2021 IRC Chapter 38
This lesson covers how wiring gets from the panel to the outlet: how nonmetallic-sheathed cable is secured and held away from the things that puncture it, how deep a buried circuit has to sit, and where a raceway is used instead of cable. These are rough-in items, checked before drywall goes up and effectively impossible to fix after it does.
NM cable (marketed as Romex) must be secured with staples, straps, or other listed fasteners every 4-1/2 feet, and within 12 inches of every box or fitting. A single-gang nonmetallic box with no internal cable clamps gets a tighter 8-inch requirement, measured along the sheath, since the shorter distance substitutes for the clamping the box itself does not provide. An overdriven staple that crushes the sheath and flattens the conductors leaves damaged insulation buried permanently in the wall, and it is one of the easiest defects to spot at rough-in once you know to look for it.
Where a cable passes through a bored hole in a stud, joist, or rafter, the hole must keep at least 1-1/4 inches of wood between its near edge and the framing edge a drywall screw or trim nail is likely to reach. Where that clearance cannot be met, the cable needs a steel plate or sleeve at least 1/16 inch thick (or other listed physical protection) installed over it within the framing member. The same rule applies to cable run parallel to the framing, stapled near the drywall face. A cable laid in a groove or notch that will be covered by wallboard needs the same plate unless 1-1/4 inches of free space is kept for the full length of the groove. In the face of a stud that space is almost never there, so a notch practically always gets a plate, since the cable sits right at the surface where fasteners land. Where cable passes through metal framing, listed bushings or grommets must be fastened in the opening before the cable is pulled, since stamped metal edges are sharp enough to cut the sheath as the building settles and vibrates.
Attic accessibility drives the protection rule. Reached only through a scuttle hole, NM cable run across the top of the joists within 6 feet of the opening needs guard strips or equivalent physical protection; farther out, protection is not required. Reached by a permanent stairway or fixed ladder, where people go regularly, the same protection is required throughout the attic, not just near the access point. In an unfinished basement, a single small cable run at an angle across the bottom of floor joists needs the backbone of a bored hole or a running board rather than a bare staple to the joist face, since nothing else protects it from a ladder or a stored bicycle catching it. Cable above a suspended ceiling must be secured independently of the ceiling grid, which is engineered to carry tiles, not wiring.
NM cable is a dry-location assembly. It may not be embedded in poured concrete or masonry fill, used in a wet or damp location, or used as service-entrance cable, because its paper-and-thermoplastic jacket is destroyed by moisture and by the alkalinity of curing concrete. Where NM would otherwise be exposed to physical damage, such as along a garage wall at bumper height, it needs protection such as rigid metal conduit, IMC, or Schedule 80 PVC; the sheath resists moisture and abrasion, not impact. UF (underground feeder) cable has a solid molded jacket rather than NM's paper-wrapped assembly, which is what lets it be buried directly; the portion above grade still needs sunlight-resistant marking and the same physical-damage protection.
| Wiring method (residential yard, general case) | Minimum cover |
|---|---|
| Direct-buried UF cable | 24 inches |
| Rigid PVC conduit | 18 inches |
| Rigid metal conduit or intermediate metal conduit | 6 inches |
| Residential 120 V, 20 A or less branch circuit, GFCI protected (UF) | 12 inches |
A metal raceway needs less cover than direct-buried cable because the raceway itself resists a shovel or a pick; the reduced 12-inch depth for a GFCI-protected 120-volt, 20-ampere branch circuit is the exception crews commonly use for landscape lighting and shed circuits, and it disappears the moment the circuit rating goes up or the GFCI protection is removed. Splices in a direct-buried cable must be made with connectors or kits listed for direct burial; ordinary wire nuts and tape wick water into the strands and corrode the connection over years. Backfill over a cable must be free of sharp rock that will cut the jacket as the soil settles; where the excavated soil is rocky, screened sand bedding or a protective board covers the cable instead. An underground raceway is always a wet location, so its conductors need wet-location-listed insulation, and the raceway is sealed at one or both ends so moist air drawn up from the trench does not condense onto live parts inside the panel.
