Review Metal-clad Cable Requirements

By Mark E. Battersby | Mar 15, 2002
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There are a number of changes in the 2002 National Electrical Code (NEC) regarding Metal-Clad (Type MC) Cable that should be reviewed before your next electrical project. In the 1999 NEC, Article 334 covered Type MC Cable; however, in the 2002 NEC, Type MC Cable has been relocated into Article 330. Until you become familiar with the new location in the NEC, it may take some searching to find it. There are, however, more significant changes than just relocating the article within Chapter 3.

Metal-Clad Cable is a factory assembly of one or more insulated circuit conductors, with or without optical fiber members, enclosed in an armor of interlocking metal tape or a smooth or corrugated metallic sheath. Each electrical conductor within the cable is individually insulated and can be copper, copper-clad aluminum, or aluminum.

Section 330.116 requires the metallic covering of the Type MC Cable to be one of three types: smooth (tube) metallic sheath, corrugated metallic sheath, and interlocking metal tape armor. The interlocking metal tape armor Type MC cable is required to have a bare or insulated equipment grounding conductor in addition to any other conductors within the cable.

The equipment grounding conductor may be one conductor or it may be sectioned into more than one conductor, which permits the cable to have a smaller overall diameter than a cable that incorporates only a single equipment grounding conductor. The equipment grounding conductor within the cable and the outside cable armor with the appropriate connector combine to form the grounding path for a fault and provide any necessary bonding for the equipment.

Where not subject to physical abuse, Type MC Cables are permitted to be installed as service, feeder, and branch circuit cables for lighting, power, control, and signaling circuits. Type MC Cable without an outside nonmetallic jacket can be installed in an “other space used for environmental air” in accordance with Section 300.22(C).

These cables can be installed either indoors or outdoors and in exposed and concealed locations. They can be direct buried, where identified for such use. They can be installed in a cable tray or in any raceway.

The cables can be installed as open runs or as an aerial cable on a messenger wire. If installed in a wet location, the metallic cover must be impervious to moisture or the insulated conductors must be listed for use in a wet location. Type MC cable with a nonmetallic jacket can also be used in a wet location.

Section 330.10(A)(13), covering the permitted uses of Type MC Cable, contains a major change for the 2002 NEC. This new subsection permits single-conductor Type MC cables to be used. Where single conductor Type MC cables are used, all phase conductors and the neutral conductor, if required for the circuit, must be grouped together to minimize the possibility of induced voltage on the sheath.

Section 330.116 has also been modified to require any single conductor Type MC Cable to be manufactured with a nonmagnetic sheath or armor. This nonmagnetic sheath will ensure that magnetic lines of flux will not cause current flow on the sheath of the cable and, where the cables are grouped together, should help ensure cancellation of the lines of flux from one conductor to the other.

Section 330.80(B) contains new text for determining the ampacity for these single-conductor Type MC Cables. Single-conductor Type MC conductors can be grouped in a triangular or square configuration and installed on a messenger wire. Or they can be grouped in a triangular or square configuration as open runs with a maintained free air space of at least 2.15 times the outside diameter of the largest conductor in the configuration, with this space being maintained between the first configuration and any additional configurations.

Table 310.20 can be used for conductors rated at zero through 2,000 volts and Tables 310.67 and 310.68 for conductors rated over 2,000 volts.

One additional change in the 2002 NEC for Type MC cable involves Section 310.15(B)(2)(a) and the adjustment factors in Table 310.15(B)(2)(a) for the ampacity Tables 310.16 through 310.21. Normally, where cables are bundled together for more than 24 inches and the conductors exceed three, the conductor ampacity within each cable must be derated based upon the number of current-carrying conductors in the bundle. A new 310.15(B)(2)(a), Exception No. 5 has been added to exclude adjustment factors for Types AC and for MC cables without an overall outer jacket under the following conditions:

(a) Each cable cannot have more than three current-carrying conductors.

(b) The conductors are No. 12 AWG copper minimum.

(c) Not more than 20 current-carrying conductors are bundled, stacked, or supported in bridle rings.

However, where cables are stacked or bundled longer than 24 inches without maintaining spacing and the number of current-carrying conductors exceeds 20, a 60 percent adjustment factor must be used to adjust the ampacity of the conductors in the Tables.

As can be seen, there are some very useful and interesting changes for Type MC cables in the 2002 NEC. EC

ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or via e-mail at [email protected].

About The Author

Mark Battersby is a freelance writer based in Ardmore, Pa. For more than 25 years, his tax and financial features have appeared in ELECTRICAL CONTRACTOR and other leading trade publications.

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