Although often used interchangeably in certain applications, metal clad cable and electrical metallic tubing have their own set of performance and installation parameters
When used with appropriate conductors, metal-clad (MC) cable and electrical metallic tubing (EMT) have nearly identical uses but quite different working characteristics. Therefore, on any given job, it's advantageous for electricians to use both types in concert. To do so most effectively, it's important to understand specific construction specifications and permitted uses for each system, but first let's begin with some basic definitions.
Definitions. Although MC resembles its close relative, armored cable, there are some distinct differences. Whereas armored cable has a grounding strip in intimate contact with the inside of the metallic sheath, which can then serve as the equipment grounding conductor, MC cable has no such strip. Therefore, the outer sheath of MC cable does not qualify as an equipment grounding conductor. MC cable contains an insulated grounding conductor which, when properly terminated at both ends, makes a reliable return path for fault current, enabling the overcurrent device to perform its protective role. The metal jacket serves to supplement the equipment grounding conductor since it is firmly clamped to grounded metal enclosures at both ends — so that the unit cannot become energized without instantly causing the overcurrent device to trip out.
MC cable comes in various versions such as with corrugated copper armor or with an outer supplementary corrosion-resistant material. Conductors are copper, aluminum, or copper-clad aluminum (solid or stranded). In the discussion that follows, we will consider the most commonly used MC cable, interlocking metal tape aluminum sheath with solid copper conductors, primarily 12-2 w/ground and 12-3 w/ground used extensively for branch circuit work.
EMT is an unthreaded thinwall raceway that is commonly seen in industrial and commercial locations. The raceway is typically made of steel with a smooth galvanized finish, or aluminum. This raceway, with appropriate fittings properly terminated, can serve as an equipment grounding conductor in most applications. However, most electricians still opt to “pull a green for everything.”
Then, if the raceway were to pull apart under unforeseen conditions, ground continuity for the circuit would not be lost. In fact, the sections of raceway on either side of a single break would still be grounded from opposite ends of the run. Also, like the metal sheath of MC cable, the raceway provides supplementary and redundant grounding and greatly lowers the overall impedance of the equipment grounding conductor.
MC cable and EMT share other traits as well, most notably in uses permitted/not permitted as noted in the NEC. Let's take a closer look at where each type of product can and can't be applied.
Permitted uses. Article 330.10 of the NEC lists 12 general uses and four specific uses for MC cable. Although the Code should be consulted when designing specific projects, MC cable is seen frequently in nonresidential feeders and branch circuits, both exposed and concealed. Some of the uses permitted, such as for direct burial or in concrete, are rarely seen because of the suitability of rigid non-metallic conduit (RNC, gray UL-listed PVC) with wet area conductors.
Article 358.10 outlines the permitted uses of EMT. It can be used indoors as well as outdoors, exposed and concealed, in applications where a less than maximum amount of protection is required. In addition, it can be used underground and in concrete with restrictions — with supplementary corrosion protection in some cases. However, rigid nonmetallic conduit (RNC — gray PVC) can be more economical and entirely suitable in these environments.
Suitable applications. As an example of the application of these two wiring methods, a reading of Art. 620, titled Elevators, Dumbwaiters, Escalators, Moving Walks, Wheelchair Lifts and Stairway Chair Lifts, is quite revealing. The rules of 620.21, Wiring Methods, require special wiring types, including MC cable and EMT associated with the types of equipment listed above. They are also required in or on elevator cars, in machine rooms and control rooms — essentially everywhere except the traveling cables.
Although the Hazardous Locations section of NEC Chapter 5 requires threaded rigid conduit (RMC) or threaded steel intermediate conduit (IMC) for the real hardcore hazardous (classified) locations, MC cable is allowed in some installations with some listing restrictions. The rest of Chapter 5 consists of 19 specific occupancies — most of which present special challenges in regard to wiring. In many of these situations, MC cable and EMT are the wiring methods of choice.
It's worth mentioning here that in commercial garages, the Class I Division 1 area includes any pit or depression up to the top of the adjacent slab. Above the top of the slab and extending upward 18 inches is defined as a Class I Division 2 space. The authority having jurisdiction (AHJ) has the final say on whether or not the pit or depression can be reclassified as a Division 2 space, and the 18-inch zone above the pit can become unclassified if a six-air-change-per-hour ventilation system that exhausts air at the bottom of the pit is installed.
Either way, all wiring above any classified area has to be installed as specified in 511.7(A), Wiring in Spaces Above Class I Locations. MC cable and EMT are a perfect fit in this environment.
Installation basics. MC cable can be manipulated with a few simple tools. An MC cable cutter will cut through the sheath quickly and easily without nicking the conductors (Photo 1). But make sure you use MC (not NM) connectors along with an insulating bushing to protect the conductor insulation from being damaged by the sharp cutoff edge of the sheath. Or use snap-on type connectors that do not require separate insulating bushings. Consult Art. 330 for supporting and securing requirements, maximum bends, and other installation details.
The installation of EMT requires a few specialized tools. This thinwall tubing can be cut with a hacksaw (Photo 2) but a big job with many cuts will justify investing in a portable bandsaw (Photo 3). The portable band saw will also come in handy when cutting the struts required to support multiple parallel runs of EMT, especially across a ceiling. After cutting the EMT, you need to ream it to achieve a smooth rounded edge that will not damage the conductors.
There are several types of reamers available, or other suitable metal tools for this work. In addition, a conduit bender is an essential tool for EMT work. If you are going to be doing quite a few of these types of installations, a box offset bender (Photo 4) is also quite useful to have in hand.
A simple EMT run is quite easy to install, but a complex job can be a big challenge. Multiple runs coming out of a panel have to be in the right order so that they can peel off as required to go to their final destinations without having to cross (Photo 5). The runs have to be straight and uniformly spaced or they will look unsightly and unprofessional. In order to maintain uniform spacing, a plywood template (Photo 6) can be used to good advantage.
Another useful idea is to rip out a V-channel (Photo 2 on page C22) in a 24-inch length of 2×4 lumber and fasten it to a short-legged sawhorse. This homemade support is a great aid in cutting EMT and other pipe on the jobsite where a bench-mounted vise is not readily available.
Bending conduit is both an acquired and innate skill. You need a good eye for what will work and look good and also a certain body of knowledge. Complex jobs even call for the use of trigonometric functions and other advanced math. An excellent reference on this topic is the well-known “Benfield Conduit Bending Manual, 2nd Edition.” On the Internet, www.porcupinepress.com also provides lots of useful information on bending conduit.
As we have seen, MC cable and EMT are the wiring methods of choice in many areas. These two wiring methods are very different physically, yet they complement one another to a remarkable degree — each satisfying a blind spot, so to speak, where it is difficult or undesirable to use the other.
Herres is a New Hampshire licensed master electrician in Stewartstown, N.H.