NEC · Article 334

NM Cable (Romex)

NM cable — the Romex found in virtually every house — looks simple until the exam asks why you use the 60°C ampacity column for conductors stamped 90°C.

NEC 2017 2020 2023 Last updated 2026-06-27

What NM cable is

Type NM cable — sold under the trade name Romex and governed by NEC Article 334 — is a factory-assembled wiring method consisting of two or more insulated conductors and a bare equipment grounding conductor, all enclosed in a flame-retardant, moisture-resistant nonmetallic sheath. It is the dominant wiring method in residential new construction because it is lightweight, easy to route, and inexpensive.

Article 334 also covers two related types: NMC, which adds a corrosion-resistant outer jacket for use in damp or corrosive locations within the permitted occupancies, and NMS, which includes communications conductors. The journeyman exam focuses almost entirely on plain Type NM.

Where NM cable is permitted — and where it is not

Section 334.10 lists the structures where NM cable is permitted: one- and two-family dwellings, multi-family dwellings, and other structures where the NEC specifically allows it. Within those structures it may be used for branch circuits and feeders in dry locations, exposed or concealed.

Section 334.12 is the prohibited-use list, and it is heavily tested. NM cable is not permitted in the following situations:

  • In wet or damp locations.
  • Embedded in concrete, aggregate, or fill.
  • Exposed to corrosive fumes or vapors.
  • In certain building construction types — specifically those where fire-resistance ratings are required, such as Type I and II noncombustible construction. NM cable is fundamentally a residential, combustible-frame wiring method.
  • As service-entrance cable.
  • In hazardous locations.
  • Where subject to physical damage.

The practical takeaway for the exam: NM cable belongs in wood-frame residential structures in dry, protected locations. Any question that introduces a commercial building, a wet space, a concrete pour, or a corrosive environment is pushing you toward a different wiring method — most often MC cable (Article 330) or conduit.

The 60°C ampacity rule under 334.80

This is the single most-tested NM cable rule on the journeyman exam because it seems to contradict what you see on the wire. The conductors inside a typical NM cable carry a 90°C insulation rating — THHN or THWN-2 inside the jacket. A candidate might assume they can use the 90°C column of Table 310.16 and get a higher allowable current.

Section 334.80 says otherwise. The ampacity of NM cable shall be determined from the 60°C column of Table 310.16. The reason is thermal: the nonmetallic sheath traps heat, so the conductors run hotter than they would in open air or in a raceway. The code addresses this by requiring the lower 60°C column.

Key exam rule: For NM cable, always start and finish with the 60°C column. If you need to derate for bundling or elevated ambient temperature, you may apply those factors to the 90°C column ampacity, but the resulting corrected value still cannot exceed the 60°C column ampacity for that conductor size. For a 12 AWG copper conductor, the 60°C ampacity is 20 A — regardless of what the 90°C column shows.

Support, securing, and installation

Article 334.30 governs how NM cable must be fastened. The two numbers to memorize are 4½ feet and 12 inches:

  • NM cable must be supported at intervals not exceeding 4½ feet.
  • It must be secured within 12 inches of every outlet box, junction box, cabinet, or fitting.

Contrast this with MC cable, which is supported every 6 feet and within 12 inches of boxes — the box distance is the same, but the mid-run interval is longer for MC. That difference is a frequent distractor question.

NM cable must also be protected where it passes through framing members and where it emerges from concealed locations and becomes exposed. Bored holes in studs and joists require the cable to be at least 1¼ inches from the edge of the framing, or a nail plate must protect the cable.

NM cable vs. MC cable — exam comparison

Feature NM Cable (Art. 334) MC Cable (Art. 330)
Primary use Residential, dry locations Commercial; broad permitted uses
Wet locations Not permitted (NM); NMC permitted in damp Listed type permitted
Ampacity column 60°C (334.80) Per 310.16 and termination rating
Support interval Every 4½ ft Every 6 ft
Within-box distance 12 in. 12 in.
Equipment ground Bare wire EGC inside sheath Dedicated EGC; armor not the ground path

Journeyman exam questions on NM cable cluster around three themes: the 60°C ampacity cap, the prohibited-use list (wet, concrete, certain occupancies), and the support interval compared against MC cable. Know those three areas cold and most NM questions become straightforward.

