NM-B stands for 'Non-Metallic sheathed cable, type B,' where the 'B' indicates the cable is rated for 90°C (194°F) in dry locations. This designation identifies the most common electrical cable used in residential wiring—what most people call 'Romex' (a brand name that has become synonymous with NM-B cable).
what does the b stand for in nm-b wire
If you've ever looked at electrical wire and seen 'NM-B' printed on the jacket alongside size markings like '14/2' or '12/2,' you're looking at the standard cable that powers outlets, lights, and appliances throughout millions of homes. In this guide, we'll explain exactly what NM-B means, how it differs from older NM cable, and when it should (and shouldn't) be used.
What Does NM-B Wire Mean?
Let's break down the NM-B meaning piece by piece:
NM-B Wire Meaning Explained
| Component | What It Means |
|---|---|
| NM | Non-Metallic sheathed cable |
| -B | Type B designation (90°C rated) |
| Non-Metallic | Outer jacket is plastic (PVC), not metal conduit |
| Sheathed | Individual wires wrapped together in a protective outer jacket |
| Type B | Modern standard with higher temperature rating |
What Does 'B' Stand For in NM-B Wire?
The 'B' in NM-B stands for the second generation of non-metallic cable with improved temperature ratings. Here's the evolution:
| Type | Temperature Rating | Status |
|---|---|---|
| NM (original) | 60°C (140°F) | Obsolete, replaced in 1984 |
| NM-B | 90°C (194°F) | Current standard since 1984 |
| NM-C | Does not exist | Not a real designation |
Important: While NM-B is rated for 90°C conductor temperature, the ampacity (current-carrying capacity) is still based on the 60°C column of the NEC ampacity tables for most applications. The higher temperature rating provides a safety margin.
NM-B vs Romex: What's the Difference?
Many people ask about NM-B vs Romex—but they're actually the same thing.
Understanding the Terms
- NM-B = The official electrical code designation (what electricians and inspectors call it)
- Romex = A brand name owned by Southwire that has become the common term (like 'Kleenex' for tissues)
All Romex is NM-B cable, but not all NM-B cable is technically 'Romex' brand.
When you go to a hardware store and ask for 'Romex,' you'll get NM-B cable. When reading electrical codes or inspection reports, you'll see it called 'NM-B' or 'Type NM-B.'
What 'Romex' Actually Means
'Romex' is simply a trade name that stuck. Other manufacturers make identical NM-B cable under different brand names:
- Southwire's Romex
- Cerrowire's NM-B
- United Copper's NM-B
- And many others
They all meet the same NEC standards for Type NM-B cable.
What Is NM-B Wire Used For?
Type NM wire is the standard cable for residential electrical installations. Here's where it's commonly used:
Common NM-B Wire Applications
| Application | Typical Wire Size | Details |
|---|---|---|
| Lighting circuits | 14/2 or 14/3 NM-B | 15-amp circuits |
| General outlets | 14/2 or 12/2 NM-B | 15 or 20-amp circuits |
| Kitchen outlets | 12/2 NM-B | 20-amp small appliance circuits |
| Bathroom outlets | 12/2 NM-B | 20-amp circuits |
| 240V appliances | 10/2 or 8/3 NM-B | Dryers, ranges, water heaters |
| Air conditioner | 10/2 or 12/2 NM-B | Based on unit requirements |
| Three-way switches | 14/3 or 12/3 NM-B | Requires 3-wire cable |
Where NM-B Cable Can Be Used
✅ Approved locations:
- Inside walls, ceilings, and floors
- Exposed in dry locations (basements, attics)
- One and two-family dwellings
- Multi-family dwellings (with restrictions)
- Apartments and condos (interior only)
❌ Not permitted:
- Exposed to moisture or wet locations
- Underground (direct burial)
- Embedded in concrete
- Exposed outdoors
- In corrosive environments
- As service entrance cable
- In commercial/industrial buildings (with exceptions)
- Exposed in garages (must be protected above 8 feet)
Understanding NM-B Wire Size Markings
When you look at NM-B cable, you'll see markings like '14/2,' '12/2,' or '10/3.' Here's what they mean:
Decoding Wire Size Numbers
Format: [Gauge] / [Number of Conductors]
| Marking | Wire Gauge | Number of Conductors | What It Contains |
|---|---|---|---|
| 14/2 | 14 AWG | 2 current-carrying | 1 hot (black), 1 neutral (white), 1 ground (bare) |
| 14/3 | 14 AWG | 3 current-carrying | 1 hot (black), 1 hot (red), 1 neutral (white), 1 ground (bare) |
| 12/2 | 12 AWG | 2 current-carrying | 1 hot (black), 1 neutral (white), 1 ground (bare) |
| 12/3 | 12 AWG | 3 current-carrying | 1 hot (black), 1 hot (red), 1 neutral (white), 1 ground (bare) |
| 10/2 | 10 AWG | 2 current-carrying | 1 hot (black), 1 neutral (white), 1 ground (bare) |
Note: The ground wire is not counted in the '/2' or '/3' designation—it's always present as an extra bare or green wire.
