Key Takeaways
- NM cable is permitted inside conduit in dry locations.
- Pulling NM cable through long or curved conduits can be difficult.
- Use individual conductors for longer or more complex wiring projects.
Overview of Non-Metallic Cable
Common Misconceptions
Non-metallic cable, often called RX, is commonly misunderstood. Many believe it is illegal to run this type of cable inside conduit. This misconception leads to countless arguments, yet you are absolutely permitted to do so under certain conditions.
General Legality of Using NM Cable in Conduit
You are allowed to run non-metallic cable inside conduit except in damp locations. Conduit in outdoor areas is always deemed damp, prohibiting the use of NM cable. Indoors, however, even areas like garages are considered dry, akin to other parts of the house.
Using conduit provides extra protection against punctures. Despite its benefits, pulling NM cable through long sections of conduit is very difficult, especially with bends. For runs exceeding 2-3 feet, individual conductors like THHN or THWN are a better choice to avoid such troubles.
Conduit fill is another critical aspect. Regulations stipulate a maximum fill of 53%, with NM cable classified as a single conductor, quickly exceeding this limit. Additionally, you cannot strip NM cable to use its internal wires individually, as they lack the necessary protection.
Conduit Usage in Different Locations
Interior Environments
When running non-metallic cable (NM cable) through an interior environment, one of the main considerations is protection. The sheath on NM cable can easily be punctured by nails or staples. By inserting NM cable into a conduit, you gain additional protection. Although it is permissible to insert NM cable into a conduit in interior spaces, it can be challenging to pull long sections through conduits due to bends and curves.
- Ease of Use: For runs longer than 2-3 feet, using individual conductors like THHN or THWN is more efficient.
- Protection: Provides an extra layer of protection against punctures.
Exterior and Damp Location Restrictions
Running NM cable in conduits in exterior environments, or any location considered damp, is strictly prohibited. Outdoors, conduits are always classified as damp locations, and you cannot run NM cable in these conditions. This rule also applies to damp indoor locations, although spaces like garages are typically not classified as damp.
- Residential Restrictions: Garages are treated the same as interior residential spaces.
- Outdoor Prohibition: NM cable cannot be used in any exterior conduits due to moisture concerns.
Benefits of NM Cable in Conduit
Enhanced Protection
Using non-metallic (NM) cable inside conduit offers superior protection. The tough jacket of NM cable is already robust, but when you add the conduit, it provides an additional layer of defense against punctures from nails, staples, or other sharp objects.
Legal and Safe Installation
You can legally run NM cable inside any type of conduit as long as it's in a dry location. Within your house, even in areas like the garage, NM cable in conduit is perfectly legal and safe. This ensures compliance with electrical codes and standards.
Challenges of Long Runs
For runs longer than 2 or 3 feet, pulling NM cable through conduit becomes difficult, especially if there are bends or curves. For these situations, using individual conductors like THHN or THWN is recommended as they are easier to pull through long distances and bends.
Conduit Fill Restrictions
Conduit fill refers to how much of the conduit can be filled with wires. With NM cable, this can quickly become an issue since it's treated as a single conductor. You can only fill a conduit up to 53% of its capacity, and a single piece of NM cable might exceed this limit.
Not Suitable for Damp Locations
NM cable inside conduit is not suitable for damp locations, like outdoors or certain indoor areas. Always ensure your installation location is dry to maintain safety and compliance.
Avoid Cutting NM Cable
Do not cut NM cable to use the individual wires inside the conduit. The individual conductors within NM cable lack the necessary protection without their outer sheathing and are not approved for such use.
Difficulties with NM Cable in Conduit
Issues with Bends and Long Runs
Using non-metallic (NM) cable in conduit becomes particularly challenging when dealing with bends and long runs. The hefty jacket on the NM cable increases the difficulty of pulling it through the conduit. With bends or curves, it can get hung up and become very difficult to manage.
For longer runs, especially those exceeding 2-3 feet, it is advised to use individual conductors, like THHN or THWN. Professional electricians prefer these because they can be pulled over long distances with ease using pulling tools.
Conduit Fill Considerations
Conduit fill refers to the volume of conductors allowed inside a conduit. The rules vary, but typically, a maximum of 53% of the conduit space can be filled. NM cable is considered a single conductor for these calculations, despite containing three wires inside.
Even one piece of NM cable can quickly exceed the 53% fill limit. Disassembling the NM cable to use the interior wires separately is not permitted, as those wires lack the necessary protection without their sheathing. Proper consideration of conduit fill is essential to ensure compliance and safety.
Alternative Wiring Methods
Individual Conductors
For longer or more complex wiring runs, individual conductors are a better choice than non-metallic sheathed cable. These conductors, often labeled THHN or THWN, are preferred by professional electricians for extended runs. You can pull these wires over 100 feet using pulling tools, and you won't face the same kind of issues as with non-metallic sheath cables.
THHN and THWN Wires
THHN (Thermoplastic High Heat-resistant Nylon-coated) and THWN (Thermoplastic Heat and Water-resistant Nylon-coated) wires are designed for use in conduit systems. They offer greater flexibility and easier handling. Here's a brief comparison:
|
Feature |
THHN |
THWN |
|
Temperature Rating |
High Heat-resistant |
Heat and Water-resistant |
|
Coating |
Thermoplastic, Nylon |
Thermoplastic, Nylon |
|
Usage |
Mostly in dry conditions |
Suitable for wet and damp conditions |
Using THHN or THWN wires within your conduit systems allows for easier pulls and complies with conduit fill regulations, avoiding the complications that arise when using bulkier non-metallic cables.
Concluding Thoughts
In electrical work, non-metallic (NM) cable plays a crucial role. This cable is sometimes referred to as RX cable and is easy to work with for many indoor applications. When considering running NM cable through conduit, it's entirely legal for dry locations. However, outdoor or damp locations present different challenges and restrictions, making it unsuitable for such environments.
Key Considerations
- Protection: Inserting NM cable into conduit provides additional protection against punctures from nails or staples.
- Cable Length: For lengths beyond 2-3 feet, individual conductors like THHN or THWN are more practical to install.
- Conduit Fill: Adhere to the rule that a conduit should not exceed 53% of its capacity with NM cable, as it is treated as a single conductor.
Remember, taking NM cable out of its sheath to use the internal wires individually is not permissible due to safety and regulatory standards. Always consider these points for both safety and efficiency in your electrical projects.