Quick answer
NM-B cable (Romex) is nonmetallic sheathed cable: two or three insulated copper wires plus a bare ground, wrapped together in a plastic jacket, and it is the most common wiring inside the walls of US homes.
Romex is a brand name that many people use for any NM-B cable. Its wires are rated 90°C, but its ampacity is limited to the 60°C value, and it belongs in dry locations only.
Quick facts
The key facts about NM-B Cable (Romex), with sources:
Key takeaways
- NM-B is the flat, plastic-jacketed cable used for most outlet and light circuits in US houses.
- "Romex" is a brand; electricians and stores use it for NM-B cable in general.
- The label "12-2 W/G" means 12 AWG, two insulated wires, with a ground.
- Its wires are rated 90°C, but the cable may only carry the 60°C ampacity 2.
- It is made for dry locations; wet or outdoor runs need a different cable or wire in conduit.
What NM-B cable is
Open an unfinished basement ceiling in most US homes built after the 1960s and you will see it: a flat, colored plastic cable stapled to the joists. That is nonmetallic sheathed cable, type NM-B. "Nonmetallic" means the outer jacket is plastic, not metal armor.
Inside the jacket are insulated wires and a bare copper ground. A "-2" cable has a black hot wire and a white neutral wire. A "-3" cable adds a red wire, used for things like three-way switches or 240-volt circuits that also need a neutral. Paper filler often sits between them.
One major maker describes its NM-B as a "nonmetallic sheathed indoor copper building cable" used mainly for residential branch circuits for outlets and switches 2.
Romex is Southwire's registered brand name for its version, sold as Romex Brand SIMpull Type NM-B 1. When people say "Romex", they almost always mean NM-B from any maker.
How NM-B carries power safely
Each wire has its own insulation, and the jacket adds a second layer that guards against nicks during installation. The bare wire is the equipment grounding conductor. It gives fault current a path back to the panel so the breaker trips.
The "B" version has conductor insulation rated 90°C dry 2. Even so, the NEC limits NM-B to the ampacity of a 60°C conductor.
The higher rating can only be used when derating, such as when many cables are bundled together.
In plain terms, 14 AWG NM-B is a 15-amp cable and 12 AWG NM-B is a 20-amp cable.
Because the jacket is plastic, it gives no physical armor. That is why the code limits where it can run, and why it must be protected where it could be damaged.
- Outer PVC jacketcolor shows size on many brands; printed with type and size
- Paper wrap or fillerkeeps the shape
- Black insulated wirehot
- White insulated wireneutral
- Bare copper wireequipment ground
Source: [2]
Common NM-B sizes, colors and uses
Jacket colors below are one major maker's scheme, which most brands follow. The printed label is what counts 2. Ampacity is from the 60°C column 3.
| Cable | Jacket color | 60°C ampacity | Typical use |
|---|---|---|---|
| 14-2 W/G | White | 15 A | Lighting and general outlets on 15-amp breakers |
| 14-3 W/G | Light blue | 15 A | Three-way switches on 15-amp circuits |
| 12-2 W/G | Yellow | 20 A | Kitchen, bath, laundry and garage outlets |
| 12-3 W/G | Purple | 20 A | Shared-neutral kitchen circuits, three-way on 20 A |
| 10-2 / 10-3 W/G | Orange / pink | 30 A | Water heaters, dryers |
| 8 and 6 AWG | Black | 40 A / 55 A | Ranges, some EV and subpanel circuits |
Example: reading an NM-B jacket label
Here is how a typical jacket marking breaks down. Printed text varies by brand, but the parts are the same.
| Marking | What it means |
|---|---|
| 12-2 | 12 AWG wire, two insulated conductors |
| W/G or WITH GROUND | Includes a bare copper equipment ground |
| TYPE NM-B | Nonmetallic sheathed cable, 90°C conductors |
| CU | Copper conductors; "AL" would mean aluminum 4 |
| UL / listing mark | Tested to UL 719 2 |
CPSC suggests shining a flashlight at a low angle to read embossed markings on older cable in basements and attics 4. Reading a jacket is safe. Opening boxes or panels to look is not.
Where NM-B is allowed and where it is not
The maker lists exposed and concealed work in normally dry locations, and air voids of masonry block walls that are not subject to excessive moisture 2. In practice that means wall and ceiling cavities, attics and basement joists in one- and two-family homes.
It is not for wet locations, outdoors, underground or inside conduit buried in the ground. A slab or exterior conduit counts as a wet location even if it looks dry.
Those runs use UF-B cable or individual wires such as THHN/THWN-2 in conduit. Many commercial buildings and some cities restrict NM cable further, so the local code decides.
Unfinished spaces
Where NM-B runs low on a basement wall or in a garage, it may need protection from damage, such as a sleeve or conduit section. If you see loose cable hanging where it can be hit, ask an electrician to look.
