In electrical wiring systems, safety and reliability always depend on choosing the right type of cable. One of the most critical parameters that determines how a wire performs is its temperature rating. Selecting a cable without considering its temperature capacity can lead to overheating, reduced lifespan, or even electrical fires.
Among the many wire types available, NM-B Cable has become one of the most widely used solutions in residential and light commercial projects. But what exactly is the temperature rating of NM-B, and how does it affect its applications? In this article, we will explore the temperature specifications of NM-B, its structure, applications, and compare it with other popular wire types such as THHN, UF-B, and XHHW. Finally, we will provide a guide on how to select the right wire for your project, ensuring both safety and efficiency.
What is NM-B Cable?
NM-B stands for Non-Metallic Sheathed Cable, type B. It is a standard cable widely used in residential wiring applications in North America. Its main functions include:
- Providing power distribution for lighting circuits
- Connecting household receptacles
- Powering residential appliances such as air conditioners, ovens, or heaters
NM-B is UL certified and complies with the National Electrical Code (NEC), making it a trusted choice for electricians and contractors. Unlike other specialty wires, NM-B is primarily designed for indoor dry locations, which makes its temperature rating an essential factor.
Structure and Materials of NM-B Cable
NM-B cable is composed of three key layers:
1.Copper Conductors – Solid or stranded copper wires that carry electrical current.
2.PVC (Polyvinyl Chloride) Insulation – Provides electrical insulation and temperature resistance.
3.Thermoplastic Sheath – A non-metallic protective outer jacket that shields the conductors from mechanical damage.
Each of these materials plays a role in determining the cable's temperature capacity. The PVC insulation can handle high temperatures, while the sheath provides extra protection in typical household environments.
Temperature Rating of NM-B Cable
The temperature rating of NM-B cable is often misunderstood. Let's clarify:
- Conductor Rating: The copper conductors inside NM-B are rated for 90°C (194°F).
- Code Limitation: According to NEC requirements, NM-B cables must be used at 60°C (140°F) when connected to devices such as receptacles, switches, or circuit breakers.
👉 This means while the cable itself can withstand higher heat, its actual usage is limited by terminal connection requirements.
For example:
- A 12 AWG NM-B cable may technically carry more current at 90°C, but NEC requires it to be sized according to its 60°C ampacity, which is 20 amps.
Why is Temperature Rating Important?
Choosing the wrong cable temperature rating can cause:
- Overheating → leading to insulation breakdown
- Shortened lifespan of the wiring system
- Fire hazards if current exceeds safe limits
Example: In a kitchen where appliances generate extra heat, using a cable with insufficient temperature tolerance can cause premature failure. Similarly, in attic installations where ambient temperatures are higher, the derating of cables becomes essential.
Specifications of NM-B Cable and Their Applications
NM-B is available in multiple gauges and configurations. Below is a table of common sizes, their ampacity, and applications (based on NEC 60°C rating):
| Wire Gauge | No. of Conductors | Ampacity (60°C) | Common Application |
|---|---|---|---|
| 14/2 NM-B | 2 conductors + ground | 15A | Lighting circuits |
| 12/2 NM-B | 2 conductors + ground | 20A | Receptacles, small appliances |
| 10/2 NM-B | 2 conductors + ground | 30A | Water heaters, dryers |
| 8/3 NM-B | 3 conductors + ground | 40A | Electric ranges |
| 6/3 NM-B | 3 conductors + ground | 55A | HVAC systems, heavy appliances |
These specifications show how temperature rating + wire gauge together determine the safe application of NM-B cable.
NM-B vs Other Wire Types: Temperature Comparison
While NM-B is common in homes, other wires are often used when higher temperature or harsher environments are involved:
| Wire Type | Temperature Rating | Environment | Key Advantage |
|---|---|---|---|
| NM-B | 90°C conductor, limited to 60°C usage | Dry indoor | Cost-effective for residential wiring |
| THHN | 90°C dry, 75°C wet | Conduit, industrial | Higher temperature & chemical resistance |
| UF-B | 90°C, direct burial rated | Outdoor, underground | Moisture & sunlight resistant |
| XHHW | 90°C dry/wet | Industrial, high-load | Excellent heat & abrasion resistance |
👉 From this comparison, NM-B is ideal for indoor residential use, while THHN, UF-B, and XHHW are more versatile in demanding conditions.
What Temperature is NM-B?
To summarize:
- NM-B Cable conductors are rated at 90°C (194°F).
- NEC requires it to be used at 60°C (140°F) for most terminations.
- Applications must follow the lower rating to ensure safety.
How to choose by scenario:
- Indoor residential (dry) → NM-B is the best choice.
- High heat areas / Industrial → THHN or XHHW is recommended.
- Outdoor / Underground → UF-B is the correct option.
Choose Greater Wire for NM-B and more
When sourcing cables for international projects, you need a manufacturer that offers quality, certification, and reliable service.
✅ Complete Product Range – NM-B, THHN, UF-B, XHHW, and more UL-listed cables.
✅ Certified Quality – UL & cUL approved, compliant with NEC and CSA standards.
✅ Flexible Supply – Full stock in 14 AWG to 6 AWG NM-B, available for bulk or small orders.
✅ Global Expertise – Over 30 years of manufacturing and export experience, serving customers across North America, Europe, and Asia.
✅ Dedicated Support – Professional engineers provide technical guidance; our sales team ensures smooth communication from inquiry to delivery.
Contact Us Today
If you are planning your next electrical project and need reliable NM-B or other UL-certified cables, we are here to support you.
Dongguan Greater Wire & Cable Co., Ltd.
📞 Tel/WhatsApp/Wechat: +86 135 1078 4550 / +86 136 6257 9592
📧 Email: manager01@greaterwire.com
👉 No matter the project size, we provide comprehensive solutions, professional technical support, and excellent after-sales service. Partner with us and ensure your wiring system is built for safety and long-term reliability.