NEC NM-B / Romex Installation Rules
NEC Article 334 governs NM-B (Romex). Permitted occupancies, prohibited uses, stapling per 334.30, holes per 300.4, and the dwelling-construction limitations.
NEC Article 334 of NFPA 70 (2023) governs Nonmetallic-Sheathed Cable (Types NM, NMC, NMS) - the cable most electricians simply call Romex after the original 1922 trade name. NM-B is the modern version, rated 90C for derating but limited to 60C ampacity. Article 334 is short by NEC standards, but its rules about where NM may be installed and how it must be secured are heavily enforced by AHJs.
What Article 334 Covers
NEC 334 defines NM cable as having an outer nonmetallic moisture-resistant flame-retardant jacket enclosing two or more insulated conductors plus a bare grounding conductor. Article 334 covers permitted/prohibited uses (334.10/334.12), construction (334.100-118), installation (334.15 through 334.130), and ampacity (334.80).
Permitted Uses - NEC 334.10
NM cable is permitted:
- One-family and two-family dwellings and their attached/detached garages and storage buildings
- Multifamily dwellings in Types III, IV, and V construction
- Other structures in Types III, IV, and V construction, where concealed within walls/ceilings/floors with a 15-minute thermal barrier, OR exposed where not subject to damage
- In cable trays in Types III, IV, V buildings where identified for the use
Prohibited Uses - NEC 334.12
NM cable shall not be used:
- In Types I and II construction (concrete/steel commercial buildings) except as permitted in 334.10(3)
- As service-entrance cable
- In commercial garages with hazardous (classified) locations
- In theaters and similar locations
- In motion picture studios
- In storage battery rooms
- In hoistways or on elevators or escalators
- Embedded in poured cement, concrete, or aggregate
- In wet or damp locations (except as identified in 334.10(3))
- Where exposed to corrosive fumes or vapors
The Types I/II prohibition is the biggest geographic limit: virtually all commercial-grade construction in major metros uses steel-and-concrete construction, ruling out NM in favor of MC or conduit.
Securing and Supporting - NEC 334.30
| Requirement | Section | Value |
|---|---|---|
| Maximum support interval | 334.30 | 4-1/2 ft (1.4 m) |
| Within 12 inches of every box/cabinet | 334.30 | 12 in (300 mm) |
| Within 8 inches when entering nonmetallic single-gang box | 314.17(B)(2) exception | 8 in (200 mm) |
| Unsupported in finished work | 334.30(B)(1) | Permitted where fished |
| Cables run parallel to framing | 334.15(C) | Secure within stud bay |
Through Bored Holes and Notches - NEC 300.4
When NM cable runs through framing:
- 300.4(A)(1): Holes through wood members must be at least 1-1/4 inches from the nearest edge, OR a 1/16-inch steel protection plate must be installed
- 300.4(A)(2): Notches in framing require a 1/16-inch steel plate covering the notch
- 300.4(D): Cables run parallel to framing members must be at least 1-1/4 inches from the nearest edge or protected by a 1/16-inch plate (this also applies to cable runs in joist bays parallel to studs)
- 300.4(E): Cables installed in shallow grooves in masonry/concrete must be protected by a 1/16-inch plate
These plates are commonly called "nail plates" or "stud guards" and exist specifically to prevent drywallers from puncturing cables with screws.
Ampacity - NEC 334.80
NM cable conductor ampacity shall be the 60C column of Table 310.16, even though the conductor insulation is rated 90C. The 90C value can be used for the derating calculations of 310.15, but the final result must be compared against the 60C ampacity. Translation:
- 14 AWG NM-B: 15A (60C)
- 12 AWG NM-B: 20A (60C)
- 10 AWG NM-B: 30A (60C)
- 8 AWG NM-B: 40A (60C)
This matches the small-conductor cap of 240.4(D), so practical residential rules align cleanly.
Bundling NM-B - NEC 334.80 / 310.15(C)(2)
Where more than two NM cables containing 2 or more current-carrying conductors are bundled together for more than 24 inches without maintaining spacing, the adjustment factors of Table 310.15(C)(1) apply. This commonly affects panel-area home runs stapled tight to a stud cavity for several feet.
Worked Example
A 1-family dwelling panel has 9 NM-B home runs stapled tightly together for 36 inches above the panel:
- 9 cables x 2 CCCs each = 18 CCCs
- 10 - 20 CCC factor = 50%
- 12 AWG NM-B at 90C (Table 310.16) = 30A
- 30 x 0.50 = 15A adjusted
- 240.4(D) cap = 20A; lower of (15A, 20A) = 15A
- 20A breaker exceeds 15A -> code violation
Fix: fan the cables out within the first 12 inches above the panel so they are no longer bundled for more than 24 inches.
Cross-References
- NEC 250.118 - bare conductor in NM serves as EGC
- NEC 300.4 - through-bored holes, notches, plates
- NEC 334.40 - boxes and fittings
- NEC 312 - panelboard cabinets
- NEC 314 - outlet/junction boxes
How WireFillChart Supports NM-B
Our conduit fill calculator handles NM-to-THHN conversion when running NM into a panel knockout with a connector and transitioning to conduit. For typical residential branch circuit sizing, see wire size for 15A, 20A, 30A, and the conduit fitment posts like 12-2 Romex conduit size.