WIRE AND CABLING

Guidelines for the Installation of Cabling Systems in Industrial Facilities

There are a variety of ways to install power distribution cables in industrial facilities. The engineer's responsibility is to select the method most suitable for each particular application. Each method has characteristics that make it more suitable for certain conditions than others; that is, each method will transmit power with a unique combination of reliability, safety, economy, and quality for a specific set of conditions. These conditions include the quantity and characteristics of the power being transmitted, the distance of transmission, and the degree of exposure to adverse mechanical and environmental conditions.

Layout
The first consideration in wiring systems layout is to keep the distance between the source and the load as short as possible. This consideration should be tempered by many other important factors to arrive at the lowest cost system that will operate within the reliability, safety, economy, and performance required. Some other factors that must be considered for various routings are the cost of additional cable and raceway versus the cost of additional supports; inherent mechanical protection provided in one alternative versus additional protection required in another; clearance for and from other facilities; and the need for future revision.

This method was used extensively in the past. Although it has now been replaced in most applications, it is still quite often used for primary power distribution over large areas where conditions are suitable.

Open-wire construction consists of single conductors on insulators that are mounted on poles or structures. The conductors may be bare or have a covering or jacket for protection against corrosion or abrasion.

The attractive features of this method are its low initial cost and the fact that damage can be detected and repaired quickly. On the other hand, the non-insulated conductors are a safety hazard and are also very susceptible to mechanical damage and electrical outage from birds, animals, lightning, etc. There is an increased safety hazard where crane or boom truck use may be involved. In some areas, insulator contamination or conductor corrosion can result in increased maintenance costs.

Due to the large conductor spacing, open wire circuits have a higher reactance than circuits with more closely spaced conductors, producing a larger voltage drop. This problem is reduced by operating at a higher voltage and higher power factor. Exposed open wire circuits are more susceptible to outages from lightning than other installation methods. The problem may be minimized through the use of overhead ground wires, surge arresters, or special insulators.

Aerial cable
Aerial cable is usually used for incoming or service distribution between commercial buildings. As a logical replacement for open wiring, it provides greater safety and reliability and requires less space. Properly protected cables are not a safety hazard and are not easily damaged by casual contact. They are, however, open to the same objections as open wire in regard to vertical and horizontal clearances. Aerial cables are frequently used in place of the more expensive conduit systems, where the mechanical protection of the conduit is not required. They are also generally more economical for long runs of one or two cables than are cable tray installations. It is cautioned that aerial cable having a portion of the run in conduit must be derated to the ampacity in conduit for this condition.

Aerial cables may be either self-supporting or messenger-supported. They may be attached to pole lines or structures. Self-supporting aerial cables have high tensile strength conductors for this application.

Multiple single conductors, Types MV, THW, RHH or RHW, both without outer braids; or multiconductor cables, Types MI, MC, SE, UF, TC, MV, or other factory-assembled multiconductor control, signal, or power cables that are identified for the use in NEC, Article 321, may be messenger-supported.

Cables may be messenger-supported either by spirally wrapping a steel band around the cables and the messenger or by pulling the cable into rings suspended from the messenger. The spiral wrap method is used for factory-assembled cable, while both methods are used for field assembly. A variety of spinning heads are available for application of the spiral wire banding in the field. The messenger used on factory-assembled messenger-supported wiring is required to be copper-covered steel or a combination of copper-covered steel and copper, and the assembly must be secured to the messenger by a flat copper binding strip. Single insulated conductors should be cabled together.

Factory-preassembled aerial cables are particularly susceptible to installation damage from high stress at support sheaves while being pulled in.

Self-supporting cable is suitable for only relatively short spans. Messenger-supported cable can span longer distances, depending on the weight of the cable and the tensile strength of the messenger. The supporting messenger provides the strength to withstand climatic rigors or mechanical shock. The messenger must be grounded in accordance with the NEC.

A convenient feature available in one form of factory assembled aerial cable makes it possible to form a slack loop to connect a circuit tap without cutting the cable conductors. This is done by reversing the direction of spiral of the conductor cabling every 10-20 ft.

Spacer cable is an electric distribution line construction that consists of an assembly of one or more covered conductors separated from each other and supported from a messenger by insulating spacers. This is another economical means of transmitting power overhead between buildings. Available for use in three-phase 5-15 kV grounded or ungrounded systems, the insulated nonshielded phase conductors provide protection from accidental discharge through contact with ground level equipment, such as aerial ladders or crane booms. Uniform-line electrical characteristics are obtained through the balanced geometric positioning of the conductors with respect to each other by the use of plastic or ceramic spacers located at regular intervals along the line. Low terminating costs are obtained because the conductors are non-shielded.

Open runs
This is a low-cost method where adequate support surfaces are available between the source and the load. It is most useful in combination with other methods, such as branch runs from cable trays, and when adding new circuits to existing installations.

This method employs multiconductor cable attached to surfaces, such as structural beams and columns. Type MC cable is permitted to be installed in this manner in industrial facilities as well as power-limited control and telephone circuits. For architectural reasons in office buildings, it is usually limited to service areas, above hung ceilings, and electric shafts.

