Residential Rough-In: Cable Installation with NM-B Cable

What Is Residential Rough-In?

Rough-in is the stage of electrical work that happens after the framing is complete but before the walls are closed with drywall. During rough-in you drill through studs, feed cable, mount boxes, and secure everything so the wiring is ready for a municipal inspection. Nothing is energized at this point — you are simply placing the conductors and enclosures in their permanent positions. Getting rough-in right the first time avoids expensive call-backs after drywall is hung.

NM-B Cable Basics

Nonmetallic sheathed cable, type NM-B, is the standard wiring method for most residential branch circuits in dry, indoor locations. The "B" suffix means the individual conductor insulation is rated for 90 °C, though the cable's overall ampacity is derated to 60 °C for NEC sizing purposes. Common sizes you will use in this lab are:

• 14/2 NM-B — two current-carrying conductors plus a bare equipment grounding conductor; used on 15-ampere general lighting and general-use receptacle circuits.
• 12/2 NM-B — same configuration; used on 20-ampere circuits such as kitchen small-appliance and bathroom receptacle circuits.
• 12/3 NM-B — three current-carrying conductors plus a bare equipment grounding conductor; used where a shared neutral is needed, such as a multi-wire branch circuit.

Never use NM-B in wet or damp locations, in concrete, or where exposed to physical damage without additional protection.

Drilling Through Framing

Holes drilled through studs must be centered when possible. The NEC requires that when a hole is drilled through wood framing members and the edge of the hole is within a specified distance of the edge of the framing member, you must install a nail plate (also called a nail guard or stud plate) to protect the cable from drywall screws and nails. Check the current edition of the NEC for the exact required distance — it is stated in Article 300 and reinforced in Article 334 for NM cable. Use a spade bit sized just large enough for the cable to pass through without binding; oversized holes weaken the stud unnecessarily. Drill at a consistent height throughout the run so the cable travels in a straight, orderly path.

Securing NM-B Cable

The NEC specifies two key stapling requirements for NM cable:

1. Staple within a required distance of every box or enclosure — the cable must be fastened close to each box so it cannot be pulled out. Check Article 334 of the current NEC for the precise measurement.

2. Staple at regular intervals along the run — beyond the first staple near the box, additional fasteners are required at maximum intervals stated in Article 334.

Staples must be driven snugly against the cable sheath without crushing it. A staple that cuts into or deforms the sheath is a code violation and a shock hazard. Use cable staples sized for the cable diameter — a staple intended for 14/2 may not properly secure 12/3.

Nail Plates

Whenever NM cable passes through a hole and the hole is closer to the face of a stud than the distance specified in the NEC, a steel nail plate at least the minimum code thickness must be installed over the hole before drywall goes up. Nail plates protect the cable from future fasteners. They are also required on notches cut in the face of a stud. In this lab, inspect every drilled location and install a nail plate wherever the geometry requires it — when in doubt, install one.

Box Installation

New-work plastic boxes are nailed or screwed directly to the stud face. Position receptacle boxes so the front edge of the box will land flush with or slightly proud of the finished wall surface — account for drywall thickness (typically ½ inch or 5/8 inch). Mark box height on your rough-in log before nailing. Allow sufficient cable length inside the box: the NEC specifies a minimum free conductor length at each outlet.

Circuit Assignments in This Lab

• General lighting circuit (14/2, 15 A): represents bedroom or living area lighting and general receptacles.
• Kitchen small-appliance circuit (12/2, 20 A): represents the dedicated small-appliance branch circuits required by the NEC in kitchen counter areas.
• Bathroom GFCI circuit (12/2, 20 A): represents the bathroom branch circuit. GFCI protection is required at bathroom receptacles; protection is typically provided at the device or at the first outlet in the circuit.

Safety Rules for This Lab

No circuits are energized during rough-in. Nevertheless:

• Wear safety glasses when drilling — wood chips and debris fly at eye level.
• Wear work gloves when handling cable ends; cut sheathing edges are sharp.
• Keep drill cords clear of the work area to prevent tripping.
• Call out "drilling" before you start the drill so nearby students can step clear.
• Store unused cable coils flat on the floor or on a reel — tripping over loose cable is a leading cause of lab injuries.
• If you find a framing member that is cracked or unstable, stop and notify the instructor before drilling.

The Rough-In Inspection

A municipal inspector checks rough-in before drywall is installed. In this lab your peer acts as the inspector using the instructor's checklist. Common inspection failures include: staples too far from boxes, missing nail plates, cable not secured at required intervals, insufficient free conductor length at boxes, and wrong cable size for the circuit. Fix every failed item before you submit for instructor sign-off. This habit — correct before covering — is the professional standard on every job site.