Yes. NM-B cable is allowed in conduit in dry, indoor locations. Conduit is not required for most typical NM-B installations and is generally added only where the cable needs protection from physical damage.

NEC Article 334 permits NM-B to be installed in a raceway, including conduit, and nothing in the cable’s UL listing prohibits this use.

The critical exception is wet or damp locations. NEC 334.12(B)(4) prohibits NM-B in these areas. This includes underground raceways, which NEC 300.5(B) treats as wet locations, as well as the interior of raceways installed outdoors above grade under NEC 300.9. Conduit does not change the location classification or make NM-B suitable for wet locations. It remains banned there. Outdoor and underground conduit can still contain moisture from condensation or water entering through joints and terminations, while NM-B is designed and listed for dry locations.

Framed as three separate questions, the answer looks like this:

• Can I sleeve NM-B in conduit indoors to protect it from impact or abrasion? Yes.

• Can I run NM-B in conduit outside the building? No.

• Can I use conduit to take NM-B underground? No.

Let's walk through when NM-B in conduit makes sense, why outdoor and underground raceways are prohibited even when installed in conduit, and what to use instead.

When NM-B in Conduit Is Appropriate

NM-B may be installed inside a conduit when all of the following conditions are met:

• The conduit is indoors and in a dry location

• The conduit is properly sized for the NM-B cable under NEC conduit-fill requirements (e.g., a single cable cannot occupy more than 53% of the conduit’s internal area)

• The cable’s ampacity does not exceed its 60°C rating, as required by NEC 334.80

• Ampacity adjustment is applied when more than three current-carrying conductors share the raceway

Typical approved installations include:

Short Protective Sleeve Where NM-B Is Exposed to Physical Damage

The most common reason to run NM-B in conduit is physical protection. NEC 334.15(B) requires protection where NM-B is exposed to physical damage, using RMC, IMC, EMT, Schedule 80 PVC, or another approved method.

Typical examples include:

• Exposed runs on garage or workshop walls

• Cable near panels or subpanels within easy reach

• Runs on exposed studs in unfinished basements

• Short drops from a wall, floor, or ceiling to a device or panel

• Masonry penetrations or other vulnerable transitions

The NEC does not define “subject to physical damage” by a fixed height or exact location, so the AHJ determines whether protection is required.

Wall, Floor, or Masonry Penetration

NM-B may pass through a short piece of conduit when crossing a masonry wall, penetrating a concrete slab, or transitioning between rooms. The conduit is used only at the penetration or transition point

Full-Length Interior Run in Conduit

NM-B can be run through conduit for its full length in a dry interior location, but it is usually unnecessary because the cable does not normally require physical protection there.

When NM-B in Conduit Is Not Appropriate

Conduit does not extend NM-B's permitted uses. It can protect the cable from impacts, but it cannot change the classification of the wiring method. NM-B inside conduit is not appropriate in the following situations:

Outdoor Conduit Above Ground

NEC 300.9 classifies the interior of raceways installed outdoors above grade as wet locations. NEC 334.12(B)(4) prohibits NM-B in wet or damp locations. NM-B in outdoor conduit is a code violation, even when the conduit is watertight PVC or EMT with weatherproof fittings.

For an outdoor above-ground run, use THWN-2 or XHHW-2 individual conductors in conduit. Both are listed for wet locations.

Underground Conduit

NEC 300.5(B) classifies underground locations as wet locations, regardless of the raceway used. NM-B is prohibited underground under any condition.

For a buried run to a subpanel, detached garage, or outbuilding, use THWN-2 or XHHW-2 in PVC. USE-2 in PVC is also an option for feeders where a separate ground is not required in the cable assembly.

Direct Burial

NM-B is never permitted for direct burial. For direct-burial applications, use UF-B, which is specifically listed for that purpose.

Wet or Damp Interior Locations

Some interior locations are classified as damp or wet, including under cooling towers, in some agricultural buildings, and in areas subject to regular water exposure. NM-B is not permitted in these locations. Use THWN-2 or XHHW-2 in the raceway instead.

Key NEC References

• 334.12(B)(4): NM-B is prohibited in wet or damp locations.

• 334.15(B): NM-B must be protected where exposed to physical damage.

• 300.5(B) and 300.9: Underground and outdoor raceways are treated as wet locations.

• 334.80: NM-B ampacity is limited to the 60°C rating.

• Chapter 9, Table 1: Governs conduit fill.

A Case for Alternatives

Conduit is useful for short protective sections of NM-B. Once the run gets longer, moves outside, or goes underground, different cables are used:

THHN/THWN-2 in EMT or PVC

For interior feeders and branch circuits through conduit, THHN/THWN-2 conductors are the standard replacement. They allow the 75°C column to be used and meet conduit fill more easily THWN-2 is also listed for wet locations, which covers outdoor conduit above ground.

XHHW-2 in PVC

XHHW-2 works in wet and dry locations and is rated for underground installation as individual conductors in the raceway. Good match for exterior conduit runs, service risers, and transitions between above-ground and underground.

UF-B for Direct Burial

For any run buried directly in soil without a raceway, UF-B is the correct cable. It is listed for direct burial and wet locations.

MC Cable

For interior runs where physical protection is the main concern, MC cable can replace both NM-B and a conduit sleeve. The interlocking armor provides its own mechanical protection, and the cable installs faster than pulling conductors through conduit.

NM-B in Conduit: Approach by Scenario

1) Inside a Dry Finished Wall

Yes. NM-B recommended. Conduit not required.

Standard residential branch circuit wiring. Cable is stapled to framing per NEC 334.30 and protected by the wall assembly itself. No sleeve needed.

2) Exposed Dry Garage Wall

Yes. NM-B in EMT or PVC sleeve, or THHN/THWN-2 in EMT.

Where NM-B is exposed and within reach of vehicles, tools, or stored equipment, a sleeve satisfies NEC 334.15(B). For long exposed runs, THHN/THWN-2 in EMT is often the cleaner choice.

3) Dry Basement or Utility Room

Yes. NM-B usually needs no conduit. Use EMT with THHN/THWN-2, or MC cable, for protection.

Cable secured to the underside of joists or along framing generally does not need a sleeve. Where the cable is exposed to impact or the AHJ requires protection, EMT with individual conductors or MC cable is the standard method.

4) Outdoor Conduit

No. Use THWN-2 or XHHW-2 in conduit.

Outdoor raceways are wet locations per NEC 300.9. NM-B is prohibited in wet locations per NEC 334.12(B)(4). Conduit does not change either rule.

5) Underground Conduit

No. Use THWN-2 or XHHW-2 in PVC.

Underground raceways are wet locations per NEC 300.5(B). NM-B is not permitted underground under any condition. Individual wet-rated conductors in PVC are the correct method.

6) Direct Burial

No. Use UF-B.

NM-B is not listed for direct burial. UF-B is the direct-burial cable in the NM family and is the correct choice for buried runs without a raceway.

At Nassau National Cable, you can find NM-B (Romex) for interior branch circuits, THHN/THWN-2 and XHHW-2 for the conduit runs where NM-B is not permitted, and UF-B for direct-burial applications.