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:
- They are lightweight and flexible.
- Easy to install and maneuvre in tight spaces.
- The nonmetallic jacket resists moisture and corrosion.
- They don't require special terminations like metal-clad cables.
- Cost-effective for most indoor wiring jobs.
Composition of Nonmetallic-Sheathed Cables
Nonmetallic-sheathed cables consist of the following components:
Inner Conductors
- Usually made of copper wire strands.
- Available in different sizes like 14 AWG, 12 AWG, 10 AWG depending on the required ampacity.
- Insulated with PVC or crosslinked polyethylene.
- Single-conductor cables have one insulated wire.
- Multi-conductor cables bundle 2 or more insulated wires together.
Insulation
- Inner wires are insulated to prevent contact between conductors.
- PVC is flexible and resistant to moisture, oils and chemicals.
- Crosslinked polyethylene has higher heat resistance.
Outer Sheath
- Made of PVC or crosslinked polyethylene.
- Provides overall protection and prevents moisture ingress.
- Comes in different thicknesses.
- Colored black, white, gray etc.
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)
- Used for indoor branch circuit and feeder wiring.
- Rated up to 600V and temperature range -20°C to 90°C.
- Available with 2-4 conductors from 14 AWG to 6 AWG.
NMC (Nonmetallic Corrosion Resistant)
- Used for highly corrosive environments like commercial kitchens.
- Sheath resistant to oils and chemicals.
- Rated 600V.
NMS (Nonmetallic Service)
- Specifically for overhead service drop cable.
- Rated up to 600V.
- Resistant to sunlight exposure and outdoor environments.
NMB (Nonmetallic Building)
- For underground feeders between buildings.
- Heavy duty moisture and crush resistant sheath.
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:
- Cables must be properly supported every 4-1/2 feet and within 12 inches from boxes. Use cable ties or staples.
- Avoid running cables through spaces where they may be damaged. Use nail shields when securing cables.
- Cables running through holes must be protected with grommets or clamps.
- Do not run cables below thermal insulation like on attic floors.
- Cables must be separated from sources of high heat like chimneys, lights etc.
- Usage limited to dwelling units and other structures adhering to residential wiring requirements.
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:
- Lightweight and easy to install. Can be pulled easily through wood framing.
- Cost-effective. Significantly lower cost compared to metal-armored cables.
- Safety. Non-conductive sheath is safer to handle during installation.
- Aesthetics. Provide a clean, professional looking installation.
- Versatility. Available in a wide range of sizes and types for various uses.
- Durability. Withstand exposure to moisture, sunlight and chemicals.
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:
- Not suitable for exposed outdoor installation. Lacks rugged armor protection.
- Fire hazard if damaged or improperly installed. Sheath and insulation can melt.
- Not recommended for hazardous locations. Plastic jacket is combustible.
- Not rated for direct burial underground. Lacks waterproof protection.
- Voltage drop over long runs. Higher resistance versus armored cables.
- Rodent damage. Susceptible to chew damage without metal armor.
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:
- Reduced ampacity in some sizes to address fire hazards from overloaded conductors.
- Added rules to properly secure cables to studs to avoid damage.
- New supports required to avoid stress at electrical boxes.
- Protection requirements when installed near edges of holes and notches.
Updated NEC rules improve safety by reducing fire risks from improper use or installation of nonmetallic cables. Electricians must comply with all code changes.
Conclusion
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.