Cables

f As an interior designer, you may find yourself involved in discussions about the type of electrical cables that can be run in a project. The type of cable that is permitted has a very significant effect on the cost of the electrical work. Contractors will frequently use these terms when discussing conditions on a site with you. There are actually many types of electrical cables, some of which are used only in specialized applications. Here are a few of the ones you are most likely to encounter.

Figure 30-1 Nonmetallic sheathed cable (type NM or Romex).

The cable types designated NM and NMC (Fig. 30-1) by the National Electrical Code (NEC) are commonly known by the trade name "Romex." They are nonmetallic sheathed cables with a plastic outer jacket. The enclosing sheath is both moisture resistant and flame-retardant. The NEC limits the use of Romex cable to residential one- and two-family dwellings not over three floors in height. These are typically wood frame buildings. Romex cables are easier to handle but are more vulnerable to physical damage than BX cables.

The most common type of exposed wiring is probably BX cable, which is frequently used in residences and in rewiring existing buildings. Officially, BX cable is called NEC type AC cable (Fig. 30-2), and it is also known as flex cable or armored cable. Two or more conductors are wrapped in heavy paper or plastic and encased in a continuous spiral-wound interlocking strip of steel tape. BX cable is common in commercial applications. It is installed with U-clamps or staples against beams and walls. In new construction, the NEC requires BX cable to be secured at specified intervals. In existing construction, it can be fished through walls, floors, and ceilings for renovations. BX cable is often used to connect rectangular fluorescent lights in suspended ceiling grids to allow for flexibility in relocation. BX is allowed only in dry locations. BX cable is restricted in some jurisdictions even where the NEC allows it.

Under Carpet Power Cable Type Fcc
Figure 30-3 Flat conductor cable (NEC Type FCC) as installed under carpet squares.

Metal clad cable is NEC type MC, and is often used where BX cable is restricted. It looks similar to BX, but has an additional green ground wire, providing extra grounding protection. MC cable may be used exposed or concealed and in cable trays. When covered with a moisture-impervious jacket, it is permitted in wet locations and outdoors.

NEC type FCC is also called flat wire and flat conductor cable (Fig. 30-3). It is a small factory-assembled cable in a flat housing and, when used under carpet tiles, it doesn't make a bump on the surface. Three or more flat copper conductors are placed edge-to-edge and enclosed in an insulating material. The assembly is covered with a grounded metal shield that provides both physical protection and a continuous electrical ground path. The bottom shield is usually heavy polyvinyl chloride (PVC) or metal. The whole assembly is only about 0.76 mm (0.03 in.) high, and is essentially undetectable under carpet.

The use of easily removable carpet squares means that the flat cable system can be repositioned to meet changing furniture layouts with minimum disruption and no structural work. The cable is designed for use under office furniture and traffic without damage to its electrical performance. Accessories are available for connection to 120 V power outlets. Flat cable systems are low cost and offer flexibility for open-plan offices. They are often used in new construction and to rework obsolete wiring systems in existing buildings. The NEC prohibits the use of flat cables in wet and hazardous areas and in residential, hospital, and school buildings. Flat cable layouts are usually shown on a separate electric plan.

Not all conductors are wires and cables. When larger amounts of current are carried from a source to a distribution point, cable assemblies are used. These come in a variety of forms. Some electrical distribution systems combine the conductor and enclosure in one piece. They include all types of factory-prepared and factory-constructed integral assemblies such as busways, bus-ducts, and cablebuses. Flat cable assemblies and lighting track and manufactured wiring systems are also considered to be in this category.

The terms "busway" and "busduct" are often used interchangeably for assemblies of copper or aluminum bars in a rigid metallic housing (Fig. 30-4). Such assemblies are preferred when it is necessary to carry a large amount of current that can be tapped at frequent intervals along its length. Light duty busduct or busway is used for feeder or branch circuits. Connections can be changed easily.

Busways offer plug-in receptacles that can be posi- ^ tioned wherever needed. You have probably seen light-duty plug-in busways in countertops and along laboratory or workbenches, where it a less expensive alternative to installing many separate convenience outlets. Busways can provide 20 to 60 A at 300V in two- and three-wire constructions. Heavier 100-A power at 600V in three- and four-wire constructions is used for direct connection of machine tools, light machinery, and industrial lighting. A plug-in busway makes connection simple and quick with a plug-in device, and saves large amounts of expensive hand labor to connect to cables or conduit frequently, as in machine shops and workshops. A plug-in busway is used for direct connection of light machinery and industrial lighting. Heavy-duty busduct (Fig. 30-5) is used for the vertical feeders in high-rise buildings, for example, to connect a basement switchboard to a penthouse machine room.

Cablebus is similar to ventilated busduct with insulated cables instead of bus bars, rigidly mounted in

Busduct supported from ceiling

Figure 30-5 Busduct.

an open space frame. This allows overhead power distribution throughout a large open space. The circulation of open air carries off heat, permitting a higher ampacity. However, cablebus is bulky, and it can be difficult to make tap-offs.

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