All metals conduct electricity but have different resistances. Some metals, like gold or silver, have very low resistance, but they do not have the tensile strength required for electrical wire and are too costly. Consequently, only two metals are used extensively in buildings as conductors, copper and aluminum. The choice of copper or aluminum will be based on installed cost, and since aluminum conductors are less costly than copper conductors, one would expect that aluminum conductors would always be chosen. However, several factors should be considered: To carry a given amount of current, a larger aluminum wire is needed, and its raceway may also need to be larger. Because aluminum conductors expand more than copper as they warm up under load, they tend to move back and forth at terminals and, unless the proper termination methods and wiring devices (marked CO/ALR) are used, the conductors may work loose and create a fire hazard. Also, some local building codes and agency standards either do not permit aluminum conductors to be used, or restrict their use to larger wire sizes.
Conductors may be solid round wire, stranded wire, or bus bars of rectangular cross section. Usually, conductors are wrapped in insulation of a type that prevents electric shock to persons in contact with it. The type of insulation also depends on the immediate environment surrounding the wire in its proposed use; dry or moist air, wetness, buried in earth, temperature, and exposure to mechanical or rodent damage.
Each size of commercial wire with a particular insulation is given by building codes a safe current-carrying capacity in amperes, called the ampacity of that wire.
The code ampacity is based on the maximum heating effect that would be permitted before damage to the insulation.
The codes also require that wires installed in a building be protected from mechanical damage by encasement in pipes, called conduits, or other metal and nonmetallic enclosures, termed raceways.
The safety regulations for use of wire in buildings are given in local building codes, which are usually based on the National Fire Protection Association ‘‘National Electric Code’’ (see Art. 15.8). These codes are revised frequently, so the engineer should determine which edition should be followed. There is the temptation to simply use the edition most recently published, but since these codes are adopted by the local authorities, the engineer should check with the authority having jurisdiction to determine the latest edition that has been adopted.
Another agency, Underwriters Laboratories, Inc., tests electric materials, devices, and equipment. If approved, the item carries a UL label of approval.
Major Distribution Conductors
Most buildings receive their electrical power supply through service conductors from the street mains and transformers of a public utility.
The service conductors may be underground or above ground if taken from a utility-system pole. If a building is set back a great distance from the street poles, additional poles can be installed on the customer’s property or the service conductors may be placed in an underground conduit extending to the building from the street pole. The engineer must design electrical services to comply with the requirements of the local electric utility.
At the building end, the service conductors come into a steel entrance box mounted on the building wall and then are brought to a service switch or circuit breaker. Service switches are commercially available up to 6000 A. Where the service load is greater, two or more service switches can be installed, up to a limit of six main service switches, called drops, on each service.
For large buildings, where six drops are not sufficient, the utility will install additional services with six drops available on each service. The utility may also provide medium-voltage service to larger buildings. In this case, the building owner must provide and maintain the building service transformer in addition to the other equipment described above.
Each service switch feeds a distribution center or groups of distribution centers, called panelboards. The connection between switch and panelboard is called a feeder. These main distribution panelboards consist of several circuit breakers or fused switches. Each of these breakers or switches feeds a load, either a motor or another remote panelboard or group of panelboards. The panelboards, in turn, serve branch circuits connected to lighting, wall receptacles, or other electrical devices.
Distribution systems in buildings are usually three-phase, four-wire. The final branch circuits are generally single-phase, two-wire. One wire in each circuit is grounded. The grounded circuit conductor in a feeder is colored white or natural gray, in accordance with the color code of the ‘‘National Electrical Code.’’ The three, phase conductors must also be color coded. This is necessary to ensure that phases are not crossed, to allow balancing of the loads on each phase, and to ensure proper rotation of motors. Colored insulation materials may be used for wires up to No. 6 size. For larger size wires, the phase wires may be identified by applying colored markings at the connections using colored tape.
In a branch circuit, the equipment grounding conductor is colored green. When several grounded conductors are in one feeder raceway, one of the grounded conductors should be colored white or gray. The other grounded conductors should have a colored stripe (but not green) over the white or gray, and a different color should be used for the stripe on each wire. For four-wire systems, the colors for ungrounded conductors are usually blue, black, and red, with white used for the grounded conductor.
Conductor ampacity depends on the accumulative heating effect of the IR power loss in the wire. This loss is different for a given size wire with different insulations and depends on whether the wire is in open air and can dissipate heat or confined in a closed conduit with other heat-producing wires. Tables in the ‘‘National Electrical Code’’ give the safe ampacity for each type of insulation and the derated ampacity for more than three current-carrying wires in a raceway.
Types of Insulated Conductors
Following is a list of the various types of insulated conductors rated in the National Electrical Code:
Type MI. Mineral-insulated cable sheathed in a watertight and gastight metallic tube. Cable is completely incombustible and can be used in many hazardous locations and underground. MI cable can also be fire-rated, making it acceptable as a fire-pump feeder.
Type MC. One or more insulated conductors, sheathed in an interlocking metal tape or a close-fitting, impervious tube. With lead sheath or other impervious jacket, Type MC may be used in wet locations.
Type AC. (Also known as BX cable.) This has an armor of flexible metal tape with an internal copper bonding strip in close contact with the outside tape for its entire length. This provides a grounding means at outlet boxes, fixtures, or other equipment. Type AC cable may be used only in dry locations.
Type ACL. In addition to insulation and covering as for Type AC, Type ACL has lead-covered conductors. This makes this type suitable for wet or buried locations.
Type ACT. Only the individual conductors have a moisture-resistant fibrous covering.