Why builders use it
- Fast to install. Hot, neutral and ground travel together in one jacket.
- Easy to identify. Color-coded jackets on most brands help catch mismatches with breaker size.
- Flexible. It bends around framing without fittings.
- Widely accepted. It is a standard wiring method for dwellings under the NEC.
Its limits
- Dry locations only; no outdoor, underground or wet runs.
- No metal armor, so it can be damaged by nails, screws or impact.
- Ampacity capped at the 60°C value even though the wires are rated 90°C 2.
- Restricted in many commercial and taller buildings under local rules.
- Bundling many cables through one hole or insulation can force derating.
What drives the cost
NM-B is usually the lowest-cost wiring method for a house because it installs quickly. Cost rises with gauge, with three-wire versus two-wire cable, and with copper prices, which move often. Labor is the larger share in finished homes, where walls must be fished or opened.
We do not print cable or labor prices without a dated, sourced quote set. See electrical cost guides or the project cost estimator. Copper NM cable that stays dry, cool and unharmed can serve for decades; the weak points are the connections.
How NM-B is installed on a permitted job
An electrician installing NM-B on a permitted job follows six steps in this order. Knowing them helps you read a quote; they are not instructions for doing the work yourself.
- Plan circuits and sizes from the loads and the breaker sizes.
- Pull a permit where required.
- Drill framing and run cable, keeping it back from the face of studs or adding plates where screws could hit it.
- Secure cable with staples or straps at set intervals and near boxes.
- Pass the rough-in inspection before walls close.
- Make up devices and connections, then pass the final inspection.
Our guide to electrical permits covers when inspections happen.
Care over the years
NM-B in a wall needs no maintenance. Damage usually comes from people: a screw through a wall, a cut during remodeling, rodents in an attic, or heat from recessed lights or flues.
Before hanging shelves or drilling, avoid the areas straight above and below outlets and switches. If a screw hits something and a breaker trips, leave it and call an electrician. A home electrical inspection is the right check for an older system.
When NM-B is a hazard
Safety and risk
Cable with a cut jacket, scorch marks, chewed insulation, or a run outdoors or in a damp crawlspace without protection is a hazard. So is cable on a breaker larger than its size allows.
Do not touch damaged cable. Turn off the circuit at the breaker if it is safe to reach, and call a licensed electrician. For smoke or fire, leave the home and call 911.
Warning signs to watch for
- Breaker trips after drilling or nailing into a wall.
- Warm or discolored outlet covers on a circuit.
- Cable stapled so tightly the jacket is crushed.
- Older cloth-jacketed NM or aluminum NM in a house from 1965 to the mid-1970s 4.
- Cable with no ground in older homes, often feeding three-prong outlets.
Code and listing
Nonmetallic sheathed cable is covered by Article 334 of NFPA 70, the National Electrical Code 5. The 2023 edition, like earlier ones, sets where types NM and NMC may be used and requires ampacity per the 60°C rating (Section 334.80). The cable itself is tested to UL 719 2.
Local adoption differs. Each state or city adopts an NEC edition and may limit NM cable further 6. The 2026 edition is published, but many places still enforce 2020 or 2023. NFPA lets anyone read the code free online 7.
NM-B compared with other wiring methods
| Method | What it is | Wet locations | Typical home use |
|---|---|---|---|
| NM-B (Romex) | Plastic-jacketed cable | No | Interior walls and ceilings |
| UF-B | Solid plastic underground feeder cable | Yes, including direct burial | Outdoor and buried runs |
| THHN/THWN-2 in conduit | Single wires inside a raceway | THWN-2 yes, in conduit | Garages, exposed runs, outdoors |
| EMT conduit | Thin steel tubing that holds wires | With rated fittings | Exposed and commercial work |
| Armored cable (AC/MC) | Wires inside metal armor | MC with jacket | Exposed work, some cities' homes |
Myths about Romex
- Myth Romex is a type of wire.
- Reality It is a brand. The type is NM-B, made by several companies.
- Myth 90°C wire means a higher amp rating.
- Reality NM-B is limited to its 60°C ampacity 2.
- Myth NM-B is fine in conduit outdoors.
- Reality Outdoor conduit is a wet location. NM-B is for dry locations.
- Myth Jacket color is a code requirement.
- Reality Color coding is a manufacturer convention. The printed size governs.
- Myth The bare wire is a neutral.
- Reality It is the equipment ground. The white wire is the neutral.
Local rules on NM cable
Where NM-B may be used is set by the NEC edition your state or city enforces, plus any local amendments. A few large cities require metal raceway or armored cable even in homes. NFPA's adoption maps show which edition applies where 6.
When NM-B matters to you
NM-B becomes your concern when you plan a basement finish, an addition, a garage circuit or any outdoor run. The quote should say which cable or method goes where, and outdoor or damp sections should not be NM-B.
