Cable & Conductor Physical-Protection Setback Calculator

The 1-1/4 inch rule

NEC 300.4 keeps cables and raceways far enough back from the faces of wood framing that a drywall screw or finish nail cannot reach them. The threshold is 1-1/4 in. (32 mm) from the nearest edge of the member. At a bored hole the distance is read from the edge of the hole; on a run parallel to a stud, joist, rafter, or furring strip it is read from the nearest outside surface of the cable or raceway. Where the 1-1/4 in. clearance cannot be held, the wiring is protected instead by a steel plate, sleeve, or bushing at least 1/16 in. (1.6 mm) thick, sized to cover the area of the wiring.

Which wiring methods are exempt

Exception No. 1 waives both the setback and the plate for the five rigid raceways, which resist penetration on their own. The flexible raceways FMC, LFMC, LFNC, and ENT are not rigid and are not on the list, so they are treated like cable.

Worked example: a shallow stud

1. A run of 12-2 NM cable is stapled to the face of a 2x3 furring strip, 3/4 in. back from the edge.
2. 3/4 in. is less than the 1-1/4 in. the NEC 300.4(D) parallel rule requires.
3. NM cable is not an exempt raceway, so a steel plate is required where nails or screws are likely to penetrate.
4. Protect the run with a steel plate at least 1/16 in. thick covering the exposed area, or reroute to gain the 1-1/4 in. clearance.

Bored holes vs notches

A bored hole keeps the cable inside the member, so the governing question is how much wood is left between the hole and the nearest face: 1-1/4 in. or more and no plate is needed. A notch is cut into the edge of the member, so the wiring sits right at the surface with no wood in front of it. There is no setback to measure for a notch; NEC 300.4(A)(2) requires a steel plate over the notch, installed before the building finish goes on. Whether a notch is allowed at all is a structural question governed by the building code, separate from this NEC protection requirement.

Frequently asked questions

Is the setback measured to the hole or to the cable?

For a bored hole it is the distance from the edge of the hole to the nearest edge of the member. For a parallel run it is from the nearest outside surface of the cable or raceway to the nearest edge of the framing member or furring strip.

Does EMT run through a stud need a nail plate?

No. EMT is one of the rigid raceways in Exception No. 1, along with RMC, IMC, rigid PVC, and RTRC, so it needs neither the 1-1/4 in. setback nor a plate.

What size steel plate does the NEC require?

A plain steel plate must be at least 1/16 in. (1.6 mm) thick and long and wide enough to cover the area of the wiring. A listed and marked plate thinner than 1/16 in. is permitted where it gives equal or better protection against nails and screws.

Does this cover cable through metal studs?

No. This tool covers the wood-framing cases of 300.4(A) and 300.4(D). NM cable or ENT through metal framing members has its own rule in 300.4(B), which calls for listed bushings or grommets and, where needed, a listed steel plate. That case is out of scope here.

Related calculators

• Cable Support Spacing Calculator: maximum support and securing intervals for NM, AC, MC, and SE cable.
• Conduit Support Spacing Calculator: maximum support spacing and box-securing distance by raceway type and trade size.
• Maximum Bends Between Pull Points Calculator: the 360-degree rule for a raceway run between pull points.
• Conduit Fill Calculator: mixed conductors in EMT, RMC, IMC, or PVC against the Chapter 9 Table 1 limits.