properly provide protection for wiring against potential heat or ember exposure. Similarly, simply reaching the nearest edge could lead to inadequate coverage and potential electrical hazards, especially in environments where combustible materials are present. Therefore, the NEC's prescription for extending to the finished surface is a well-considered requirement to ensure safety and compliance in electrical installations.