Type NM or NMC. Nonmetallic-sheathed cables (also known as Romex). This type may be used in partly protected areas. The New York City Code permits BX (Type AC) but does not allow Romex because it is not rodentproof and is subject to nail damage in partitions.
Type SE or USE. Service-entrance cable has a moisture-resistant, fire-resistant insulation with a braid over the armor for protection against atmospheric corrosion. Type USE is the same as Type SE, except that USE has a lead covering for underground uses.
Type UF. This type is factory assembled in a sheath resistant to flames, moisture, fungus, and corrosion, suitable for direct burial in the earth. The assembly may include an uninsulated grounding conductor. Cables may be buried under 18 in of earth or 12 in of earth and a 2-in concrete slab.
Two insulated conductors within a nonmetallic jacket or extruded thermoplastic cover may be used for surface extensions on walls or ceilings or as overhead cable with a supporting steel cable made part of the assembly. Extensions may be used in dry locations within residences or offices.
Aerial cables may be used only for industrial purposes. At least 10 ft should be provided above the floor as clearance for pedestrians only, 14 ft for vehicular traffic.
Cable Bus and Busways
Busways are bar conductors of rectangular cross section, which are assembled in a sheet-metal trough. The conductors are insulated from the enclosure and each other.
Busways must be exposed for heat dissipation. They are arranged with access openings for plug-in and trolley connections.
For heavy current loads, such as services, several insulated cables may be mounted in parallel, at least one diameter apart, within a ventilated metal enclosure with access facilities. Cable bus costs less than bus bars for the same load but generally takes up more space. Use is limited to dry locations.
A variety of devices are commercially available for connecting two or more wires.
One type, a pressure connector, called a wire nut, may be screwed over two or three wires twisted together. Another type consists of end lugs attached to wires by squeezing them together under great pressure with a special tool. The lugs have a flat extension with a bolt hole for connection by bolts to a switch or busway. As an alternative, two wires may be joined together in a similar manner with a barrelshaped
All metal connectors should be insulated with either tape or manufactured insulated covers and should be enclosed in a metal box with cover. Several connections properly insulated can be enclosed in the same metal box if the box is adequate in size. The number of spliced conductors in a box is limited by building codes.
A raceway is a general term used to describe the supports or enclosures of wires.
For most power distribution systems in buildings, rigid conduit or tubing is used.
The dimensions of such conduit or tubing and the number of wires of each size permitted is fixed by tables in the ‘‘National Electrical Code.’’ Three or more conductors may not occupy more than 40% of the interior area, with some exceptions for lead-sheathed cable. All metallic raceways must be continuously grounded.
One wide use of rigid steel conduit, galvanized, is for branch circuits buried in the concrete slabs of multistory buildings.
Electrical metallic tubing is a thin-walled tube that is permitted by codes in locations where the raceway is not subject to physical damage.
For economy in industrial installations, a continuous, rigid structure may be designed to carry both power and signal wiring. This structure may be in the form of a trough, a ladder run, or a channel. It is limited in use to certain cables specifically approved by Underwriters Laboratories for such use.
Flexible metallic conduit, also known as Greenfield, is a continuous winding of interlocking metal stripping similar to that used for Type AC metal-clad cables (BX). These conduits are often used in short lengths at the terminal connection of a feeder to a motor. For wet locations, a watertight-type (Sealtite) is available.
Surface raceways are usually oval shaped and flat. When painted the same color as the wall or ceiling, they are less conspicuous than round pipe conduit. Surface raceways with a larger, rectangular cross section may be used to mount receptacles or telephone or data outlets, in addition to housing wiring.
Access Floor Systems
In large computer rooms and in offices with heavy computer or communications
usage, such as a brokerage or a data center, an access floor system may be used.
This offers a false floor above the structural floor. The system consists of 2-ft by 2-ft removable panels, topped with a floor covering, which are supported from 6 to 36 in, or more, above the structural floor by pedestals and stringers. The space below the access floor is used for routing electrical, computer, and communication wiring. It is also used as a plenum for distributing conditioned air to the equipment and the occupied space. Since virtually the entire underfloor space is available and accessible, this system, though relatively expensive, offers flexibility for making changes in space use, such as adding equipment or rearranging room layouts.
Most modern offices undergo frequent relocation of staff due to workload, project teaming, or organizational changes. This high ‘‘churn rate’’ is made less of a burden to building managers by the use of system furniture. System furniture is a coordinated system of components including partitions, work surfaces, and storage elements that can be assembled into a variety of workstation configurations. Although, design of system furniture is not an electrical item of work, the task lighting, power, and voice / data elements are integral to the system. Individually controlled task lighting is provided for each workstation, as are power and voice / data outlets. To accommodate the required services, the specifications must include clear definition of the types and configuration of the electrical components. Furniture specifications will include wiring harness, power, lighting, and voice / data distribution as integral parts of the system. Particular attention should be paid to the method for feeding the system furniture from building services, capacity and bending restrictions of voice / data raceways (network cables) (see Fig. 15.7), and increased neutral currents caused by harmonic loads. Often, a wiring harness will have eight conductors; three phases, three neutrals (one per phase), an equipment ground conductor, and an isolated ground conductor.
Flat Conductor Cables (FCC)
These offer similar flexibility to that of an access floor system in that such cables permit outlets to be located anywhere in a room and allow easy relocation of an outlet. Flat conductor cables are available not only as power circuits but also in multiconductor, twisted pair, coaxial, and fiber-optic cables for use in communication and data systems. Manufacturers offer complete lines of power, data, and communication floor fittings for FCC system use. Use of FCC is limited to installation under carpet squares and is most commonly used in renovation work.