Where a raceway replaces cable, EMT must be secured within 3 feet of every box and supported every 10 feet. PVC is more flexible and sags with heat, so small trade sizes need support every 3 feet, and a long exposed run needs an expansion fitting where temperature swings are large. Exposed to physical damage, PVC uses Schedule 80 instead of the thinner Schedule 40, most often on a service or feeder raceway up an exterior wall. Between pull points, the total of all bends in one run may not exceed 360 degrees, four quarter bends, or pulling friction strips insulation off the conductors. The IRC handles raceway fill through conductor-count tables (E3904.6) that rest on the NEC fill limits: 53 percent of the raceway area for a single conductor, 31 percent for exactly two, and 40 percent for three or more. Insulated conductors 4 AWG or larger entering a cabinet or box through a raceway need a fitting with a smooth, rounded insulating surface, such as an insulating bushing, since a stiff conductor pulled over a raw raceway edge shaves off insulation. Splicing inside a raceway is never permitted; splices belong in boxes.
Where a cable penetrates a fire-rated wall assembly, such as the dwelling unit separation wall in a two-family house, the penetration must be closed with an approved firestop system so the assembly still performs for its rated duration. Where power conductors and Class 2 or communications cables (doorbell wiring, thermostat cable, low-voltage data) share an outlet box, a barrier or separation is required, because the low-voltage cabling's thin insulation is not rated for contact with power conductors. A bundle of NM cables passing through a single fire-caulked hole and sealed together loses its ability to shed heat, bringing the ampacity adjustment factors for bundled conductors into play; the usual fix at rough-in is spreading the penetrations across separate holes.
A worked example on conduit fill: a raceway has a cross-sectional area of 0.30 square inch, and four 12 AWG conductors are pulled into it, so the 40-percent limit applies (three or more conductors). The maximum area those conductors may occupy is 0.30 times 0.40, or 0.12 square inch. If their combined insulated area is 0.14 square inch, the raceway is undersized and the next larger trade size is needed; there is no field fix for an overfilled raceway short of pulling it apart.
In the field
On a GCHS rehab rough-in, the most valuable five minutes an inspector spends is checking every place a cable crosses within 1-1/4 inches of a framing edge for a nail plate. A screw through an unprotected cable does not usually trip a breaker outright; it can create a slow-arcing fault behind finished drywall for years. It costs almost nothing to fix before the wall closes and is effectively unfindable afterward, which is why it gets checked first, ahead of counting receptacles or box covers.
1. NM cable must be secured with staples or listed fasteners at intervals not exceeding:
2. NM cable entering a single-gang nonmetallic box with no internal cable clamps must be secured within:
3. An inspector finds a bored hole only 3/4 inch from the face of a stud. What does code require?
4. A cable is run in a groove notched into the face of a stud, to be covered by drywall, with less than 1-1/4 inches of free space along the groove. Code requires:
5. In an attic reached only by a scuttle hole, NM cable run across the top of the joists must be protected by guard strips:
6. Which use of NM cable is NOT permitted?
7. An inspector finds NM cable run exposed along a garage wall at bumper height, with no additional protection. The correct disposition is:
8. The minimum cover for direct-buried UF cable in a residential yard, general case, is:
9. A 120-volt, 20-ampere residential branch circuit run in UF cable in the backyard, with GFCI protection, needs a minimum cover of:
10. A direct-buried cable rises out of the ground to feed a yard light. It must be protected from below grade up to a height of:
11. An inspector finds a splice in a direct-buried cable made with ordinary wire nuts and electrical tape. This is a problem because:
12. Between pull points, the total of all bends in one raceway run may not exceed:
13. A raceway has a cross-sectional area of 0.30 square inch, and four current-carrying conductors are being pulled into it. What is the maximum area those conductors may occupy?
14. An inspector finds six NM cables bundled together and passing through a single hole sealed tightly with fire caulk. Beyond the firestop rating itself, what else should the inspector consider?