Next, list the new circuits you want, check panel space with the panel space tool, and ask an electrician to price the job with permit.
- If Dry interior wall or ceiling in a house
- Then NM-B is typical
- If Outdoors, underground or damp
- Then UF-B or THWN-2 in conduit
- If Exposed where it could be hit
- Then Protect it or use conduit
- If Commercial or multi-story building
- Then Check local rules first
Source: [2]
Questions about NM-B Cable (Romex)
Is Romex the same as NM-B?
Yes, in everyday use. Romex is Southwire's brand of nonmetallic sheathed cable, and the cable type is NM-B. Other companies make NM-B too. When an electrician or store says "Romex", they mean NM-B cable from any maker. The type name printed on the jacket is the one that matters for code.
What does 12-2 Romex mean?
12-2 Romex means 12 AWG wire with two insulated conductors, usually black and white, plus a bare ground if the label says "W/G". It is the standard cable for a 20-amp outlet circuit. A 12-3 cable adds a red conductor.
On most brands, 12 AWG NM-B has a yellow jacket, and 12-3 has a purple one. A 12-2 cable on a 15-amp breaker is allowed, but 14-2 on a 20-amp breaker is not.
Can NM-B cable be used outside?
No. NM-B is listed for normally dry locations. Outdoor runs, even inside conduit, are treated as wet locations. Electricians use UF-B cable or THWN-2 wires in conduit instead. The same goes for underground runs and damp crawlspaces.
If an existing outdoor run is gray or white NM-B stapled to siding, have an electrician look at it, because NM-B is listed for normally dry locations only.
Why is NM-B limited to 60°C ampacity?
The NEC limits NM cable to the 60°C ampacity because many device terminals and the cable jacket itself cannot safely run hotter. The 90°C rating of the wires is allowed only as a starting point when derating for bundling or heat.
So 12 AWG NM-B stays a 20-amp cable. In the manufacturer chart, 12 AWG copper reads 20 amps at 60°C and 30 amps at 90°C, and only the first number sets the breaker for NM-B.
What do the colors of Romex mean?
On most brands, color shows wire size: white for 14 AWG, yellow for 12 AWG, orange for 10 AWG and black for 8 AWG and larger. Some makers use other colors for three-wire cable, such as light blue for 14-3. Color is a convenience. The printed size on the jacket is the reliable mark.
Can Romex be run inside conduit?
Inside a dry building, NM cable may pass through short conduit sleeves for protection. Running it through long conduit is allowed in some cases but can force derating and is hard to pull.
Conduit outdoors or underground is a wet location, where NM-B is not allowed. If a quote shows NM-B inside a buried or exterior pipe, ask the electrician to switch that section to UF-B or THWN-2 wires.
How long does NM-B cable last?
There is no fixed lifespan. Copper NM-B that stays dry, cool and undamaged can last many decades. The parts that fail first are connections at outlets, switches and splices.
Older cable with cloth jackets or no ground may need replacement for other reasons. Signs worth a call are a brittle jacket that cracks when flexed, scorch marks near a box, and outlets that feel warm under normal load.
Is NM-B allowed in commercial buildings?
Sometimes, but with more limits. The NEC restricts NM cable by building type and construction, and some cities ban it in commercial work or require metal raceway.
A licensed electrician checks local rules before using it outside a home.
If you own a mixed-use building, such as a shop with an apartment above, expect the inspector to look closely at the wiring method in each part.
What is the difference between NM-B and UF-B?
NM-B has separate insulated wires inside a jacket and is for dry indoor use. UF-B has its wires molded into a solid plastic jacket and is rated for wet locations and direct burial.
UF-B is used for outdoor runs to sheds, lights and pumps. You can tell them apart on site: NM-B wires slide apart inside a loose jacket, while UF-B wires sit inside one solid, gray molded body.
Does old Romex need to be replaced?
Age alone is not a reason to replace it. Replacement is worth a look if the cable has no ground, has a cloth jacket that is cracking, is aluminum from 1965 to the mid-1970s, or shows heat damage.
An electrician can assess it during an inspection. Copper NM-B that is dry, grounded and undamaged is usually left in place.
Sources
- Southwire, Romex Brand SIMpull Copper Type NM-B Cable (SPEC 10028), retrieved .
- Cerrowire, CerroMax NM-B product page, retrieved .
- Cerrowire, Ampacity Charts (2017 NEC basis), retrieved .
- U.S. CPSC, Publication 516: Repairing Aluminum Wiring (June 2011), retrieved .
- NFPA, NFPA 70 (National Electrical Code) code development page, retrieved .
- NFPA, Where the NEC is enforced (adoption maps), retrieved .
- NFPA, Free access to NFPA codes and standards, retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Ratings, colors and permitted uses read from the manufacturer page; ampacity from the manufacturer chart; NEC article named by edition. No prices (G-COST).
Suggest a correction. We fix errors and say what changed.