Practice questions on this rule

Real journeyman-exam practice questions from Loomi's bank — answer first, then reveal the explanation.

Q1 NEC 334.80

Four NM cables, each with two current-carrying conductors, are run without maintained spacing through a single opening in a wood-framed wall that will be sealed with spray foam insulation. Which statement is correct?

  1. A Adjustment only applies if the cables also share a cable tray
  2. B Ampacity adjustment per Table 310.15(C)(1) applies, and the 310.14(A)(2) Exception does not apply
  3. C The 310.14(A)(2) Exception applies and allows the cables to be counted as one raceway
  4. D No adjustment is needed because only two conductors per cable are current-carrying
Show answer & explanation

Correct: B — Ampacity adjustment per Table 310.15(C)(1) applies, and the 310.14(A)(2) Exception does not apply

334.80 requires ampacity adjustment under Table 310.15(C)(1) when more than two NM cables with two or more current-carrying conductors pass through the same sealed opening without spacing, and explicitly states the 310.14(A)(2) Exception does not apply here.

Q2 NEC 334.15

In an unfinished basement, a 12/2 NM cable is run at an angle across the joists. What installation method is required?

  1. A Secure it directly to the lower edges of the joists
  2. B Suspend it freely below the joists with cable ties
  3. C Run it through bored holes in the joists or on running boards
  4. D Staple it flat across the top edges of the joists
Show answer & explanation

Correct: C — Run it through bored holes in the joists or on running boards

334.15(C) allows only cables not smaller than two 6 AWG or three 8 AWG conductors to be secured directly to the lower edges of joists; 12/2 NM is smaller than that, so it must run through bored holes in the joists or be supported on running boards.

Q3 NEC 334.17

How much nonmetallic sheath must extend beyond a cable clamp or cable entry point?

  1. A Not less than 300 mm (12 in.)
  2. B Not less than 6 mm (1/4 in.)
  3. C No minimum is specified, as long as some sheath is visible
  4. D Not less than 25 mm (1 in.)
Show answer & explanation

Correct: B — Not less than 6 mm (1/4 in.)

334.19 sets a specific minimum: the sheath must extend not less than 6 mm (1/4 in.) beyond any cable clamp or cable entry. Vaguer 'some sheath visible' answers and the larger 25 mm or 300 mm figures overstate the actual requirement.

Q4 NEC 334.104

What insulation temperature rating must the power conductors in NM cable have, per 334.112?

  1. A 60°C
  2. B 105°C
  3. C 75°C
  4. D 90°C
Show answer & explanation

Correct: D — 90°C

334.112 requires power conductor insulation rated 90°C (194°F), consistent with Table 310.4(1) branch-circuit types, even though NM cable's ampacity is later limited by the 60°C column elsewhere in the Code. Cables marked NM-B or NMC-B meet this insulation requirement.

More practice questions: Wiring Methods and Materials (Articles 332–344)

Frequently asked questions

Why does NM cable use the 60°C ampacity column when its conductors are stamped 90°C?
Section 334.80 requires it. The conductors inside NM cable may be 90°C rated, but heat builds up within the nonmetallic sheath, so the NEC caps the allowable ampacity at the values in the 60°C column of Table 310.16. You may start from the 90°C column to apply correction and adjustment factors during derating, but the final corrected ampacity cannot exceed the 60°C value for that conductor size.
Can NM cable be run exposed in an unfinished basement?
It depends on the occupancy. In one- and two-family dwellings, NM cable is permitted on the surface of wooden framing members in unfinished basements. In multi-family dwellings and most other structures, the rules tighten. Check Article 334.15 for exposed installation requirements and 334.10 for the list of permitted structures — the occupancy type is the key variable.
What is the support interval for NM cable?
NM cable must be supported at intervals not exceeding 4½ feet and within 12 inches of every outlet box, junction box, cabinet, or fitting. That 12-inch securing distance at the box is one of the most-tested NM cable facts on the journeyman exam. MC cable, by comparison, is supported every 6 feet.
Related articles
MC Cable The metal-clad alternative permitted where NM cable is not. Conductor Ampacity The 60/75/90°C columns and why termination ratings cap the final value. Direct Burial Wire UF cable — the burial-rated counterpart to NM for outdoor runs. Branch Circuits NM cable feeds most branch circuits in residential wiring.
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