Cable tray
A cable tray is defined in the NEC as "a unit or assembly of units or sections, and associated fittings, forming a rigid structural system used to support cables and raceways." These supports include ladders, troughs, and channels, and have become very popular in industrial electric systems for the following reasons: low installation cost, system flexibility, improved reliability, accessibility for repair or addition of cables, and space saving when compared with conduit where a larger number of circuits with common routing are involved.

Cable trays are available in a number of styles, materials, and mechanical load-carrying capabilities. Special coatings or materials for corrosion protection are available. Initial planning of a cable tray should consider occupancy requirements as given in the NEC and also allow additional space for future system expansion.

Covers, either ventilated or nonventilated, may be used when additional mechanical protection is required or for additional electrical shielding when communication circuits are involved. Where cable trays are continuously covered for more than 6 ft with solid, unventilated covers, the cable ampacity rating must be derated as required by the NEC, Section 318.

A solid fixed barrier is required for separation of cables rated over 600 V from those rated 600 V or less. Barrier strips are not required when the cables over 600 V are Type MC.

Seals or fire stops may be required when passing through walls, partitions, or elsewhere to minimize flame propagation.

In stacked tray installations, it is good practice to separate voltages, locating the lowest voltage cables in the bottom tray and increasingly higher voltage cables in ascending order of trays. In a multiphase system, all phase conductors should be installed closely grouped in the same tray.

A cable tray provides a convenient economical support method when more than three cables are being routed in the same direction. Single conductors of size 1/0 AWG and larger, that are identified for the use, are permitted in cable tray in industrial establishments. Type MC cable can be installed in cable tray and, when only one or two cables have to be routed to a separate location, the cable can then be installed as open runs of cable. Type TC cable, as well as single conductors, requires the use of a raceway between the cable tray and the termination point.

The steel or aluminum metal in a cable tray can also be used as an equipment grounding conductor when the tray sections are listed by a nationally recognized testing laboratory as having adequate cross-sectional area and are bonded using mechanical connectors or bonding jumpers. Refer to the NEC, Section 3 18-7, for complete requirements.

Cable bus
Cable bus is used for transmitting large amounts of power over relatively short distances. It is a more economical replacement of conduit or busway systems, but more expensive than cable tray. Cable bus is also more reliable, safer, and requires less maintenance than openwire or bus systems.

Cable bus is a hybrid between cable tray and busway. It uses insulated conductors in an enclosure that is similar to cable tray with covers. The conductors are supported at maintained spacings by nonmetallic spacer blocks. Cable buses are furnished either as components for field assembly or as completely assembled sections. The use of completely assembled sections is recommended when the run is short enough that splices may be avoided. Multiple sections requiring joining may preferably employ the continuous conductors.

The conductors are generally spaced one cable diameter apart so that the rating in air may be attained. This spacing is also close enough to provide low reactance, resulting in minimum voltage drop.

Conduit
Among conduit systems, rigid steel provides the greatest degree of mechanical protection available in above-ground conduit systems. Unfortunately, this is also a relatively high cost system. For this reason, it is being replaced, where possible, by other types of conduit and wiring systems. Where applicable, rigid aluminum, rigid nonmetallic conduit (NMC), electrical metallic tubing (EMT), intermediate metal conduit (IMC), electrical nonmetallic tubing (ENMT), and plastic, fiberglass, and cement ducts may be used. Cable trays and open runs of Type MC cable are also being utilized.

Conduit systems offer some degree of flexibility in permitting replacement of existing conductors with new ones. However, in case of fire or short-circuit current faults, it may be impossible to remove the conductors. In this case, it is necessary to replace both conduit and wire at great cost and delay. Also, during fires, conduits may transmit corrosive fumes into equipment where these gases can do a lot of damage. To keep flammable gases out of such areas, seals must be installed.

With magnetic conduits, an equal number of conductors of each phase must be installed in each conduit; otherwise, losses and heating will be excessive. For example, a single conductor should not be installed in steel conduit.

Refer to the NEC for regulations on conduit use.

Underground ducts are used where it is necessary to provide good mechanical protection. For example, when overhead conduits are subject to extreme mechanical abuse or when the cost of going underground is less than providing overhead supports. In the latter case, direct burial (without conduit) may be satisfactory under certain circumstances.

Underground ducts use rigid steel, plastic, or fiberglass conduits encased in concrete, or precast with multihole concrete duct banks with close fitting joints. When the added mechanical protection of concrete is not required, heavy wall versions of fiberglass conduits are direct buried as are rigid steel and plastic conduits. Medium-voltage, low-voltage, signal and communications systems should not be installed in the same manhole. Manholes intended for cable splices or for drain provisions on long length cables should have adequate provisions for grounding.

Cables used in underground conduits must be suitable for use in wet areas. Some cost savings can be realized by using flexible plastic conduits with factory installed conductors.

Where a relatively long distance between the point of service entrance into a building and the service entrance protective device is unavoidable, the requirements of the NEC, Section 230-6, apply. The conductors must be placed under at least 2 inches of concrete beneath the building; or they must be placed in conduit or duct and enclosed by concrete or brick not less than 2 in ches thick. They are then considered outside the building.

