This question pertains to Protection Against Physical Damage, Art. 300.4, and in particular the proximity of an installed cable to a 'likely' nailing surface; e.g. the edge/face of a stud, or furring strip. The language in both Articles 300.4(A)(1) and (D) essentially establishes a 1-1/4" boundary around a potential (or likely) nailing surface (or wood edge ...where nails or screws are likely to penetrate.) within which a cable should not be routed or installed, unless the cable or raceway is protected from penetration by nails or screws [(A)(1)] "by a steel plate(s) or bushing(s), at least 1.6 mm (1⁄16 in.) thick, and of appropriate length and width installed to cover the area of the wiring." or [(D)] "by a steel plate, sleeve, or equivalent at least 1.6 mm (1⁄16 in.) thick." The intent here seems clear; i.e. to protect the cable from the most likely type of nail or screw penetration (drywall/sheetrock, picture hangers, etc.), by either distance or shielding/physical barrier, but the products I most often see used as the physical barrier (metal plates) typically only protect the cable either within the stud (A)(1), or very near its surface in the case of (D), while the cable is still quite vulnerable further away.
The language in (A)(1) at least includes "appropriate length and width installed to cover the area of the wiring.", which in my mind, considering the article's intent, surely extends beyond the sides of the stud - consider how many homeowners try to nail into a stud to hang something on the wall, and miss. Shouldn't the Code help protect those who want to hang a picture, even if they can't precisely find the stud? Based on my understanding of the 'intent', I interpret the 1-1'4" boundary more as a radius, or boundary out from all surfaces of the stud. This interpretation would at least protect the cable and the homeowner a bit more by giving the screw driver and hammer a greater margin of error. As for the 1-14" boundary for parallel runs to framing members, would this apply to horizontal members (e.g. firestops, braces) as well as vertical studs, in which case, doesn't that cover everything? And if I were to run a 1-1/2" diameter cable through, or alongside/parallel to a 2x4 stud, would I only be required to install a protective plate over 1-1/4" of its width/diamer, at most? Am I allowed to run a 1-1/2" cable (or even 1/2" cable) such that its surface is within 1/4" of drywall if I'm beyond 1-1/4" from the stud, assuming new construction/rough-in? I see where a Canadian company makes plates wide enough to cover the vulnerable cable beyond the stud, but I don't see that in common use here in the southeast. I'd appreciate any feedback, and education, on these concerns from anyone willing to share. I'm truly only seeking a safe installation ....because I've seen some things!