Can You Use Romex Under a Trailer or Mobile Home?
A user asked the app whether Romex can be run underneath a trailer. The answer is no. The open space beneath a trailer or mobile home is at minimum a damp location, and NM cable is limited to dry locations inside normally dry buildings. Running Romex under the unit exposes it to moisture, physical damage, and in many cases sunlight, and it fails on all three counts.
Why NM cable fails there
NEC 2023 334.10 permits NM cable in dry locations of one and two family dwellings and similar structures, and 334.12(B) prohibits it in wet or damp locations. The Code defines a damp location as one protected from weather but subject to moderate moisture, which describes the underside of a trailer with skirting exactly. Without skirting, wind driven rain makes a case for calling it a wet location outright. The paper filler inside NM wicks water, the jacket is not listed for moisture exposure, and corrosion at terminations follows. On top of that, cable hanging below the frame is exposed to damage, and 334.15(B) requires protection from physical damage even where NM is otherwise allowed.
What to use instead
UF cable is the direct substitute. It is listed for wet locations per Article 340, looks and terminates like NM, and can be secured to the underside of the structure following the same support intervals of 12 inches from boxes and 4.5 feet between staples. Where the run is exposed to damage, sleeve it in conduit. The other clean solution is THWN or XHHW conductors in PVC or liquidtight conduit, which are wet rated systems from end to end. For manufactured homes specifically, Article 550 governs, and 550.15 requires wiring exposed underneath the home to be protected and suitable for the conditions, which the listed methods above satisfy.
Feeder runs to a mobile home
If the question is really about getting power to the trailer, the typical compliant approach is an underground feeder of UF or individual THWN conductors in conduit from the service pole or pedestal to the home, with a 4 wire feeder keeping the neutral isolated from ground at the home panel per 550.33 and 250.32. Direct buried UF needs 24 inches of cover per Table 300.5, and PVC conduit needs 18 inches. Anything emerging above grade must be protected to 8 feet.