What are the disadvantages of Romex wire?

The disadvantages of Romex wire, or NM-B non-metallic sheathed cable, relate primarily to durability, location restrictions, and safety limitations compared to other wiring methods. First, it is not rated for wet, damp, or outdoor locations, so it cannot be used underground, outside, in direct contact with concrete without protection, or in any area where moisture may accumulate, unlike UF-B or THWN in conduit. Second, its PVC jacket and thin insulation are vulnerable to physical damage from nails, screws, rodent chewing, and abrasion, and it must be protected from damage where exposed, making it unsuitable for unfinished garages, basements, or workshops without additional protection. Third, it has lower heat resistance than individual THHN conductors and can be damaged by prolonged high attic temperatures or being covered with insulation in a way that traps heat. Fourth, it is not as resistant to chemicals, gasoline, or oils often found in garages. Fifth, because it is a bundled assembly, it is subject to ampacity derating when multiple cables are bundled together for more than 24 inches, which reduces its safe current-carrying capacity. Sixth, it cannot be used in commercial buildings above three floors in many jurisdictions and is not allowed in plenum ceilings without being specifically rated. Finally, older Romex without ground or with undersized ground, and improper installation practices like over-stapling that pinches the jacket, have contributed to historical safety concerns, though modern NM-B is much improved when installed correctly.