Direct burial
Cables may be buried directly in the ground where permitted by the NEC when the need for future maintenance along the cable run is not anticipated nor the protection of conduit required. The cables used must be suitable for this purpose; that is, they must be resistant to moisture, crushing, soil contaminants, and insect and rodent damage. Direct buried cables rated over 600 V must be shielded and provide an exterior ground path for personnel safety in the event of accidental dig-in. Multiconductor nonshielded Type MC cables rated up to 5000 V are also permitted to be direct buried. Refer to the NEC, Tables 300-5 and 7 10-3(b), for minimum depth requirements.

The cost savings of this method over duct banks can vary from very little to a considerable amount. Cable trenching or burying machines, when appropriate, can significantly reduce the installation cost of direct buried cable, particularly in open field construction, such as in industrial parks. While this system cannot readily be added to or maintained, the current-carrying capacity of a cable of a given size is usually greater than that for cables in ducts. Buried cable must have selected backfill for suitable heat dissipation. It should be used only when the chances of its being disturbed are minimal or it should be suitably protected. Relatively recent advances in the design and operating characteristics of cable fault location equipment and subsequent repair methods and material have diminished the maintenance mean time to repair.

Hazardous (classified) locations
Wire and cable installed in locations where fire or explosion hazards may exist must comply with the NEC, Articles 500 through 517. The authorized wiring methods are dependent upon the Class and Division of the specific area (see table 12-7). The wiring method must be approved for the class and division, but is not dependent upon the group, which defines the hazardous substance.

Equipment and the associated wiring system approved as intrinsically safe is permitted in any hazardous location for which it has been approved. However, the installation must prevent the passage of gases or vapors from one area to another. Intrinsically safe equipment and wiring is not capable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific flammable or combustible atmospheric mixture in its most easily ignitable concentration.

Seals must be provided in the wiring system to prevent the passage of the hazardous atmosphere along the wiring system from one division to another or from a Division I or II hazardous location to a nonhazardous location. The sealing requirements are defined in the NEC, Articles 501 through 503. The use of multiconductor cables with a gas/vapourtight continuous outer sheath, either metallic or nonmetallic, can significantly reduce the sealing requirements in Class 1, Division 2 hazardous locations.

Installation procedures
Care must be taken in the installation of raceways to ensure that no sharp edges exist to cut or abrade the cable as it is pulled in. Another important consideration is to not exceed the maximum allowable tensile strength or the manufacturer's recommendation for the maximum sidewall pressure of a cable.

These forces are directly related to the force exerted on the cable while it is being pulled in. The forces can be decreased by shortening the length of each pull and reducing the number of bends. The force required for pulling a given length can be reduced by the application of a pulling compound on cables in conduit and the use of rollers in cable trays.

When the cable is to be pulled by the conductors, the maximum tension in pounds is limited to 0.008 times the area of the conductors, in circular mils, within the construction.

The allowable tension should be reduced by 20-40 per cent when several conductors are being pulled simultaneously since the tension is not always evenly distributed among the conductors. This allowable tension must be further reduced when the cable is pulled by a grip placed over the outer covering.

A reasonable figure for most jacketed constructions would be 1000 lb. per grip; but the calculated conductor tension should not be exceeded. Pulling eyes, connected to each conductor, provide the maximum allowable pulling tension. Reusable pulling eyes are available.

Sidewall pressures on most single conductors limit pulling tensions to approximately 450 lb. times the cable diameter (inches) times the radius of the bend (feet). Triplexed and paralleled cables would use their single conductor diameters and a factor of 225 lb. and 675 lb. respectively, instead of the 450 lb. factor for a single conductor.

For duct installations involving many bends, it is preferable to feed the cable into the end closest to the majority of the bends (since the friction through the longer duct portion without the bends is not yet a factor) and pull from the other end. Each bend gives a multiplying factor to the tension it sees; therefore, the shorter runs to the bends will keep this increase in pulling tensions to a minimum. However, it is best to calculate pulling tensions for installation from both ends of the run and install from the end requiring the least tension.

The minimum bending radii is 8 times the overall cable diameter for nonshielded single and multiconductor cables and 12 times for metal tape shielded or lead covered cables. The minimum bending radius for nonshielded Type MC cable with interlocking armor or a corrugated sheath is 7 times the overall diameter of the metallic sheath; for shielded cables, the minimum bending radius is 12 times the overall diameter of one of the individual conductors or 7 times the overall diameter of the multiconductor cable, whichever is greater. Type MC cable with a smooth metallic sheath requires a greater minimum bending radius; refer to the NEC, Section 334-11. The minimum bending radius is applicable to bends of even a fraction of an inch in length, not just the average of a long length being bent.

When installing cables in wet underground locations, the cable ends must be sealed to prevent entry of moisture into the conductor strands. These seals should be left intact or remade after pulling if disrupted, until splicing, terminating, or testing is to be done. This practice is recommended to avoid unnecessary corrosion of the conductors and to safeguard against entry of moisture into the conductor strands, which would generate steam under overload, emergency loadings, or short-circuit conditions after the cable is energized.