You're right, your hunch is good! Generally, the use of NM-B (Romex) cable in conduit is allowed in certain situations, but with important limitations. It's not a general "free pass" to run Romex inside conduit whenever you feel like it.Here's a breakdown, leaning towards "Yay, but with HUGE caveats and restrictions": Protection is Key (but not the whole story)

The primary reason to put conductors in conduit is for physical protection. If the Romex is in a location where it's likely to be damaged, conduit can offer protection. However, this doesn't automatically make it code-compliant.

NEC (National Electrical Code) Limitations are Crucial

The NEC is the guiding document here. While specific local codes may differ, the NEC is the starting point.

NEC 334.12 Uses Not Permitted outlines very specific situations where NM cable is not allowed. If your situation falls into any of these categories, conduit or no conduit, you cannot use NM cable. Common examples include: In wet or damp locations. Where exposed to corrosive fumes or vapors. In hazardous (classified) locations. Embedded in poured concrete or aggregate. NEC 334.15(B) Protection From Physical Damage

While NM cable can be run in conduit for protection from physical damage, the conduit itself MUST be installed according to its own article in the NEC. This means proper supports, fittings, etc. It is more common and accepted to use conduit and THHN/THWN conductors than using NM-B inside a conduit.

Derating

Putting multiple current-carrying conductors inside conduit can lead to heat buildup. NEC 310.15 requires ampacity derating if there are more than three current-carrying conductors in a raceway (conduit). NM-B already has a limited temperature rating (typically 90°C, but often used at the 60°C ampacity), and further derating could significantly reduce its allowable current. This is a very common reason to avoid using NM-B in conduit.

Fill Capacity

Conduit has a limited fill capacity. You can't just stuff as many cables as you want into it. NEC Chapter 9 and the relevant conduit article will dictate the maximum allowable fill. Romex cables, being flat and wide, take up a lot of space in the conduit compared to individual conductors.

Conduit Type

The type of conduit matters. EMT (Electrical Metallic Tubing - thinwall), PVC, and other types have different requirements for installation and suitability for certain environments.

Practicality and Cost

It's often more expensive and more difficult to run Romex in conduit than to simply use individual THHN/THWN conductors. THHN/THWN is designed for conduit, has a higher temperature rating, and allows for easier pulling.

In Summary (The "Yay, but...") Yes, you can use NM-B cable inside conduit in some situations, primarily for physical protection. However, you MUST comply with all applicable NEC regulations, including those for NM-B cable, the type of conduit you're using, derating, and fill capacity. In many cases, it's more practical and cost-effective to use individual THHN/THWN conductors designed for conduit.* The vast majority of electrical work calls for THHN/THWN when utilizing a conduit.

Important Disclaimer: I am an AI chatbot and cannot provide electrical advice. This information is for general knowledge and discussion only. Always consult with a qualified electrician for any electrical work. They will be able to assess your specific situation and ensure that the work is done safely and according to code. Incorrect electrical work can be dangerous and even deadly. Flag for review