When planning an electrical project, whether it’s a home renovation, a new construction, or an upgrade, choosing the correct type of cable is crucial for safety, compliance, and performance. Two common terms you’ll encounter in residential and commercial wiring are NM cable and NM-B cable. While they look similar, understanding their differences is key to a successful and code-compliant installation. This guide will break down what NM and NM-B wires are, compare their similarities and differences, and help you select the perfect cable for your needs.
What is NM Wire?
NM stands for Non-Metallic. It is a type of electrical cable featuring two or more insulated conductors (typically THHN wires) and a bare ground wire, all wrapped in a flexible plastic (PVC) jacket. Introduced in the early 20th century, traditional NM cable was the standard for decades in dry, indoor locations. Its design offered a cost-effective and easier-to-install alternative to metallic conduit systems. However, older NM cable had limitations, particularly in its heat resistance.
What is NM-B Wire?
NM-B is the modern, upgraded version. The “B” stands for a higher temperature rating. NM-B cable is constructed with improved insulation materials (typically rated at 90°C for conductors) and a tougher, flame-retardant PVC outer jacket. This evolution was driven by updated electrical codes (like the NEC in the U.S.) demanding higher safety standards. NM-B is now the standard and most commonly used type of non-metallic sheathed cable for new installations. When people refer to “Romex®” (a popular brand name) today, they are almost always referring to NM-B cable.
Similarities Between NM and NM-B Wire
Before diving into the differences, it’s important to know what they share:
- Basic Construction: Both consist of insulated copper conductors and a bare ground wire within a non-metallic sheath.
- Primary Use: Designed for branch circuit wiring in residential and commercial buildings for outlets, switches, and lighting.
- Standard Sizes: Available in common gauges like 14, 12, and 10 AWG for 15-amp, 20-amp, and 30-amp circuits, respectively.
- Installation Ease: Both are simpler and faster to install compared to wiring in conduit.
Key Differences Between NM and NM-B Wire
The main distinctions lie in material improvements and safety ratings. The table below summarizes the critical differences:
|
Feature
|
Traditional NM Cable
|
Modern NM-B Cable
|
|
|---|---|---|---|
|
Temperature Rating
|
typically 60°C
|
90°C
|
|
|
Insulation & Jacket
|
Standard PVC insulation and jacket.
|
Improved, flame-retardant PVC. More resistant to heat and moisture.
|
|
|
Usage Environment
|
Dry locations only.
|
Primarily dry, indoor locations. The “B” upgrade offers a slight safety buffer but is not for damp/wet areas.
|
|
|
Voltage Rating
|
600 Volts (same as NM-B).
|
600 Volts.
|
|
|
Code Compliance
|
Obsolete. Not permitted in new installations.
|
Required for all new construction and renovations per modern electrical codes (NEC).
|
|
|
Cost
|
Historically cheaper, now largely unavailable.
|
Slightly higher due to enhanced materials.
|
How to Choose: NM vs. NM-B?
The choice is straightforward: Always choose NM-B cable for any new electrical project.
- For New Construction or Renovation: NM-B is the only compliant and safe option. It meets or exceeds all current National Electrical Code (NEC) requirements.
- If You Find Old NM Cable: Do not purchase or install it. It is an outdated product. If encountered during a repair, it is best to replace it with NM-B, especially if the circuit is being modified or extended.
- For Specific Applications: Remember that neither NM nor NM-B is suitable for damp locations (like basements or exteriors), direct burial, or areas with high physical damage risk. In these cases, you would need other cables like UF-B, THWN/THHN in conduit, or MC cable.
Your Reliable Partner for Premium Wiring Solutions
At JZD Cable, we understand that quality and compliance are non-negotiable in electrical work. That’s why our NM-B cables are manufactured to the highest industry standards, ensuring superior safety, durability, and performance for your residential and commercial projects. We offer a full range of gauges and configurations to meet your specific needs.
