NEC Article 334: Romex NM Cable Guide - Uses, Limitations & Installation
NM-B cable (commonly called Romex) is the most widely used wiring method in residential construction. Understanding NEC Article 334 is essential for every residential electrician.
In This Guide
NM Cable Types & Construction
Nonmetallic-sheathed cable (NM) consists of two or more insulated conductors and an equipment grounding conductor enclosed within an overall nonmetallic jacket. The "NM" designation stands for nonmetallic, referring to the outer sheath material.
Cable Designations
| Type | Outer Jacket | Temperature Rating | Typical Use |
|---|---|---|---|
| NM-B | PVC jacket, rated 90°C conductors | 90°C (but limited to 60°C ampacity) | Standard residential, dry locations |
| NMC-B | Corrosion-resistant jacket | 90°C (limited to 60°C ampacity) | Damp or corrosive locations |
| NMS | Includes signal/data conductors | 90°C (limited to 60°C ampacity) | Power + communications combined |
Understanding the "-B" Suffix
The "-B" in NM-B indicates the cable meets the 2002 NEC and later requirements for 90°C rated conductors. Older "NM" cable without the B suffix used 60°C insulation. All NM cable manufactured today is NM-B. Despite the 90°C insulation rating on the individual conductors, NEC 334.80 limits the ampacity of NM cable to the 60°C column of Table 310.16 unless the cable is subject to ambient temperatures above 30°C, in which case the 90°C rating is used only for temperature correction calculations.
Common NM-B Cable Sizes
| Cable | Conductors | Ampacity (60°C) | Typical Use |
|---|---|---|---|
| 14/2 NM-B | 2 #14 + ground | 15A | General lighting |
| 12/2 NM-B | 2 #12 + ground | 20A | Receptacles, kitchen, bath |
| 10/2 NM-B | 2 #10 + ground | 30A | Dryer, water heater |
| 10/3 NM-B | 3 #10 + ground | 30A | Electric dryer (with neutral) |
| 6/3 NM-B | 3 #6 + ground | 55A | Range, cooktop |
Where NM Cable Is Permitted (334.10)
NEC 334.10 lists the locations and building types where NM cable may be used. The key provisions include:
Permitted Uses
- • One- and two-family dwellings and their garages and accessory buildings
- • Multifamily dwellings permitted by the building code (typically Types III, IV, and V construction)
- • Other structures permitted by the building code (not exceeding three floors above grade)
- • Cable trays in structures where NM is permitted
- • Concealed or exposed installations in normally dry locations
- • To be fished in the air voids of masonry block or tile walls (dry locations)
Where NM Cable Is NOT Permitted (334.12)
NEC 334.12 strictly prohibits NM cable in several locations. These restrictions are among the most commonly tested on licensing exams and the most frequently violated in the field.
Prohibited Uses - Do NOT Install NM Cable:
- • As service entrance cable
- • In commercial garages with hazardous locations
- • In theaters, assembly halls, or similar locations (except as allowed in Article 518)
- • In motion picture studios
- • In storage battery rooms
- • In hoistways or on elevators
- • Embedded in poured concrete or aggregate
- • In wet or damp locations (use NMC or UF cable instead)
- • In any hazardous (classified) location per Articles 500-516
- • Exposed in dropped or suspended ceilings in other than one- and two-family and multifamily dwellings
Important Clarification: Exposed vs Concealed
NM cable can be installed exposed (visible) in normally dry locations, but it must be protected from physical damage. When run across the face of studs or joists in unfinished areas like basements, it should follow the building surface. When it would be subject to physical damage, it must be protected by a raceway, guard strip, or other approved means.
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Securing & Support Requirements (334.30)
NEC 334.30 establishes specific requirements for how NM cable must be secured and supported. These rules ensure the cable stays in place, maintains its integrity, and remains accessible for future maintenance.
Key Support Rules
Within 12 Inches of Every Box
NM cable must be secured within 300mm (12 inches) of every outlet box, junction box, cabinet, or fitting. Measure from the point where the cable enters the box or connector.
Every 4.5 Feet Along Runs
Support NM cable at intervals not exceeding 1.4m (4.5 feet) along the entire run. Use approved cable staples, straps, or other listed means.
