If you're tackling a residential wiring project or trying to choose the right cable, you may have come across the terms NM and NM-B. Though they look similar, there's an important difference between these two types of cables — especially in terms of temperature rating and code compliance.
What Is the Difference Between NM and NM-B Wire
In this article, we’ll break down what NM and NM-B mean, how they differ, and which one you should use.
What Is NM Cable?
NM stands for Non-Metallic cable, also commonly called Romex® (a brand name). It is a type of electrical cable used for interior residential wiring. NM cables typically contain:
One or more insulated conductors
A bare ground wire
A non-metallic PVC jacket for protection
Before 1985, NM cable came with a temperature rating of 60°C (140°F), which was adequate at the time but no longer meets modern code for most applications.
What Is NM-B Cable?
NM-B stands for Non-Metallic – Type B, the current standard for residential non-metallic sheathed cable in the U.S. The “B” designation refers to the upgraded temperature rating of the cable.
Key features:
Rated for 90°C (194°F) in dry locations
Meets modern NEC (National Electrical Code) requirements
Available in common sizes like 14/2, 12/2, 10/2, etc.
Today, NM-B has replaced NM in almost all standard applications.
NM vs NM-B: What’s the Real Difference?
| Feature | NM Cable | NM-B Cable |
|---|---|---|
| Temperature Rating | 60°C (140°F) | 90°C (194°F) |
| NEC Compliance | Outdated (pre-1985) | Fully compliant with modern NEC |
| Availability | Obsolete | Industry standard |
| Jacket Material | PVC | Improved PVC with color coding |
| Common Use Today | No longer recommended | Widely used in residential wiring |
Why NM-B Is the Current Standard
Because of its higher temperature rating, NM-B is safer and more versatile. It can handle higher current loads in small-gauge wires without overheating and is required by NEC for most dry-location residential installations.
🔧 Applications:
Interior branch circuits
Lighting and receptacle circuits
Residential and light commercial buildings
⚠️ Note: NM-B is only suitable for dry, indoor locations. It is not rated for outdoor or wet environments — for that, use UF-B (Underground Feeder) or other appropriate cables.
Common Questions
Q: Is NM cable still used today?
A: No, NM cable with 60°C rating is now obsolete. All new residential installations use NM-B.
Q: What does the “B” stand for in NM-B?
A: It signifies an upgraded thermal rating and improved jacket, per NEC code updates.
Q: Can NM-B be used in wet areas or outdoors?
A: No. NM-B is for dry, indoor use only.
Summary: Which One Should You Use?
If you're wiring a home or updating an old system, NM-B is the correct and code-compliant choice. NM cable is outdated and no longer meets safety standards.
At TOT Wire & Cable, we stock a full range of NM-B cables including 14/2, 12/2, 10/2 and more — all compliant with UL and NEC standards.
2 comments
Paul 08/02/2025
Thanks for the information! I live in a home built in 1961 and would like to replace some light fixtures, but I'm not sure it is safe to do so, given that the wiring is NM, not NM-B. To further complicate matters I can no longer get standard incandescent bulbs, so need to use LED bulbs. I have been told that replacing my existing fixtures with modern fixtures and using LWD bulbs could lead to overheating and risk of a fire. HELP, PLEASE!!
Totcables 08/04/2025
This is an excellent question and a very common concern for homeowners with older homes. The short answer is that, in most cases, replacing your incandescent fixtures with modern ones and using LED bulbs is not only safe but actually **safer** from a fire risk perspective, provided you follow some key guidelines. Here's a breakdown of the issues you've raised and the reasoning behind it: ### NM vs. NM-B Wiring and Overheating * **The Difference:** The core difference between NM (Non-Metallic) wiring from 1961 and modern NM-B wiring is the temperature rating of the insulation. Older NM wiring typically has a temperature rating of $60^\circ C$ ($140^\circ F$), while modern NM-B wiring is rated for $90^\circ C$ ($194^\circ F$). * **The Overheating Risk with Incandescents:** The overheating risk you've heard about is very real, but it's specifically tied to **incandescent bulbs**. Incandescent bulbs are extremely inefficient, converting most of their energy into heat rather than light. A high-wattage incandescent bulb can generate enough heat to cause the insulation on older NM wiring to become brittle and degrade over time, leading to a fire hazard. This is why many modern fixtures have a warning to use wiring rated for $90^\circ C$. * **Why LEDs Solve This:** LED bulbs are a game-changer. They are incredibly efficient and produce a fraction of the heat of incandescent bulbs. When you replace a 100-watt incandescent bulb with a 15-watt LED bulb that provides the same amount of light, you are dramatically reducing the amount of heat generated inside the fixture and the junction box. This significantly reduces the risk of overheating the old NM wiring. ### The Problem with "Modern Fixtures" and LEDs The concern about a "modern fixture" and "LWD bulbs" (presumably you mean LED bulbs) causing a problem is a bit of a misunderstanding. The risk isn't from the modern fixture itself, but from the fact that a new fixture is designed for modern wiring standards. Here's what you need to be aware of: * **Code Compliance:** While using an LED bulb in a modern fixture with your old wiring is technically safer in practice, it may not be in strict compliance with current electrical codes. The concern is that a future homeowner might come along and put a high-wattage incandescent bulb in the fixture, which could then overheat the older wiring. * **The Correct Path:** The safest and most by-the-book approach is to use fixtures that are specifically rated for $60^\circ C$ wiring, or to use LED-specific fixtures that don't have an Edison base socket (the standard screw-in socket) that would allow an incandescent bulb to be installed. However, many people safely and successfully install modern fixtures with LED bulbs in older homes, knowing they will only ever use LEDs. ### Your Best Course of Action 1. **Use LED Bulbs:** The switch to LED bulbs is a fantastic and safe choice. They are far more efficient, last much longer, and, most importantly for you, generate very little heat. 2. **Choose Your Fixtures Wisely:** When you replace your fixtures, look for ones that are well-ventilated. Using LED bulbs is key, as they are not a heat risk. If you are concerned about code compliance, you can look for fixtures that are designed specifically for LED lights, which often have integrated LED chips and no traditional bulb socket. 3. **Inspect Your Wiring:** Before installing any new fixture, it's a good idea to inspect the visible wiring in the junction box. If you see any signs of degradation, such as cracked, brittle, or discolored insulation, it's time to have a licensed electrician replace the wiring. 4. **Consult a Professional:** For peace of mind and to ensure everything is done correctly, it's always a good idea to consult a licensed electrician. They can assess your specific situation, confirm the condition of your wiring, and ensure your new fixtures are installed safely and to code. In summary, the fear of fire comes from the heat generated by old incandescent bulbs on old wiring. Since you are moving to LED bulbs, which generate minimal heat, you are actually **mitigating** that risk. The key is to be aware of the "what if" a future user puts a hot incandescent bulb in the fixture and to address that either by using a dedicated LED fixture or simply being confident you will only use LED bulbs.