One aspect of home wiring that electrical inspectors will pay close attention to is the improper use of non-metallic cable (NM) in exposed locations. Common exposed locations where this is found are on basement foundation walls or across the faces and ends of studs and joists in unfinished framed spaces. Learn if exposed wire is allowed, its dangers, and workarounds like THHN wire for exposed sheathed cables in your home.
Why Exposed Wire Should be Avoided
The National Electrical Code, the basis for all local building codes, has specific regulations for installing electrical wires so that they are protected from damage to avoid fire, shocks, and trips or surges.
- Fire: Exposed wire or any other wire that is insufficiently covered can cause fires if the wires short and result in a spark.
- Shock: Exposed wire where the conductor is bare can shock a user, resulting in injury or death.
- Trips, Surges: Exposed wires may not deliver sufficient power to devices, causing a higher power draw and tripped circuit breakers or short circuits.
NM Electrical Cable Proper Uses
Non-metallic or NM cable, also known by the trade name Romex, is the most common form of electrical wire used in residential electrical work.
NM cable is a bundle of individual conducting wires wrapped in a plastic vinyl outer sheathing. Normally the cables carry 10-, 12-, or 14-gauge conducting wires for individual house circuits.
The sheathing on NM cable does offer some protection against incidental physical damage. 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.
Typical NM Wire Locations
Typical locations for NM cable include but are not limited to:
- Wall cavities that are covered with finished wall materials such as drywall or plaster, but only in cavities where the cable will not be accessible.
- Inside the air voids of masonry block or tile walls but only where the air voids are dry; the walls cannot be damp or wet.
- Between exposed joists or studs, provided the cables are recessed away from the face of the framing members. This is normally done by boring holes in the centers of the framing members and running the cables through the holes.
NM Wire Improper Exposed Uses
The National Electrical Code forbids the use of NM cable in situations where it is exposed in a manner where physical damage is possible.
One example of this is where a do-it-yourself electrician attaches NM cable across the front face of studs or ceiling joists or where it is attached across the face of concrete foundation walls.
While it is allowable for NM cable to run through holes bored in the centers of exposed framing members, it cannot be attached across the front face of studs, since this creates the possibility of snagging or damage to the cables.
Similarly, NM attached to the face of concrete walls is susceptible to damage and thus is not allowed.
Wiring for Exposed Locations
One approved way to run wiring across exposed surfaces is to mount an approved rigid conduit across the framing members or wall, then run individual THHN conductor wires inside the conduit. Such wiring is well protected against physical damage.
The citation from the National Electrical Code is as follows:
Protection from Physical Damage: Cable shall be protected from physical damage where necessary by rigid metal conduit, intermediate metal conduit, electrical metallic tubing, Schedule 80 PVC conduit, Type RTRC marked with the suffix -XW, or other approved means.
The most common type of conduit is known as electrical metallic tubing (EMT), but other types of conduit are also used, including flexible metal conduit (FMC), intermediate metal conduit (IMC), which is sturdier and offers more resistance to corrosion, plastic conduit (PVC), most often used underground, and rigid metallic conduit (RMC), the heaviest gauge available.
The electrical conduit is designed for use with specific types of individual conductor wires. The most common wires are THHN wires, which are individual copper conducting wires with a color-coded, heat-resistant plastic insulation around them.
Can NM Cable be Run Inside Conduit?
If an electrical inspector has flagged NM cable that has been run incorrectly, you might be tempted to install the conduit, then reuse the same NM cable by running it through the new conduit.
This is a subject of some debate among electricians. Some pros argue that NM cable inside conduit may be susceptible to heat build-up and is therefore not allowed.
Others take a more lenient view. While the National Electrical Code does not expressly forbid inserting NM cable inside conduit, it is, in practice, very difficult to do so and very few professional electricians will do such an installation.
It is the accepted practice to run only individual THHN conductors (or another approved form of wire) inside the conduit. THHN wires are relatively inexpensive and are much easier to run through conduit because they are thin and have less coating on them.
Can you put Romex in conduit?
It is not good practice to put Romex in conduit. Romex is the brand name of a type of NM or non-metallic cable that is sheathed in tough, flexible plastic. Individually coated bundled wires are contained within the sheathing. It's generally best to run only separate THHN wires inside the conduit. THHN wires are inexpensive, easy to run through the conduit, and provide more space in the conduit since there is no cable sheathing or paper liner.
Can an exposed wire cause a fire?
An exposed wire can cause a fire. If the coating is nicked or if the live and ground wires touch, the resulting spark can cause a fire.
334.15 Exposed Work. National Electric Code of Minnesota.
Conduit Fill Table. Elliott Electric Supply.