What Is Romex (NM-B) Cable?

Romex is the flat, plastic-jacketed cable that wires almost every home in North America, the white, yellow, or orange bundle you see running through the studs of any framed wall. Strictly speaking, though, “Romex” is a brand name, and the cable's real designation is NM-B. Knowing what that means, how to read its jacket color and markings, and, most importantly, where the electrical code will and won't let you use it, is essential whether you're planning a project or just understanding your home's wiring. This guide covers all of it, including the connectors and boxes that terminate it, since the wire connectors and enclosures matter as much as the cable.

Romex Is a Brand; NM-B Is the Real Name

Romex is a trademarked product name (owned by Southwire) that has become the generic term for the cable, the way “Kleenex” stands in for tissue. It dates back to the 1920s and the Rome Wire Company, which gave it the “Rome-X” name. The actual cable type is NM-B, short for Non-Metallic sheathed cable, and the “B” indicates that its conductors carry 90°C-rated insulation. It's governed by Article 334 of the National Electrical Code, which spells out exactly how and where it can be used.

What's Inside

Peel back the jacket and an NM-B cable is simple: a flexible PVC outer sheath, a paper or tape separator, two or three insulated current-carrying conductors (black for hot, white for neutral, and red for a second hot when present), and a bare copper equipment grounding conductor. That combination, several conductors and a ground wrapped in one easy-to-handle jacket, is what made NM-B the standard for residential branch-circuit wiring: you staple one cable through the framing instead of pulling individual wires through pipe.

Reading the Label: X/Y and the Color Code

NM-B is described with an “X/Y” shorthand, where X is the wire gauge (AWG) and Y is the number of insulated conductors, with the bare ground implied and not counted. So “14/2 with ground” means 14 AWG wire, two insulated conductors (a hot and a neutral) plus a bare ground, and “12/3” means 12 AWG with three insulated conductors (used for three-way switches, multiwire circuits, and 240V circuits that need a neutral).

The jacket color is a helpful field shortcut, because it's coded to the wire gauge. Introduced by manufacturers in 2001, the convention was expanded in 2024 to give three-conductor cables their own distinct colors, so you can tell a 14/2 from a 14/3 at a glance:

One important caveat: this color coding is an industry convention, not an NEC requirement, so a manufacturer can legally produce NM-B in other colors. Always confirm the actual gauge by reading the printed legend on the jacket rather than trusting color alone, especially with older or odd-lot cable.

Gauge, Ampacity, and the 60°C Rule

Wire gauge determines how much current the cable can safely carry, and there's a subtlety here that trips people up. Even though NM-B's conductors carry 90°C-rated insulation, NEC 334.80 requires that its ampacity be figured from the 60°C column of the ampacity tables. That gives the familiar limits: 14 AWG is good for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. The 90°C rating isn't wasted, it provides extra headroom when you have to derate for high ambient temperature or bundling, since you start the calculation at 90°C, but the final answer can never exceed the 60°C value. In practice that means matching the breaker to the wire: 14 AWG on a 15-amp breaker, 12 AWG on a 20-amp breaker, never a smaller wire on a larger breaker.

Where You Can Use Romex

NM-B is made for the dry, protected interior of buildings, above all one- and two-family dwellings and light commercial construction (NEC construction Types III, IV, and V). It's run concealed inside walls, ceilings, and floors, stapled through studs and joists, and that's where the vast majority of home wiring lives. If the location is dry, indoors, and protected from damage, NM-B is almost certainly the right and permitted choice.

Where You Can't (the Part That Matters Most)

This is where costly and dangerous mistakes happen, because NM-B's jacket is not built for moisture or abuse. NEC 334.12 prohibits it in a long list of locations:

• Wet or damp locations, or anywhere exposed to weather. The PVC jacket is for dry areas only.
• Underground or direct burial. Use UF-B cable, which is molded for burial and wet conditions.
• Embedded in concrete, masonry, or aggregate.
• Exposed where subject to physical damage, and in plenums or air-handling ceiling spaces.
• Buildings taller than three floors above grade, and commercial construction Types I and II (steel and concrete frame).
• Inside conduit as a general practice, with only narrow exceptions such as short protective risers.

For those situations, the code points you to other wiring methods: UF-B cable for underground and wet locations, individual THHN/THWN conductors pulled through conduit for exposed, wet, or commercial runs, and NMC cable for damp or corrosive indoor areas like agricultural buildings. Local jurisdictions can add further restrictions, so confirm with your authority having jurisdiction.

Installation Rules

When NM-B is permitted, the code still governs how it's installed. Per NEC 334.30 and 300.4, the cable must be secured within 12 inches of every box or enclosure and supported at least every 4.5 feet along the run, with a cable clamp at each box entry. Where it passes through framing closer than 1.25 inches to the edge of a stud, a steel nail plate protects it from drywall screws and nails. Don't crush the jacket with over-driven staples, run the sheath about a quarter inch into the electrical box, and keep the bare ground continuous throughout. Securing hardware such as straps and supports and proper junction boxes for every splice are part of a compliant job.

Romex vs Other Cable Types

NM-B is one member of a family of wiring methods, and choosing the right one is mostly about the environment:

Safety and When to Call a Pro

The cardinal rules are simple: match the wire gauge to the breaker (never protect 14 AWG with a 20-amp breaker), never use NM-B where the code prohibits it, and replace any cable with a nicked or damaged jacket rather than taping it. NM-B terminates on devices such as receptacles and is protected at the panel by properly sized circuit breakers. Installing new circuits or working at the panel is licensed-electrician work in most jurisdictions and usually requires a permit and inspection, so bring in a professional for anything beyond understanding what's already in your walls.

The Bottom Line

Romex is the everyday name for NM-B, the non-metallic sheathed cable that forms the backbone of residential wiring: a PVC jacket around insulated conductors and a bare ground, sized by gauge and identified by a jacket color code that, since 2024, even distinguishes two- from three-conductor cable. Remember that its ampacity follows the 60°C column (14 AWG at 15 amps, 12 at 20, 10 at 30), match the breaker accordingly, and above all respect where it's prohibited, no wet, outdoor, buried, or exposed-to-damage locations, where UF-B or conduit belongs instead. Install it to the securing and protection rules, and leave new circuits and panel work to a licensed electrician.