Sheathing on Romex is made of tough polyvinyl chloride (PVC) thermoplastic. The sheathing on both 14 and 12 AWG wire is 19 mils thick. Still, no matter how strong the NM sheathing is, it is not meant for exposed applications.
While Romex wiring is useful for many indoor applications, it is unsuitable for outdoor use.
Romex (aka NM cable) should never be run exposed, EXCEPT between joists. That includes between studs. If it is exposed within easy access, it should be in conduit. And all circuits with outlets should be protected by a GFCI.
Noting that Romex is the name of a popular brand of Non Metallic cable, of the type specified in the NEC (US National Electrical Code) as type NM. (Like “Kleenex” is a brand of facial tissue.) Romex is not permitted anywhere that it is exposed to risk of damage, to water (dampness), and/or to sunlight.
However, leaving exposed wires running along the outside of a home or building is never a safe idea. To protect these wires from the elements, many owners elect to run them through an exterior wall with a conduit for an added layer of protection and simplified installation.
The idea is that it needs to be protected from abrasion. Also: this is an exception to other code standards, which says Type NM cable (aka Romex) should not be run through conduit.
Romex wire is designed for in-wall use and therefore is not designed to be seen. It is unsightly underneath cabinets when it is exposed. As a result, lights are typically mounted all the way towards the back of the cabinet (near backsplashes) so no one can see the wire as it comes out from the wall.
The following NEC regulations apply to Romex conductors:
They are not permitted in residential construction higher than three stories, or in any commercial construction. They must be protected, secured and clamped to device boxes, junction boxes and fixtures.
The NEC has specific definitions of wet, dry and damp locations, and NM cable can only be used where it's dry. This means you can't run it along the ceiling of your porch or patio, even if the area is covered. Concealed. Concealed means that you can run Romex behind walls, floors and ceilings.
UF-B: Ideal for underground setups, including landscape lighting, outdoor receptacles, and other exterior electrical systems. MC: Suitable for areas where you may expose the wire to potential physical damage or need to be installed in challenging environments, such as outdoor machinery or equipment.
Even if the device does not complete the ground, Romex or NM wiring can always be used with metal electrical boxes by attaching the bare or green grounding wire to the box with a screw. Many metal electrical boxes can be ganged, while most plastic boxes cannot.
For commercial buildings, the most common mistake has to do with romex cable, or non-metallic (NM) sheathed cable. Using this wiring where exposed, above drop ceilings or in walls where it's not supposed to be is not only a code violation but a safety hazard.
Romex is allowed to be installed in commercial applications in accordance with 334.10(3), but only if the cable is concealed within walls, floors, or ceilings that provide a thermal barrier of material that has a least a 15-minute finish rating as identifed in listings of fire-rated assemblies.
The THWN wire meaning is short for Thermoplastic High Water-resistant Nylon-coated. In THHN, water is replaced with heat.
UF cable designation is allowed for outdoor & underground use, romex is an indoor use only wire.
Outdoors, conduits are always classified as damp locations, and you cannot run NM cable in these conditions. This rule also applies to damp indoor locations, although spaces like garages are typically not classified as damp.
It is often asked whether it is fine to use a 12-gauge wire on a 15-amp circuit, and the answer is yes, it is actually fine. At 75°C, 12 AWG copper wire is ranked 25 Amps.
Common sense and practice says you cannot run romex exposed on the walls in a residential house - only allowed to do that in attics, crawl spaces, garages, unfinished basements, etc.
Life Expectancy of Electrical Wiring
However, the commonly agreed-upon general time frame of electrical wiring being effective is around 50 years.
14-gauge is used for lamps, light fixtures, and lighting circuits with 15 amps. 16-gauge is best for extension cords supporting 13 amps. 12-gauge is ideal for use in the bathroom, kitchen, outdoor receptacles, and 120-volt AC's supporting 20 amps.
Wire and cable manufacturers at NEMA and Underwriters Laboratories (UL), state that Type NM-B cable (which includes Southwire's Romex® Brand) needs to be removed if the cable has been submerged in flood waters.
Overall, while running Romex wire between joists is possible, it should be done with careful attention to safety and code compliance.
Type NM cable shall be permitted as follows: For both exposed and concealed work in normally dry locations except as prohibited in 334.10(3) To be installed or fished in air voids in masonry block or tile walls.
If indoors, a standard 18 AWG wire may suffice. Basic LED lighting will be a two-wire (Positive and Negative), while RGB will be a four-wire configuration (Red, Green, Blue, and Negative), and RGBW five wires (Red, Green, Blue, White, and Negative).