Is it acceptable to install nonmetallic sheathed cable (NM) exposed on a wall or in an attic, a basement, or a residential garage?
Where would physical protection of the NM cable be required, and where is physical protection of NM cable often used but not necessarily required?
Is it acceptable to install NM cable in a raceway and, if so, how is the ampacity determined?
What lengths of raceway are considered to be sleeves for physical protection and at what length is the sleeve really considered to be a true raceway?
NM cable is permitted as a wiring method in one- and two-family dwellings, many multifamily dwellings, and commercial buildings with some restrictions. It is not allowed (1) in multifamily dwellings or commercial buildings that exceed three floors above grade, (2) where it would be used as service entrance cable, in commercial garages having hazardous (classified) areas, and (3) in certain other locations as detailed in Section 336-5 of the NEC.
Due to the varied locations and types of construction in which NM cable is used, it may be necessary to install the cable either exposed on or in the building or installed in a sleeve or raceway for physical protection.
Section 336-6 permits NM cable to be installed in exposed locations and has permitted this installation technique since the introduction of NM cable into the Code in the 1930s. If installed exposed, the cable must closely follow the building surface or be installed on running boards. The NM cable must also not be installed in an area where the cable may be damaged or subjected to any physical abuse that might affect the safe operation of the system.
For example, NM cable could be installed on the surface of an existing plaster- or wallboard-covered wall in a building where NM cable is allowed. It could be installed exposed in a residential garage or in an attic or basement. In most cases, the NM cable must closely follow the building surface or another means of support for the cable must be provided.
Many basements, attics, and storage areas do not have drywall applied so the trusses, joists, or framing members are left exposed. In some residential garages and attics, the construction may consist of unfinished walls and ceiling structures with high peaks or gables. In these cases, it may not be feasible to install the cables along the building structure because it would involve much longer lengths of cable and more labor for installation. A running board could be installed and supported from the building structure at a lower height with the cables attached to the running board.
Installing NM cable exposed in the basement of a structure is so common that Section 336-6(c) provides guidance on the installation of the cables in a basement. In the unfinished basement, an NM cable containing conductors smaller than two No. 6 AWG or three No. 8 conductors must be installed through bored holes in the framing members. Cables consisting of two No. 6 or three No. 8 or larger conductors can be stapled directly to the bottom of the framing members in the basement.
The restriction for the smaller size conductors to be installed through bored holes and not to the lower edges of the joists is to ensure better physical protection for these cables. Section 300-4 provides various specific requirements for providing physical protection for cables installed in wood framing members.
Where the NM cable may be exposed to physical damage, Section 336-6(b) requires that the cable be protected. This protection can be by installing it in conduit, electrical metallic tubing (EMT), Schedule 80 PVC, pipe, guard strips, listed surface metal or nonmetallic raceways, or by any other means that will provide suitable protection for the cable.
If the NM cable is installed where it passes through a floor, the cable must be protected for at least 6 inches above the floor by enclosing it in rigid metal conduit, intermediate metal conduit, or one of the other methods previously mentioned. This protection will permit the cable to be fished from floor to floor without having to worry that a nail attaching wood moulding to the wall (close to the floor) would penetrate the cable.
NM cable protection is also often used where the cable must exit the interior of the building structure to provide power for devices on the exterior of the building. Floodlights on exterior eaves will often be powered using NM cable that has been sleeved to protect the cable from physical damage and from deterioration from the outside elements, such as dampness or sunlight.
Section 336-6 does not limit the length of sleeve that can be used to protect the cable, but this sleeve is normally only used for very limited lengths of cable runs. For example, the sleeve enclosing NM cable exiting the attic and feeding the floodlights mounted on the eaves of a building may only be 12 to 24 inches in length, although there may be construction applications where the sleeve is much longer.
The question is often raised in the field "What is considered to be a sleeve and what is considered to be a raceway installation?"
A sleeve is not required to be continuous and can be whatever length is needed to provide physical protection. A raceway installation must be installed complete between outlet, junction, or splicing points before the installation of conductors in accordance with Section 300-18(a).
If a metal raceway is used, Section 300-10 requires the metal raceway, cable armor, or other metal enclosures be metallically joined together for effective electrical conductivity. There is an exception to Section 300-10 that permits short sections of raceways used to provide support or protection of cable assemblies from physical damage to not be electrically continuous.
If the NM cable leaves the interior of the building and is enclosed in a sleeve or raceway on the exterior of the building for a short distance, do the conductors in the cable have to be derated for the higher ambient temperature often found outside the building.
Section 310-15(a)(2) provides some guidance on this issue. It states "where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest ampacity value shall be used." This would apply to the conductors within the cable leaving a cooler area within the building and entering a higher temperature environment on the outside of the building. The higher ambient temperature surrounding the cable located on the outside of the building would require that a lower ampacity be used for the conductors in the cable so the lower ampacity would then be used for the entire cable.
Obviously, the temperature on the outside of the building and the temperature of the cable would depend upon the location of the building and cable and the time of year. Table 310-16, including the ambient temperature correction factor portion of the Table, could be used to determine the allowable ampacity for the cable conductors.
There is an exception in Section 310-15(a)(2) that does help with short runs, such as the short sleeves feeding out to floodlights and similar equipment. The exception permits some relaxation on the general rule for derating of conductors. It states: "where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition up to a distance of 10 feet or up to 10 percent of the circuit length figured at the higher ampacity, whichever is less."
This exception could then apply to the cable sleeved to the outside of the building allowing the higher ampacity for the conductors using the interior temperature. The reasoning for this exception is that the cable installed in the cooler area will act as a heat sink to draw excess heat from the short run of conductors exposed to the higher ambient heat.
Most installers are probably using some or all of these techniques explained here but a review of the Code Sections that permit these practices can be helpful for a better understanding of a common practice.
ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at firstname.lastname@example.org.