I would think its ok to run NM wire under a house trailer with skirting (not in the ground) but along the bottom, because its not exposed to any weather conditions.

2002 NEC

Answer: No

See Section 550.15(H) Under-Chassis Wiring (Exposed to Weather). Where outdoor or under-chassis line-voltage (120 volts, nominal, or higher) wiring is exposed to moisture or physical damage, it shall be protected by rigid metal conduit or intermediate metal conduit.

The conductors shall be suitable for wet locations.

Exception: Electrical metallic tubing or rigid nonmetallic conduit shall be permitted where closely routed against frames and equipment enclosures.

Section 551.47(N) and 552.48(M) contain the same restrictions.

[This message has been edited by Joe Tedesco (edited 03-05-2002).]