What is the difference between NM cable and Romex?

Romex is a brand name owned by Southwire for their nonmetallic-sheathed cable. NM cable is the generic term used in the NEC for this type of wiring. They refer to the same product category.

Can NM cable be used in a garage?

NM cable can be used in a finished garage if it is protected from physical damage, which usually means running it inside the wall cavity behind drywall. Exposed runs in an unfinished garage may need protection or a different wiring method depending on local code.

What does 12/2 mean on NM cable?

The first number is the wire gauge and the second is the number of insulated conductors. So 12/2 contains two 12-gauge insulated wires plus a bare ground, and is rated for 20-amp circuits.

Can NM cable be used outdoors or underground?

No. Standard NM cable is rated for dry interior locations only. Outdoor and underground installations require cable types such as UF-B that are listed for wet locations and direct burial.

Do I need a permit to replace NM cable in my house?

Yes. Replacing branch circuit wiring is electrical work that requires a permit and inspection in most jurisdictions, whether it is a single run or a whole-house rewire.

NM Cable (Nonmetallic Sheathed Cable) — Residential Wiring

An NM cable is a nonmetallic-sheathed electrical cable that bundles insulated conductors and a bare ground wire inside a flexible plastic jacket for interior residential wiring.

NM Cable diagram — labeled parts and installation context

What It Is

NM stands for nonmetallic-sheathed cable, commonly known by the brand name Romex. It is the standard wiring method for most branch circuits in wood-framed homes across the United States. The cable contains two or three insulated copper conductors plus a bare copper equipment grounding conductor, all enclosed in a flat thermoplastic outer jacket.

NM cable is designed for dry interior locations where it is protected from physical damage. The jacket color indicates the conductor gauge: white for 14 AWG rated at 15 amps, yellow for 12 AWG rated at 20 amps, and orange for 10 AWG rated at 30 amps. The printed markings on the jacket identify the wire size, number of conductors, voltage rating, and listing information. NM-B, the current standard designation, has thermoplastic insulation rated for 90 degrees Celsius on the individual conductors.

The NEC covers NM cable under Article 334, which specifies where it may and may not be used, how it must be supported and protected, and the types of buildings and locations where it is permitted. NM cable must be supported within 12 inches of every box and at intervals not exceeding 4-1/2 feet along the run, using staples, straps, or other approved fasteners.

Types

NM-B is the current standard with thermoplastic insulation rated for 90 degrees Celsius, though the ampacity is limited by the 60-degree column for termination purposes. Two-conductor cable with ground is used for standard 120-volt circuits such as lighting and receptacles. Three-conductor cable with ground is used for 240-volt circuits, three-way switch legs, and multi-wire branch circuits.

Common residential sizes include 14/2 for 15-amp lighting, 12/2 for 20-amp receptacles, 12/3 for three-way switches and split receptacles, 10/2 for dryer and water heater circuits, 10/3 for ranges and dryer circuits on four-wire connections, and 6/3 for large appliance and sub-panel feeders. UF-B cable is the underground-rated cousin of NM cable, with conductors embedded directly in the jacket material for moisture resistance.

Where It Is Used

NM cable is used for lighting circuits, receptacle circuits, dedicated appliance circuits, and other branch wiring inside finished walls, ceilings, attics, and floor cavities. It runs through bored holes in studs and joists, along the sides of framing members, and across attic floor joists where it is protected from physical damage.

It is not permitted in wet locations, outdoors, underground, in commercial buildings above three stories, or in any location where it is exposed to physical damage and not protected by conduit or other means. Some jurisdictions, notably parts of Chicago and New York City, do not permit NM cable at all and require metallic wiring methods for all residential work.

How to Identify One

NM cable has a flat or slightly oval plastic jacket that is flexible but not metallic. Stripping the jacket reveals individually insulated wires in standard colors: black for hot, white for neutral, red for a second hot in three-conductor cable, and bare copper for ground. The jacket is printed with the gauge, conductor count, cable type, voltage rating, and UL listing mark at regular intervals along its length.

Older NM cable from before the 1960s may have a cloth-wrapped jacket instead of plastic and may lack a grounding conductor. These older cables are still found in homes that have not been rewired, and their identification is important during inspections because the lack of a ground wire limits what devices and fixtures can be installed.

Replacement

Replacement is needed when the jacket is melted, cut, or brittle from age, when individual conductor insulation is damaged or crumbling, or when the cable was installed in a location where it is not code-compliant, such as an exposed run in a finished basement or a damp crawl space. Rerouting cable typically requires opening walls or ceilings, and an electrical permit is required for the work. A whole-house rewire from older ungrounded cable to modern NM-B is one of the most significant residential electrical upgrades and typically requires extensive drywall repair afterward.