When NM cable is run on the surface of ___ facing the attic, the cables require protection from physical damage by using substantial guard strips.

The requirement for protection of NM cable with substantial guard strips is primarily due to the potential for physical damage when these cables are exposed. When NM cable is run on the surface of insulation, walls, or floors, it is particularly vulnerable to impact or abrasion from other materials or activities in the vicinity.

Each of these surfaces has its own risks; for instance, cables near insulation may be subject to movement or contact with tools, while those on walls could be accidentally bumped or scraped during normal activities. Similarly, cables running along floors may be tripped over or subject to weight from objects being placed upon or moved around them.

By requiring substantial guard strips, the code aims to ensure that the NM cables maintain their integrity and continue to function safely over time, preventing electrical hazards that could arise from damaged insulation or exposed conductors. This protective measure is essential regardless of the surface on which the cables are run, making the answer applicable to any of these scenarios.