Yes, Romex (Non-Metallic sheathed cable or NM cable) can be run in conduit under certain conditions, despite a common misconception that it is illegal or strictly prohibited. While permissible, it is often not the most practical or efficient method for various electrical installations.
Many believe it is against electrical code to run NM cable inside conduit, leading to frequent misunderstandings. However, electrical codes do not forbid this practice outright. The key lies in understanding the practical limitations and code requirements that often make it an less ideal choice compared to using individual conductors (like THHN or THWN wires) within conduit.
Understanding the Permissibility
The ability to run NM cable in conduit primarily hinges on meeting the specific requirements of the National Electrical Code (NEC) regarding conduit fill and cable protection. When considering this option, remember that the purpose of the conduit is typically for physical protection of the cable, especially in areas where it might be exposed to damage.
Why It's Often Avoided (Even If Allowed)
While allowed, several factors make running NM cable in conduit less common or practical:
- Conduit Fill Limitations: This is the most significant hurdle. NM cable, being flat or oval and having an outer jacket, takes up significantly more space than individual, round conductors (like THHN/THWN) with the same wire gauge.
- The NEC limits the percentage of conduit fill to prevent overheating and allow for easy future wire replacement. For three or more conductors, conduit is typically limited to 40% fill.
- Due to its bulk, a single NM cable can quickly exceed the allowable fill capacity of a standard conduit size, often requiring a much larger and more expensive conduit than anticipated.
- Difficulty in Pulling: NM cable is relatively stiff and less flexible than individual wires. Pulling it through long runs or conduits with multiple bends can be extremely difficult, if not impossible, without damaging the cable's outer sheath.
- Cost Inefficiency: Because a larger conduit size is often required for even a single NM cable, the material costs for conduit and fittings can quickly outweigh any perceived savings from using NM cable over individual conductors.
- Heat Dissipation: While generally not a primary concern for short runs, enclosing NM cable in conduit can slightly impede heat dissipation compared to an open-air installation, though this is usually accounted for in conduit fill calculations.
Comparing NM Cable vs. Individual Conductors in Conduit
| Feature | NM Cable in Conduit | Individual Conductors (e.g., THHN) in Conduit |
|---|---|---|
| Permissibility | Permitted under certain conditions | Standard and preferred method |
| Conduit Fill | High, often requiring larger conduit sizes | Low, allowing for smaller conduit and more wires |
| Ease of Pulling | Difficult, especially with bends | Easy, highly flexible |
| Cost | Potentially higher due to large conduit size | Generally more cost-effective for conduit runs |
| Flexibility | Limited | High |
| Protection | Good physical protection within the conduit | Excellent physical protection within the conduit |
Practical Applications and Considerations
Despite the challenges, there are specific scenarios where running NM cable in conduit might be considered:
- Short Exposed Runs for Physical Protection: For very short distances where an NM cable needs protection from physical damage, such as exiting a wall to a surface-mounted box, a piece of conduit can be used as a sleeve.
- Transition from Concealed to Exposed: Where NM cable transitions from inside a wall or ceiling to an exposed area (e.g., an unfinished basement or garage), a short section of conduit can provide the necessary protection.
- Outdoor or Wet Locations: While NM cable itself isn't rated for wet locations, running it inside watertight conduit can offer additional protection from moisture and physical damage when it's necessary to extend a circuit to an outdoor fixture or location. However, individual THWN conductors specifically rated for wet locations are generally a superior choice.
Key Considerations Before Using NM Cable in Conduit:
- Consult the NEC: Always refer to the latest edition of the National Electrical Code (NEC) for specific conduit fill tables (Annex C) and other relevant articles to ensure compliance.
- Manufacturer Specifications: Check the NM cable manufacturer's specifications for allowable bending radius and any recommendations regarding conduit installation.
- Conduit Sizing: Be prepared to use a much larger conduit size than you might initially expect. It's often more economical and code-compliant to switch to individual THHN/THWN conductors once a run enters conduit.
In summary, while it is not illegal to run Romex in conduit, the practical challenges and code requirements, particularly concerning conduit fill, often make it an inefficient and sometimes impractical choice for most applications. For runs requiring conduit, using individual conductors is almost always the preferred and more compliant method.
