Understanding the Intricacies of Nonmetallic-Sheathed Cables

What are Nonmetallic-Sheathed Cables?

Nonmetallic-sheathed cables, also known as NM cables or Romex cables, are types of electrical wiring used primarily in North America for indoor residential and light commercial wiring. They consist of two or more insulated conductors wrapped in an outer nonmetallic sheath.

The nonmetallic sheath is made of plastic materials like PVC or crosslinked polyethylene. This provides protection and insulation for the inner conductor wires.

Nonmetallic-sheathed cables provide many advantages over traditional metal-armored cables:

Composition of Nonmetallic-Sheathed Cables

Nonmetallic-sheathed cables consist of the following components:

Inner Conductors


Outer Sheath

The combination of conductors, insulation and outer sheath provide the complete nonmetallic cable assembly.

Types of Nonmetallic-Sheathed Cables

There are several types of NM cables used for different purposes:

NM (Nonmetallic)

NMC (Nonmetallic Corrosion Resistant)

NMS (Nonmetallic Service)

NMB (Nonmetallic Building)

So in summary, there are NM cables designed for various indoor and outdoor applications depending on the service environment.

Installation Guidelines

There are certain guidelines to follow when installing nonmetallic-sheathed cables:

Properly installed nonmetallic cables should last the lifetime of the building while providing safe and reliable power delivery.

Advantages of Nonmetallic-Sheathed Cables

Some notable benefits of using NM cables:

In summary, nonmetallic-sheathed cables combine safety, performance and cost-efficiency for residential and light commercial wiring applications. Their flexibility and ease of installation make them the preferred choice over traditional armored cables.

Limitations of Nonmetallic Cables

While NM cables provide many benefits, some limitations include:

Nonmetallic cables must be used appropriately by following all electrical codes and installation best practices. Qualified electricians can ensure safety and reliability.

Recent Code Changes for NM Cables

The National Electrical Code (NEC) includes updates and new rules for NM cables in recent editions:

Updated NEC rules improve safety by reducing fire risks from improper use or installation of nonmetallic cables. Electricians must comply with all code changes.


In conclusion, nonmetallic-sheathed cables provide a flexible, cost-effective wiring method for indoor residential and commercial buildings. Their non-conductive jacket makes installation safer. While subject to certain limitations, NM cables continue improving through NEC code enhancements. Following proper installation guidelines ensures they deliver safe and reliable electrical power.