334
Nonmetallic-Sheathed Cable (NM Cable / Romex)
NM cable is the backbone of residential electrical work. It is affordable, easy to install, and available at every supply house. But the NEC places significant restrictions on where and how it can be used. Section 334.10 permits NM cable in one- and two-family dwellings, multifamily buildings of Types III, IV, and V construction, and certain other structures of those same construction types. Section 334.12 is equally important — it lists where NM cable is NOT permitted: buildings of Type I or II construction (generally steel-framed high-rises and large commercial buildings), exposed locations in dropped or suspended ceilings in non-dwelling buildings, commercial garages, hazardous locations, or wet and damp locations. One of the most practically important rules is 334.80, which limits the ampacity of NM cable to the 60-degree-C column of Table 310.16 for final circuit ampacity — even though modern NM-B cable contains conductors with 90-degree-C rated insulation (THHN equivalent). You can use the higher 90-degree-C ampacity as a starting point for derating calculations (temperature correction or bundling), but the final derated ampacity can never exceed the 60-degree-C value. Section 334.30 requires NM cable to be secured within 12 inches of every box or enclosure and supported at intervals not exceeding 4.5 feet. These securing requirements are among the most commonly cited violations during rough-in inspections.
When You Need This
- Wiring any residential new construction project — NM cable will be the primary wiring method for branch circuits
- Determining whether NM cable is permitted in a commercial renovation — check the building construction type against 334.10 and 334.12
- Calculating ampacity for NM cables bundled through a fire-stopped hole in framing — start at 90-degree-C for derating, cap at 60-degree-C
- Preparing for rough-in inspection — securing and supporting per 334.30 is the most frequently failed item
- Answering exam questions about NM cable restrictions — uses not permitted (334.12) and the 60-degree-C ampacity cap (334.80) are heavily tested
Key Points
Common Mistakes
Using the 90-degree-C ampacity as the final circuit rating for NM cable — 334.80 caps it at the 60-degree-C column
Installing NM cable in a Type I or II commercial building because it seems faster than running MC or EMT — 334.12 prohibits it
Running NM cable exposed in a dropped ceiling of a commercial space — even in Type III/IV/V buildings, exposed NM in suspended ceilings of non-dwelling spaces is not permitted
Failing to secure NM cable within 12 inches of boxes — inspectors measure this and it is the top rough-in rejection item
Running NM cable in a damp basement without switching to NMC or another approved wiring method — standard NM (Type NM-B) is for dry locations only
Forgetting nail plates when the cable passes within 1.25 inches of the edge of a stud or joist — this invites a drywall screw through the cable
Exam Tip
The two most tested concepts from Article 334 are: (1) the 60-degree-C ampacity cap from 334.80 — if the exam asks for the ampacity of 12 AWG NM cable, the answer is 20 amps (the 60-degree-C value), not 25 or 30; and (2) uses not permitted from 334.12 — know that NM cable cannot go in Type I or II construction, commercial garages, wet locations, or exposed in suspended ceilings of commercial buildings.
Frequently Asked Questions
The outer nonmetallic sheath of NM cable traps heat more than individual conductors in a raceway. Section 334.80 conservatively limits the final ampacity to the 60-degree-C column to account for this reduced heat dissipation. However, you CAN use the higher 90-degree-C ampacity as a starting point for derating calculations — this is the main benefit of NM-B's 90-degree insulation.
It depends on the construction type. NM cable is permitted in Types III, IV, and V construction (typically wood-framed or light steel buildings) but is prohibited in Types I and II (heavy steel, concrete, and non-combustible high-rise construction). Even in permitted building types, NM cable cannot be run exposed in dropped or suspended ceilings of non-dwelling spaces.
Per 334.80 and 310.15(C), you can bundle up to two NM cables (each containing two or more current-carrying conductors) through a fire-stopped or sealed framing hole without triggering the bundling derating requirement. Once you have three or more NM cables bundled together in contact for more than 24 inches, you must derate the ampacity using the 310.15(C) adjustment factors, starting from the 90-degree-C column and capping at the 60-degree-C value.
Related Code Sections
This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.