NEC Article 334: NM-B Cable (Romex) Rules

334.10 Where NM-B Is Permitted

NM-B (commonly called Romex) is permitted in one- and two-family dwellings and multifamily dwellings of Types III, IV, and V construction (generally wood-frame buildings 3 stories or fewer). It can be run concealed within walls, floors, and ceilings, or exposed in dry locations.

NM-B may also run through air voids of masonry block walls if the location is not subject to dampness. NM-B is the most common residential wiring method in the United States because it is inexpensive, quick to install, and includes the ground conductor built into the cable assembly.

A single run of 12/2 NM-B provides hot, neutral, and ground in one pull.

334.12 Where NM-B Is NOT Permitted

NM-B is NOT permitted in several important locations: any building exceeding 3 floors above grade, commercial buildings of Types I and II construction (steel and concrete frame), embedded in poured concrete or aggregate, exposed in dropped or suspended ceilings used as plenums, in wet or damp locations, in hazardous (classified) locations, or underground (direct burial).

NM-B also cannot be run inside conduit as a general rule, though 334.15 provides specific allowances for short riser protection. These restrictions are why commercial and industrial work relies on conduit with individual THHN conductors or MC cable.

334.30 Securing and Supporting

NM-B must be secured within 12 inches of every box entry and supported at intervals not exceeding 4.5 feet along the run per 334.30. Cable clamps are required at all box entries. Flat NM-B cables (like 14/2 and 12/2) must be stapled flat against the framing member, not on edge, to prevent damage.

Staples must be driven snug but not so tight that they deform or crush the cable jacket. In accessible attics with permanent stairs, cables within 6 feet of the scuttle opening must be protected by guard strips or run through bored holes in joists. Cables running across the tops of ceiling joists must be protected if the attic is accessible.

334.15 Protection from Damage

NEC 300.4 and 334.15 work together to protect NM-B from physical damage. When boring through wood studs, the edge of the hole must be at least 1-1/4 inches from the nearest edge of the stud per 300.4(A)(1). If the hole is closer than 1-1/4 inches, a 1/16-inch thick steel nail plate covering the bored area is required.

Cables run in notches in studs or joists must also be protected by steel plates. In unfinished basements, NM-B may be run along the underside of floor joists if the cable follows the surface of the framing, but cables run at an angle to joists must be protected by running boards or bored through joists.

In garages, cables within 8 feet of the floor are considered subject to physical damage and must be protected by conduit, guard strips, or concealment within the wall.

334.80 Ampacity

NM-B cable contains conductors rated 90C (THHN-equivalent insulation) inside an outer sheath rated 60C. Per NEC 334.80, the ampacity of NM-B must be determined using the 60C column of Table 310.16: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A, 6 AWG = 55A.

However, the 90C individual conductor rating provides a critical advantage when derating is required. For temperature correction or bundling adjustment calculations, you start with the 90C ampacity value, apply the correction factors, and then check that the result does not fall below the 60C column ampacity.

This gives NM-B significantly more derating headroom than wire types with only 60C ratings. See our NM-B wire type page for complete ampacity tables with derating pre-computed.

Related WireRef Guides

• Romex vs conduit
• MC cable vs NM-B
• NM-B wire type

Frequently Asked Questions

NEC references verified May 2026