NEC Article 334 — Nonmetallic-Sheathed Cable (Romex): The Complete Exam Guide

Article 334 covers Type NM, NMC, and NMS nonmetallic-sheathed cable — the wiring method that runs through the walls of virtually every house built in the last 60 years. While NM cable (sold under the Romex brand name) is the most common residential wiring method in North America, it has strict limits: where it can be used, how it must be supported, and where it is absolutely prohibited.

Residential journeyman exams test NM cable rules heavily. Know this article well.

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What Is Nonmetallic-Sheathed Cable?

Nonmetallic-sheathed cable consists of two or more insulated conductors wrapped together in a nonmetallic outer jacket. The most familiar form is Type NM-B — the yellow, white, or gray-jacketed cable sold as "Romex" at every hardware store.

Cable types:

The -B suffix (NM-B, NMC-B) indicates conductors rated at 90°C, but because most device terminals are rated 60°C, the ampacity is limited to the 60°C column of Table 310.16 for most applications.

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NEC 334.10 — Where Permitted

Type NM cable is permitted in:

• One- and two-family dwellings and their attached or detached garages, and associated storage buildings
• Multifamily dwellings (apartments) permitted to be of Types III, IV, and V construction — up to three floors above grade
• Other structures where not prohibited by 334.12

Exam tip: The three-floor limit is frequently tested. NM cable is NOT permitted in multifamily buildings that exceed three floors above grade. This is why apartment buildings taller than three stories must use other wiring methods (EMT, MC cable, etc.).

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NEC 334.12 — Where NOT Permitted

Type NM cable shall not be used in:

• Buildings more than three floors above grade (multifamily)
• Commercial garages (not residential garages)
• Theaters and similar assembly occupancies
• Motion picture studios
• Storage battery rooms
• Hoistways and elevator shafts
• Hazardous locations (Articles 500–504)
• Embedded in poured concrete or aggregate
• Where exposed to sunlight (unless listed for the purpose)

Exam tip: The most-tested prohibition is commercial garages and buildings over three floors. If a question describes a four-story apartment or a commercial auto shop, NM cable is the wrong answer.

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NEC 334.15 — Installation in Exposed Work

When NM cable is run on the surface of a building (exposed, not concealed in walls):

• Cable must closely follow the surface of the building finish or running boards
• Where running at angles with joists in unfinished basements or crawl spaces, cable must be run through bored holes in joists or attached to the lower edges of joists — it cannot sag freely between supports
• Cable must be protected from physical damage where subject to damage

Where run in unfinished basements, NM cable larger than two 6 AWG or three 8 AWG conductors may be secured directly to the lower edges of floor joists. Smaller cables (like 14/2 or 12/2) must be run through bored holes or on running boards.

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NEC 334.17 — Through or Parallel to Framing Members

The 1.25-inch rule — one of the most-tested provisions in the article:

When NM cable passes through a bored hole in a wood framing member:

• The edge of the hole must be at least 1¼ inches (1.25 inches) from the nearest edge of the wood member
• If 1.25 inches of clearance cannot be maintained, a steel plate (nail plate) at least 1/16 inch thick must protect the cable

Where NM cable is run parallel to framing members (alongside a stud or joist), the cable must be held securely to the framing — it cannot be run loosely between framing.

Exam tip: The 1.25-inch clearance + steel nail plate rule appears on almost every residential exam. The plate must be at least 1/16 inch thick and must cover the full cable run within the framing zone.

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NEC 334.30 — Securing and Supporting

This is the most-tested section of Article 334:

Key rules:

• NM cable must be secured within 12 inches of every outlet box, junction box, cabinet, or fitting — no exceptions for concealed runs in finished walls
• Cable must be secured at intervals not exceeding 4½ feet
• When run through bored holes in framing, the framing counts as a support point — the 4.5-foot rule applies to the exposed runs between framing members
• Cables run through conduit or raceway do not need additional stapling — the raceway provides support

Stapling:

• NM cable must be secured with approved staples, cable ties, straps, or other fittings designed not to damage the cable
• Drive staples over (not through) the cable — a staple that punctures the outer jacket can damage insulation

Exam tip: 12 inches from boxes + 4½-foot maximum intervals. These numbers are on every residential exam without exception.

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NEC 334.80 — Ampacity of NM Cable

The ampacity of conductors in NM cable must be determined from Table 310.16, using the 60°C column regardless of the conductor's 90°C rating:

Why 60°C when the conductors are rated 90°C?

NM cable generates heat inside the jacket, and most terminations (receptacles, breaker lugs, wire nuts) are rated only for 60°C. Using the 90°C column would overload the terminations even if the conductors themselves could handle more.

Exception: Where NM-B conductors terminate on equipment marked 75°C or 90°C, the higher ampacity may be used — but this is uncommon in standard residential work.

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NEC 334.104 — Conductor Identification

The conductors in NM cable are color-coded:

• Black — ungrounded (hot) conductor
• White — grounded (neutral) conductor
• Bare or green — equipment grounding conductor

In 3-wire NM cable (e.g., 12/3 with ground), a red conductor is added as the second hot. This is used for 240V circuits and for switch loops in 3-way switch wiring.

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Common Exam Mistakes on Article 334

1. Using NM cable above three floors in a multifamily building — the limit is three floors above grade; anything higher requires a different wiring method
2. Forgetting the 12-inch rule near boxes — the most-missed measurement; applies even in concealed locations
3. Confusing the 1.25-inch edge clearance with the steel plate thickness — clearance = 1.25 inches; plate thickness = 1/16 inch. Don't mix them up
4. Using 90°C ampacity with NM-B — always use 60°C column for NM cable unless the termination is specifically rated for higher temperature
5. Running NM cable in a commercial garage — the tenant operating an auto repair business in a detached structure makes it a commercial garage; NM is prohibited

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Key Article 334 Numbers

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Ready to Practice Article 334 Questions?

GetLicenseReady includes residential wiring questions covering NM cable support intervals, prohibited locations, the 1.25-inch rule, and ampacity — the exact scenarios that appear on journeyman exams.

Related reading: NEC Article 300 — Wiring Methods | NEC Table 310.16 — Wire Sizing & Ampacity | NEC Article 210 — Branch Circuits | NEC Articles 404 & 406 — Switches & Receptacles