My steel building has 2"x6" wall studs and 2"x4" exterior purlins. The steel siding is attached with screws to the purlins. I've run my wiring behind the studs and attached it to the top of the purlins. Thus the wiring lays on the purlins behind the 2"x 6" studs. The inspector stopped by and informed me that nail plates are required where less than 1 1/4" of wood is between the wall face and the wiring. Since there is actually 5 1/2" from the inside wall face to the wiring, he's now referring to the exterior wall. I thought the purpose of nail plates was for protection from nails or screws being inserted from the inside of the building. Why would anyone be needing protection from nails or screws being inserted from the outside wall? I didn't argue but I don't think he's correct. I guess I'm wondering if he NEC defines the wall face where nail plates are required? I'd be surprised if the face wall refers to the outside exterior side of the building wall. Thanks
The code does not differentiate which face of the stud. It simply says:
In both exposed and concealed locations, where a cable- or raceway-type wiring method is installed through bored holes in joists, rafters, or wood members, holes shall be bored so that the edge of the hole is not less than 32 mm (1¼ in.) from the nearest edge of the wood member. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by screws or nails 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."
If you are in WA then in WAC 296-46B-010 General it states that: (a) The exterior shear panel/sheathing nail inspection must be completed by the building code inspector and, where siding nails or fasteners which penetrate into the wall cavity are to be used, all siding must be installed; or (b) All wiring and device boxes must be a minimum of 2 ½ inches from the exterior surface of the framing member; or (c) All wiring and device boxes must be protected by a steel plate a minimum of 1/16 inch thick and of appropriate width and height installed to cover the area of the wiring or box.
If your cables don't go through drilled holes in the framing members (studs or purlins), 300.4(A)(1) would not apply in that case. It only applies to cables passing through a hole in the member.
It sounds like in your case, the cables are run on top of the purlins, so technically, you do not need nail plates there.
The thing is, the steel siding does not get attached with puny, short drywall screws, but rather long ones probably. So you may still want to consider changing your plan and running the cables either through drilled holes in the studs, or adding some kind of protection. If that siding ever gets replaced, an errand screw can quickly go through your cables.