When it comes to residential and light commercial electrical wiring, choosing the right cable is not only about performance-it is about safety, compliance, and long-term reliability. Among the many wire types available in the U.S. and Canadian markets, NM-B wire (often referred to as "Romex") is one of the most widely used options indoors.
However, a common question arises among contractors, engineers, and homeowners: Can NM-B wire be used outside?
We will take a detailed look at what NM-B wire is, its applications, how it compares with other cable types, the sizes available, specific scenarios for each size, and whether NM-B can truly be applied in outdoor environments. Finally, we will highlight why Dongguan Greater Wire & Cable Co., Ltd., as a UL & cUL certified manufacturer, is your trusted partner for NM-B and other electrical cable solutions.
1. What is NM-B Wire?
NM-B stands for Non-Metallic Sheathed Cable, Type B. It is specifically designed for indoor dry location wiring in residential and light commercial projects.
Construction of NM-B Wire:
- Conductors: Made of solid or stranded copper, depending on size
- Insulation: PVC insulation for each conductor
- Ground Wire: Bare copper ground included
- Outer Jacket: Non-metallic PVC sheathing
The "B" designation indicates that the insulation is rated for up to 90°C (194°F) in dry conditions. This heat rating allows NM-B to handle a wide range of branch circuits safely.
⚠️ Important: The outer sheath of NM-B is not moisture-resistant and not sunlight-resistant, which is why its application is strictly limited to indoor spaces.
2. Where is NM-B Wire Commonly Used?
NM-B is one of the most popular wires in residential construction across the United States. It is lightweight, flexible, and easy to install, which makes it a cost-effective choice for contractors.
Common Applications:
- Indoor branch circuits for outlets and receptacles
- Lighting circuits in bedrooms, living rooms, and hallways
- Small appliances such as microwaves, refrigerators, or dishwashers
- Interior utility rooms (as long as conditions remain dry)
- Farm or workshop interiors where moisture exposure is controlled
Electricians choose NM-B for its ease of installation, UL compliance, and affordability, making it the standard solution for most indoor wiring needs.
3. How Does NM-B Differ from Other Wire Types?
To understand where NM-B fits in the market, it's important to compare it with similar cables such as UF-B, THWN, and XHHW.
| Wire Type | Key Features | Suitable Environment | Outdoor Use |
|---|---|---|---|
| NM-B | Non-metallic sheath, 90°C dry rating, easy to strip | Indoor dry locations only | ❌ Not allowed |
| UF-B | Solid thermoplastic sheath, moisture resistant | Direct burial, wet areas, outdoor lighting | ✅ Yes |
| THWN | Single conductor, moisture/heat resistant, conduit installation | Wet/damp/outdoor conduit | ✅ Yes |
| XHHW | Cross-linked polyethylene insulation, durable, chemical resistant | Industrial wet/dry areas | ✅ Yes |
👉 Conclusion: NM-B is designed only for indoor wiring, while UF-B, THWN, and XHHW are engineered to handle outdoor or wet conditions.
