By McKinney TX Electrician | Published March 11, 2025 | Originally Posted June 5, 2022
Source: ABRelectric.com | Video Views: 174,576 | Collin County, TX
When it comes to electrical wiring, one question frequently arises among homeowners and electricians: Can Romex cable be run through conduit legally? Romex, a common brand name for Non-Metallic Sheathed Cable (Type NM or NMC), is a staple in residential wiring, but its use in conduit often sparks debate. To settle the matter, let’s turn to the National Electrical Code (NEC) and break down the rules, practical considerations, and best practices.
What Does the NEC Say?
The NEC provides clear guidelines for using Romex (Type NM or NMC) in various conditions, particularly under Article 334, which governs its installation. Two key factors determine whether Romex can be run through conduit: protection from physical damage and suitability for wet or damp locations. Additionally, conduit fill requirements from Chapter 9 of the NEC play a critical role. Let’s explore these in detail.
1. Protection from Physical Damage
According to NEC Section 334.15(B), Romex must be protected from physical damage where necessary. This applies to scenarios like running cable along a wall, through a floor, or in exposed areas such as a garage. Approved protective methods include:
- Rigid Metal Conduit (RMC)
- Intermediate Metal Conduit (IMC)
- Electrical Metallic Tubing (EMT)
- Schedule 80 PVC Conduit
- Type RTRC marked with the suffix -XW
- Other NEC-approved means
For example, when Romex passes through a floor, it must be sleeved in one of these conduits, extending at least 150 mm (6 inches) above the floor surface. Similarly, if installed in shallow grooves or chases in masonry, concrete, or adobe, Type NMC cable must comply with NEC 300.4(F) and be covered with plaster, adobe, or a similar finish for added protection.
So, can you run Romex through conduit for physical protection? Yes, as long as the conduit is appropriately sized and installed in a dry, indoor location (more on that below).
2. Wet or Damp Locations
Here’s where things get tricky. NEC Section 300.5(B) classifies the interior of underground enclosures or raceways as a wet location. This means any cable or conductor installed underground—even inside conduit—must be listed for wet locations per NEC 310.10(C). Romex, however, is not designed or listed for wet or damp environments, as noted in Article 334.12(B), which prohibits its use outdoors or in damp/wet conditions.
For instance, if you’re running power underground to a kitchen island, Romex isn’t allowed. Instead, you’d need individual conductors like THHN or THWN, or a cable like UF (Underground Feeder), which are rated for wet locations. Even if the conduit is sealed, the NEC considers underground runs inherently wet, so Romex will fail inspection in these cases—expect a red tag!
3. Conduit Fill Requirements
Running Romex through conduit also requires compliance with NEC Chapter 9, Table 1, which governs conduit fill percentages. Unlike individual conductors (e.g., THHN), Romex is a multi-conductor cable with a sheathed design, meaning its entire cross-sectional area counts as one conductor. The NEC limits fill to:
- 53% for a single conductor
- 40% for multiple conductors (if over two)
Practically, this means a single 12/2 Romex cable (two conductors plus ground) requires at least a 3/4-inch conduit to stay within the 53% fill limit. If you’re running multiple Romex cables through a conduit longer than 24 inches, you’ll also need to consider derating—reducing the ampacity of the conductors due to heat buildup—per NEC guidelines. Overfilling a conduit isn’t just a code violation; it can make installation difficult and risky.
So, Is It Legal to Run Romex in Conduit?
Yes, Romex can be run through conduit, but only under specific conditions:
- Dry Locations Only: It’s fine for indoor, surface-mounted runs (e.g., EMT in a garage), but not for wet or damp areas like underground or outdoor installations.
- Proper Conduit Sizing: The conduit must be large enough to meet fill requirements (e.g., 3/4-inch for 12/2 Romex).
- Physical Protection: Use conduit to shield Romex where damage is a concern, following NEC 334.15(B).
However, just because you can doesn’t always mean you should. Pulling individual conductors (like THHN) through conduit is often easier, allows for future upgrades, and avoids fill or derating headaches. Romex in conduit is most practical for short transitions—like from an attic to a surface-mounted pipe—where splicing in a junction box would be overkill.
Practical Tips for Homeowners and Electricians
- Homeowners: If you’re DIY-ing, stick to the NEC. Running Romex in conduit might save you a call to an electrician, but a failed inspection will cost more in the long run. When in doubt, consult the code book or a pro.
- New Electricians: Don’t rely on “if it fits, it ships” logic. Inspectors enforce the NEC, not old-school habits. Check Article 334, 300.5, and Chapter 9 for clarity.
- Old-Timers: The code has evolved—Romex in conduit isn’t outright banned, but it’s tightly regulated. Let’s argue in the comments!
Conclusion
The NEC doesn’t prohibit running Romex through conduit, but it sets strict boundaries to ensure safety and compliance. Stick to dry locations, size your conduit correctly, and protect the cable where needed. Whether you’re a homeowner tackling a project or an electrician brushing up on code, the key is simple: Go to the book. The National Electrical Code is your guide—use it wisely.
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