NM-B Wire Ampacity Chart
The NM-B wire ampacity determines how much current the wire can safely carry. This is critical for proper circuit protection.
Standard NM-B Ampacity Ratings
| Wire Size | Maximum Ampacity | Circuit Breaker Size | Common Use |
|---|---|---|---|
| 14 AWG | 15 amps | 15-amp breaker | Lighting, general outlets |
| 12 AWG | 20 amps | 20-amp breaker | Kitchen, bathroom, general outlets |
| 10 AWG | 30 amps | 30-amp breaker | Water heater, window AC, dryer (partial) |
| 8 AWG | 40 amps | 40-amp breaker | Electric range, large AC units |
| 6 AWG | 55 amps | 50-amp breaker | Electric range, large appliances |
Important Ampacity Notes
Temperature derating: When multiple NM-B cables are bundled together or pass through insulation, ampacity must be reduced. Consult NEC Article 310 for specific derating factors.
Voltage drop: For long runs (over 50-100 feet), consider using larger wire to minimize voltage drop, even if ampacity is adequate.
Always use proper breaker size: The breaker must match the wire size, never exceed it:
- 14 AWG → Maximum 15-amp breaker
- 12 AWG → Maximum 20-amp breaker
- 10 AWG → Maximum 30-amp breaker
NM vs NM-B: What Changed?
Understanding NM vs NM-B helps explain why the 'B' designation was added.
Evolution of Non-Metallic Cable
| Feature | Original NM | Modern NM-B |
|---|---|---|
| Introduced | 1920s | 1984 |
| Conductor insulation | 60°C rated | 90°C rated |
| Jacket material | Paper/fabric early, then PVC | Modern PVC compound |
| Conductor material | Copper | Copper (aluminum available in some sizes) |
| Ampacity | Based on 60°C | Rated for 90°C, used at 60°C ampacity |
| Code status | Obsolete | Current standard |
| Color coding | Varies | Standardized (black=hot, white=neutral, red=second hot) |
Why the Change to NM-B?
The upgrade to NM-B in 1984 provided:
- Better heat resistance: Safer in bundled applications
- Improved safety margin: More tolerance for temperature rise
- Modern insulation materials: Better durability and flexibility
- Standardization: Consistent specifications across manufacturers
NM vs NMC Cable
Another common question: NM vs NMC cable—what's the difference?
NM-B vs NM-C Cable Comparison
| Feature | NM-B | NM-C |
|---|---|---|
| Full name | Non-Metallic sheathed cable, Type B | Non-Metallic sheathed cable, Type C (corrosion-resistant) |
| Jacket | Standard PVC | Reinforced, moisture-resistant |
| Approved locations | Dry locations only | Damp, moist, or corrosive locations |
| Cost | Standard | 30-50% more expensive |
| Common use | Interior residential wiring | Barns, garages, crawl spaces |
| Availability | Widely available | Less common |
When to use NM-C:
- Unfinished basements with potential moisture
- Attached garages
- Agricultural buildings
- Any location subject to dampness or corrosion
Most residential work uses NM-B because interior spaces are considered dry locations.