4. Available Sizes of NM-B Wire
NM-B wire is manufactured in a range of gauges to meet different amperage requirements. Below are the common sizes available in the U.S. and Canadian markets:
| NM-B Size | Description | Conductors | Typical Ampacity | Notes |
|---|---|---|---|---|
| 14/2 NM-B with ground | 14 AWG, 2 conductors + ground | Black, White, Bare Ground | 15 amps | Standard lighting, receptacles |
| 12/2 NM-B with ground | 12 AWG, 2 conductors + ground | Black, White, Bare Ground | 20 amps | Kitchens, bathrooms, outlets |
| 12/3 NM-B with ground | 12 AWG, 3 conductors + ground | Black, Red, White, Bare Ground | 20 amps | Multi-way switches, split circuits |
| 10/2 NM-B with ground | 10 AWG, 2 conductors + ground | Black, White, Bare Ground | 30 amps | Water heaters, AC units |
| 10/3 NM-B with ground | 10 AWG, 3 conductors + ground | Black, Red, White, Bare Ground | 30 amps | Dryers with 120/240V |
| 8/3 NM-B with ground | 8 AWG, 3 conductors + ground | Black, Red, White, Bare Ground | 40 amps | Ovens, electric stoves |
| 6/3 NM-B with ground | 6 AWG, 3 conductors + ground | Black, Red, White, Bare Ground | 55 amps | Indoor sub-panels, heavy appliances |
| 4/3 NM-B with ground | 4 AWG, 3 conductors + ground | Black, Red, White, Bare Ground | 70 amps | Large sub-panels, heavy-duty systems |
5. Application Scenarios for Different NM-B Sizes
| NM-B Size | Ampacity | Typical Applications | Example Scenarios |
|---|---|---|---|
| 14/2 NM-B | 15 amps | Lighting circuits, standard outlets | Powering bedroom outlets, ceiling fans |
| 12/2 NM-B | 20 amps | Kitchen/bathroom receptacles, appliances | Wiring outlets for coffee machines or toasters |
| 12/3 NM-B | 20 amps | Multi-way switch circuits, split receptacles | Controlling hallway lights from 3 switches |
| 10/2 NM-B | 30 amps | Water heaters, small AC units | Dedicated water heater circuit |
| 10/3 NM-B | 30 amps | Dryers (120/240V) | Laundry dryer outlet requiring neutral |
| 8/3 NM-B | 40 amps | Kitchen ranges, ovens | Double oven or large electric stove |
| 6/3 NM-B | 55 amps | Indoor sub-panels, heavy appliances | Garage sub-panel wiring |
| 4/3 NM-B | 70 amps | Large sub-panels, high-load equipment | Basement workshop powering multiple tools |
6. Can NM-B Wire Be Used Outside?
Returning to our original question-Can NM-B wire be used outside?
The answer is NO. According to the National Electrical Code (NEC), NM-B is not permitted in outdoor, damp, or wet environments. The reasons are:
- Its sheath is not sunlight-resistant → UV rays degrade insulation
- It is not water-resistant → moisture leads to corrosion and hazards
- It is designed strictly for dry, protected indoor conditions
Outdoor Alternatives:
- Use UF-B cable for direct burial or outdoor lighting circuits
- Use THWN or XHHW conductors inside conduit for wet/outdoor applications
By following these standards, you ensure both safety and compliance with NEC codes.
7. The best choice for buying NM-B wire--Dongguan Grete Wire and Cable Co., Ltd.
When selecting electrical wires, compliance and quality certification are critical. That's why choosing a trusted manufacturer is essential for contractors, wholesalers, and distributors.
Why Choose Us?
Dongguan Greater Wire & Cable Co., Ltd. is a professional manufacturer of NM-B cables with both UL and cUL certification, serving the U.S. and Canadian markets for decades.
✅ Certifications: UL & cUL approved, ensuring compliance with North American standards
✅ Product Range: Full NM-B series (14/2 up to 4/3 with ground)
✅ Traceability: Printed identification and UL file number on every cable
✅ Quality Assurance: 100% copper conductors, durable PVC sheathing
✅ Global Supply: Long-term partnerships with contractors, distributors, and retailers
Our Advantage
- Over 30 years of production experience
- Reliable bulk supply at competitive pricing
- Strict quality control for consistent performance
- Customized solutions to meet different project needs
📩 Contact Us Today:
Tel / WhatsApp / WeChat: +86 135 1078 4550 / +86 136 6257 9592
Email: manager01@greaterwire.com
Whether you are a wholesaler, contractor, or project manager, Dongguan Greater Wire & Cable Co., Ltd. is your trusted partner in safe and certified electrical wiring solutions.
👉 Ensure safety, ensure compliance, ensure reliability-choose Greater Wire & Cable for your next NM-B cable solution