Single Staple for Each Cable
NM cable must not be stapled on edge. Use the correct size staple for the cable diameter. Do not over-drive staples - this can damage the outer jacket and the conductors inside.
Exception for Concealed Work
Where NM cable is fished into finished walls, ceilings, or floors, it is not required to be secured. This exception applies to renovation and retrofit work where access to the framing is not available.
Stacking Cables
When multiple NM cables run parallel through the same area, do not stack them on top of each other under a single staple unless using a stacker-type staple rated for multiple cables. Bundling cables together increases heat buildup and may require derating per NEC 310.15(C). Many inspectors look for this violation specifically.
Bending Radius Requirements
NEC 334.24 references NEC 300.34 for bending requirements. For NM cable, the minimum bending radius is 5 times the cable diameter. This means the inside edge of the bend must not be tighter than 5 times the overall cable diameter.
| Cable Size | Approx. Diameter | Min. Bend Radius (5x) |
|---|---|---|
| 14/2 NM-B | 0.44" | 2.2" |
| 12/2 NM-B | 0.49" | 2.45" |
| 10/2 NM-B | 0.55" | 2.75" |
| 6/3 NM-B | 0.98" | 4.9" |
Sharp bends damage the conductors inside the cable and can crack the outer jacket. This is especially important around corners and where cable enters boxes. Take care to make gentle bends and avoid kinking.
Through Studs, Joists & Rafters (334.17)
When NM cable passes through wood framing members, it must be protected from physical damage (particularly from nails and screws driven during drywall and trim installation). NEC 300.4 provides the specific requirements.
Holes Through Studs
- • 1-1/4" minimum clearance: The edge of the hole must be at least 1-1/4" from the nearest edge of the stud (NEC 300.4(A)(1))
- • If less than 1-1/4": A steel nail plate at least 1/16" thick must be installed to protect the cable
- • Hole size: Should not exceed 40% of the stud width to maintain structural integrity (per building code)
- • Centered is best: Drill holes in the center of the stud whenever possible
Notches in Studs
Avoid Notching When Possible
While the NEC allows cable in notches with a nail plate installed, notching weakens the stud structurally. Drilling holes is always preferred. When notching is unavoidable, use a 1/16" steel nail plate and verify that notch depth does not exceed building code limits (typically 25% of stud depth for bearing walls, 40% for non-bearing walls).
Through Joists and Rafters
The same 1-1/4" clearance rule applies when NM cable passes through joists and rafters. When running cable along the face of joists in an unfinished basement, NM cable may be attached directly to the bottom of the joists. When run at angles to the joists, the cable must be run through bored holes or on running boards.
Temperature Ratings & Ampacity
The temperature rating of NM-B cable is one of the most commonly misunderstood aspects. While the individual conductors inside NM-B cable are rated at 90°C, NEC 334.80 provides a critical limitation.
NEC 334.80 - The 60°C Rule
"The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C rated conductor." This means you must use the 60°C column of Table 310.16 for NM cable ampacity, even though the wire inside is rated 90°C. The 90°C rating may only be used for applying ambient temperature correction factors.
Practical Impact
| Wire Size | 60°C Ampacity (NM cable limit) | 90°C Ampacity (conductor only) |
|---|---|---|
| #14 | 15A | 25A |
| #12 | 20A | 30A |
| #10 | 30A | 40A |
| #6 | 55A | 75A |
The green values are the maximum ampacity you can use for NM cable in normal conditions.
Common Code Violations
These are violations that inspectors frequently find on job sites. Avoid these to pass inspections and maintain safe installations.
Missing nail plates
Holes or notches within 1-1/4" of the stud edge without a protective steel plate. This is the single most common NM cable violation found during rough-in inspections.
Cable not secured within 12" of box
NM cable must be stapled or otherwise secured within 12 inches of every outlet box, junction box, or fitting.
NM cable in wet or damp locations
NM cable running through exterior walls, unfinished below-grade areas, or other damp locations. Use UF cable or run NM in a raceway.
Over-driven staples
Staples driven too tight that crush the cable jacket. This damages insulation and can create a short circuit over time. The staple should be snug but not compress the cable.
Cable not protected from physical damage
NM cable run exposed where it is subject to damage (across floor surfaces, through unprotected holes in metal framing, etc.) without adequate protection.
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