NM-B Installation Requirements
Proper installation of Type NM cable ensures safety and code compliance.
Securing and Supporting
| Requirement | Specification |
|---|---|
| Support spacing | Within 12' of boxes, every 4.5 feet along runs |
| Staple type | Insulated staples that don't damage cable |
| Bending radius | Minimum 5× cable diameter |
| Protection | Required where subject to physical damage |
Running NM-B Through Framing
- Holes in studs: 1-1/4' minimum from edge of stud
- Notches: Protect with 1/16' steel plate
- Multiple cables: Maintain spacing when possible to prevent overheating
Exposure and Protection
| Location | Requirements |
|---|---|
| Attics | Protected from damage, run along joists or through holes |
| Basements | Can be exposed on walls, must be supported properly |
| Garages | Protected above 8 feet from floor |
| Crawl spaces | Protected where accessible |
Common NM-B Wire Questions
Can I use NM-B wire outdoors?
No. NM-B is rated for dry locations only. For outdoor use:
- Use UF (Underground Feeder) cable
- Use individual THWN wires in conduit
- Use direct burial rated cable
Can NM-B wire be used in conduit?
Technically yes, but it's not ideal. NM-B can be run through conduit for physical protection, but:
- Individual THWN wires are preferred for conduit runs
- NM-B jacket doesn't slide easily through conduit
- Multiple NM-B cables in conduit require derating
What's the difference between with ground and w/ground?
They mean the same thing. Modern NM-B always includes a ground wire:
- 'With ground' or 'w/ground' on the jacket
- Always includes bare or green ground conductor
- Ground wire not counted in the /2 or /3 designation
Can I bury NM-B wire?
No. NM-B is not rated for burial. Use:
- UF (Underground Feeder) cable for direct burial
- Schedule 40 PVC conduit with THWN wire
- Metal conduit with appropriate wire
Safety Considerations
Identifying NM-B Cable Problems
| Warning Sign | What It Means | Action |
|---|---|---|
| Melted or burned insulation | Overheating, overloaded circuit | Replace immediately |
| Exposed conductors | Damaged jacket | Repair or replace |
| Water exposure | Cable compromised | Replace with appropriate cable type |
| Rodent damage | Chewed insulation | Replace affected sections |
| Old fabric-wrapped wire | Pre-NM-B cable | Consider upgrade during renovation |
When to Call an Electrician
DIY electrical work is legal in most areas for homeowners working on their own property, but consider professional help for:
- Service panel work
- 240V circuits
- Any work requiring permits
- Situations where you're unsure about code requirements
- Complex three-way or four-way switch circuits
Conclusion
NM-B stands for Non-Metallic sheathed cable, type B—the 'B' designation indicating the modern 90°C temperature-rated version that's been the residential wiring standard since 1984. Whether you call it NM-B, Romex, or 'that wire with the plastic jacket,' it's the backbone of home electrical systems across North America.
Key takeaways:
- NM-B means Non-Metallic sheathed cable, type B (90°C rated)
- The 'B' distinguishes it from the older 60°C rated NM cable
- 'Romex' is a brand name that's become synonymous with NM-B cable
- Use 14 AWG for 15-amp circuits, 12 AWG for 20-amp circuits
- Only use NM-B in dry locations—never outdoors or buried
- Always follow NEC and local codes for proper installation
- Wire size markings (14/2, 12/3) indicate gauge and number of current-carrying conductors
Understanding NM-B cable helps you make informed decisions about home wiring projects and communicate effectively with electricians and inspectors.
Have electrical wiring questions? Always consult a licensed electrician for work beyond your skill level and follow all local electrical codes.
