What Is A Romex Cable? | Plain English Guide

Home wiring in the United States often uses a flat, plastic-jacketed cable that many people call “Romex.” The name is a brand, not a generic type. The generic term in codes and on packaging is “Type NM-B nonmetallic-sheathed cable.” Most houses use it for branch circuits to lights, switches, and receptacles.

This guide explains what the cable is made of, where it’s allowed, where it isn’t, how sizes map to breakers, and the right way to handle and fasten it. You’ll find clear tables, plain terms, and links to the official sources that set the rules.

What romex means and where the name comes from

Romex® is a trademark used by Southwire for its NM-B product line. The cable itself is a factory-assembled bundle: copper conductors with PVC insulation and a thin nylon layer, a bare equipment grounding conductor, and a flexible outer PVC jacket. The full marking on the jacket lists conductor count, gauge, type, and voltage rating, such as “14/2 NM-B 600 V.” The family is evaluated to UL requirements for nonmetallic-sheathed cable, and the design aligns with the installation rules in the National Electrical Code.

What a romex cable includes and labels

A typical cable labeled “14/2 WG NM-B” contains two insulated 14 AWG copper conductors, a bare grounding wire, and a PVC jacket. The “/2” means two current-carrying conductors; the “WG” tag means the grounding wire is included. Many makers use jacket colors for quick size ID: white for 14 AWG, yellow for 12 AWG, orange for 10 AWG, and black for 8 AWG and 6 AWG. Packaging and jacket print also list temperature rating, voltage rating, and any special features such as easier pulling jackets.

Where romex NM-B is allowed

Codes treat NM-B as a wiring method for dwellings and similar spaces. Article 334 permits Type NM in one- and two-family dwellings and in multifamily buildings of Types III, IV, and V construction. In Type I or II construction, NM may be used when placed within raceways that are permitted in those structures. The same article links to rules for fastening, protection, routing, and box entries.

Use also depends on the space. NM-B may be concealed in walls, floors, or ceilings where required, and it may run through stud bays, floor cavities, and similar chases. Short sleeves of conduit often protect the cable where it emerges from a wall into a garage, basement, or utility room. Recent NEC text adds detail about abrasion protection where a cable enters or exits a raceway and about minimum sheath length at clamps, which keeps edges from nicking insulation. Local jurisdictions may adopt earlier NEC editions or add amendments, so read the wording that applies where you live before starting any work.

Where romex NM-B is not allowed

There are clear limits. NM-B is for dry locations. It is not for wet locations, not for outdoor runs, and not for spaces with corrosive vapors. Direct burial is out; that job goes to UF-B or to individual conductors in approved raceways. Drop ceilings used as air plenums need plenum-rated wiring methods, not NM. Areas subject to physical abuse call for protection or for a different wiring method altogether. Always check your local adoption of the NEC and any city or state amendments before you pull cable.

What is romex wire used for at home

Most homeowners see NM-B feeding bedroom and hallway lighting on 15-amp circuits, general-purpose receptacle circuits on 20 amps, and larger loads like electric water heaters or baseboard heaters on 30 amps when permitted by the equipment listing. Three-conductor cable such as 12/3 handles multiwire branch circuits or 3-way switching. Two-conductor cable such as 14/2 handles simple switched legs and standard receptacle runs. Panels and devices list their ratings, and the cable choice follows those ratings and the code language for the space.

Sizing romex cable the right way

NM-B conductors carry a 90 °C insulation rating, yet the usual practice for dwelling branch circuits is to size by the 60 °C column, because common breakers, switches, and receptacles use 60 °C terminations in the ampere range that matches NM sizes. That is why 14 AWG pairs with a 15-amp breaker, 12 AWG pairs with 20 amps, and 10 AWG pairs with 30 amps. The marking on the device and the code language about termination ratings drive this choice. Larger feeders and equipment with 75 °C terminations follow their own markings and tables.

Picking the right conductor count

The jacket print lists conductor count before the gauge. “12/2 WG” means two insulated conductors plus a ground. “12/3 WG” adds a third insulated conductor. That third conductor lets you share a neutral on a multiwire branch circuit when installed with a common-trip breaker and handle tie, or it lets you run travelers for a 3-way switch pair without adding a separate cable. For split receptacles on a shared neutral, use a double-pole breaker so both hot legs trip together.

Routing that keeps the cable safe

Keep NM-B out of harm’s way. Where a bored hole leaves the cable closer than 1¼ inches to the face of a stud, add a listed steel plate or sleeve to stop screws and nails. Where the cable passes through a floor, sleeve it in a permitted raceway that rises above the floor surface, and use a bushing at each end to protect the jacket. Where the cable drops down a masonry wall, fasten it neatly to the surface or run it in conduit for abrasion protection.

Fastening so the run stays neat

Fasten flat cable on its flat face, not on edge. Use staples or straps sized for the jacket. Place a fastener within 12 inches of each box that lacks an internal clamp, and keep the run held at intervals that match your local rule based on NEC 334.30. Keep kinks out of the jacket and avoid crushing the outer sheath. Leave enough free conductor in the box to make clean terminations, and cap the bare ground when you’re not ready to land it.

Boxes, clamps, and sheath length

Use clamps or fittings that are listed for nonmetallic-sheathed cable. Bring the sheath slightly into the box so the clamp grips the jacket, not the bare or insulated conductors. Recent code language calls for a minimum sheath extension beyond an entry clamp, which keeps sharp edges away from insulation. If a box has integral clamps, follow the molded diagram or the instruction sheet and avoid feeding too many cables through one opening.

Grounding and bonding with NM-B

The bare copper in NM-B is the equipment grounding conductor. Tie grounds together with listed connectors, and add a pigtail to metal boxes and yokes that need one. Use green screws or listed clips for the bond in metal boxes. Do not cut the bare wire short; leave enough length to land the bond and still allow device replacement later. Where a cable feeds a plastic box, tie the grounds and add a pigtail only when the device needs it.

Common mistakes to avoid

Mixing 14 AWG and 12 AWG on the same 20 A circuit creates a weak link; size the breaker to the smallest gauge in the run. Staples that dent the jacket can nick insulation and should be replaced. Sheath cut back too far leaves conductors exposed to clamp edges; bring the jacket into the box. Crowding many NM cables through one bored hole can cause heat buildup; spread the holes or use stackers. Leaving cables loose near a water heater or furnace invites abrasion; shield or reroute.

Checklist before you pull

Plan the run

Sketch the path on framing. Mark box locations and heights. Count how many cables enter each box and confirm box volume for the conductor count and device bodies. Stage nail plates for tight studs and mark spots where the 1¼-inch set-back will be tight.

Prepare the cable

Cut to length with a bit extra for clean box work. Pre-curve the end so it enters the box without scraping. Strip the jacket with a knife or a purpose-made jacket tool that rides the outer sheath and avoids cut marks on insulation.

Pull and dress

Run straight lines and gentle bends. Keep bends wider than five times the cable’s thickness. At boxes, leave at least 6 inches of free conductor beyond the sheath. Shape pigtails now so devices land cleanly later.

Tools that make clean work

A light hammer and insulated-saddle staples keep the jacket tidy. A jacket ripper saves time on long runs. A right-angle drill with a sharp ¾-inch bit makes smooth bores that don’t chew the sheath. A bright marker for labeling cables neatly at the panel and at boxes pays off when circuits get energized and tested.

Color cues, but always read the print

Color helps with quick ID at the store and on site. White jackets usually mark 14 AWG, yellow marks 12 AWG, and orange marks 10 AWG. Black jackets often mark larger NM sizes, while some specialty cables use different colors or stripes. Always read the jacket print since color is a convenience, not a rule in the code.

Common pairings that work well

Bedrooms and hallways: 14/2 on 15 A runs to lights and general receptacles where local rules allow. Kitchens and dining areas: 12/2 on 20 A small-appliance circuits, often in dedicated pairs. Laundry areas: a 20 A branch circuit with 12/2. Garage receptacles: usually 20 A with 12/2 in many places, with added mechanical protection where exposed. Electric water heater: 10/2 on a 30 A two-pole breaker sized to the nameplate.

Buying tips that save time

Match the reel label to your plan before checkout. Confirm gauge, conductor count, and “WG” for the grounding wire. Check that the jacket says “NM-B 600 V.” For long pulls across open framing, the SIMpull-type jacket on some brands glides easier and resists scuffing. Keep a few feet extra in the estimate for neat box work, and store leftover coils with the ends taped to avoid splayed conductors in the next project.

Safety habits that pay off

Kill power at the breaker and lock out the panel if anyone else is on site. Test the circuit before touching conductors. Keep staples straight and snug, not smashed. Use listed cable connectors at boxes and enclosures. Respect bend radius at corners, and pull gently around framing. Follow your jurisdiction’s permit and inspection steps from rough-in through final, and keep the code links handy when a question comes up.

Where to read the official rules

The product standard for nonmetallic-sheathed cable is UL 719. The installation rules live in NFPA 70, mainly Article 334 plus general wiring rules that apply to every method. For construction detail from a leading maker, see the Southwire Romex NM-B spec.