cable, vdsl, patch
What is a Cable Tray System? Per
the National Electrical Code, a cable tray system is "a unit or assembly
of units or sections and associated fittings forming a rigid structural system
used to securely fasten or support cables and raceways."
* Cable trays support cable the way that roadway bridges support traffic. What standards / guidelines are available for cable tray systems?
1. The National Electrical Code publishes the standards for all types of electrical
applications. Articles 318, 250, and 800 cover various aspects of cable tray systems. NEMA VE1 covers general cable tray definitions, manufacturing standards, performance standards, test standards, and application information. Free download of this document is available on the NEMA website. NEMA FG1 addresses the standards for fiberglass cable tray systems. Free download of this document is available on the NEMA website.
NEMA VE2 is a cable tray installation guideline which covers receiving and unloading
material, storage of material, and general installation practices. Free download
of this document is available on the NEMA website. What types of Cable Tray are available?
1. Ladder Tray How do I know what type of cable tray is right for my application?
1. Ladder Cable Tray provides: What materials / finishes are available for the various cable tray systems?
1. Steel (Min. Yield = 33KSI) (35 KSI for Stainless) Now that I know what types of cable trays are available, what configurations are available?
1. Straight sections are available to route cables in a horizontal or vertical
plane. After selecting the type of cable tray and configuration required, what support methods are available?
1. Trapeze Support (Single or Multi-tier) Each
of these support methods are preferable in different applications. For instance,
trapeze supports may be desired in an application where cables will be pulled
through the cable tray. Center hung supports, on the other hand, are generally
used when cables will be installed from the side of the cable tray. Center hung
supports are especially useful when future cable additions are desired. Wall supports
and underfloor supports are useful when ceiling structure is not available or
undesired. Outdoor installations are controlled by the structures available to
support the cable tray. Where? Job site and installation considerations
2. What? Cable Tray Systems in Ducts, Plenums and Other Air Handling Space The objective of this article to provide clear information as to the use of cable tray in those areas covered by Section 300-22 of the 1996 National Electrical Code. Section 318-4 Uses Not Permitted states that "Cable tray systems shall not be used in environmental air spaces except as permitted in Section 300-22 to support wiring methods recognized for use in such spaces. The wiring methods allowed under Section 300-22 that utilize cable tray must follow the installation and safety requirements as covered in Section 318 - Cable Tray." Many
of the misinterpretations about cable tray are due to the fact that those misinterpretations
are made with the thought that cable tray is a raceway. It is not a raceway and
it has never been a raceway in the National Electrical Code. Cable tray is a mechanical
support system just as strut is a mechanical support system. To install a metal
support system in an area rarely presents a fire safety problem. It is the cables
that are being supported by the cable trays that limit where a cable tray wiring
system may be installed. The only limitation on the cable tray is that it can't
be used in hoistways or where subject to severe physical damage. Any type of cable
tray may be installed in the areas covered by Sections 300-22(b), 300-22(c) and
300-22(d). The section states that Type MI (Mineral Insulated) cable or Type MC (Metal Clad) cable employing an impervious metal sheath without an overall non-metallic covering may be installed in Ducts or Plenums Used for Environmental Air. For such installations, both of these cable types may be supported by cable tray. Section 318-3(a)(1) states that Type MI cable may be installed in cable tray for support. Section 330-12. Exception No. 2. states that "Type MI cable installed in cable trays shall comply with Section 318-8(b)." Ladder or ventilated trough cable tray is an ideal support system for Type MI cable. Where small numbers of Type MI cables are involved, ventilated channel cable tray is the ideal support system. Type MI cable is an excellent cable for critical circuits. It has a UL two hour fire resistive rating when properly installed. It is safest wiring method available. Sections
318-3(a)(4) and 334-3(6) state that Type MC cable may be installed in cable tray
for support. Section 334-10(b) states that "Type MC cable installed in cable
tray shall comply with Article 318." Large amounts of the various types of
Type MC cable have been installed in cable tray. The performance record has been
excellent. The Cable Tray Institute's Hot Line has received many requests for technical clarification assistance concerning Section 300-22(c). There are two problems with the material relating to cable tray in this section. 1. The wording in the second paragraph "or solid bottom metal cable trays with solid metal covers" implies that the types of insulated single conductors that are installed in raceways may also be installed in solid bottom cable trays with solid metal covers. Due to the present wording of Section 300-22(c), there have been some installation made that are not in compliance with Article 318. The cable tray was basically used as a wireway and in such cases the rules of Article 362 (Wireways) should apply. Depending on the specific installation, there may or may not be safety problems with such installations but Section 318-3(b) doesn't allow insulated single conductors to be installed in solid bottom cable trays.
Single conductor installations in cable tray have the following limitations: 2. Some individuals have made erroneous interpretations of Section 300-22(c) concerning the types of cable tray that may be installed in "Other Space Used for Environmental Air." They assume that the wording of the second paragraph means that only solid bottom metal cable tray with solid metal covers may be installed in these installations. This is incorrect. Ladder, ventilated trough, ventilated channel or solid bottom cable tray may be installed to support the applicable types of cables specifically listed for the use. Allowable Wiring Methods that may be supported by Cable Tray for Section 300-22(c) Installations. Type
MI cables, Type MC cables without an overall non-metallic covering, Type AC cables
and other factory-assembled multiconductor control, power and signal cables that
are specifically listed for the use. Some of the multiconductor cables that are
listed as plenum cables with adequate fire-resistance and low smoke producing
characteristics are Type PLTC Cables (Article725), Fire Protective Signaling Cables
(Section 760), Optical Fiber Cables (Article 770) and Communication and Multipurpose
Cables (Article 800). The appropriate types of cables that are used for branch circuit conductors and data handling or signal cables may be supported by cable tray under raised floors. The branch circuit cables in Section 645-5(d)(2) that may be supported in cable trays are Type MI cable, Type MC Cable and Type AC Cable. Section 645-5(d)(5) and Section 645-5(d)(5) Exception No. 3. list the various types of data and signal plenum cables with adequate fire-resistance and low smoke producing that may be installed in data processing facilities. These cables can be installed in any cable tray type. Due to the high wiring density, most raceway wiring methods are impractical for use in such installations while cable trays have the features which make them ideal for modern wiring methods. Wiring changes can be made easily where the wiring method is cables in cable trays. Cable trays are the way to go for a state of the art wiring method that can easily accommodate changes at minimum cost in short time schedules. Cable Tray Type Selection What type of cable tray should be used for the main runs of a cable tray wiring system? The cable tray types to choose from are ladder, ventilated trough, or solid bottom. What are the reasons for selecting a specific type of cable tray? The engineer or designer should select the type of cable tray that has the features which best serve the project's requirements. For a few types of installations, the National Electrical Code (NEC) specifies the cable tray type to be used: Single conductor cables and Type MV cables must be installed in ladder or ventilated trough cable trays. Single conductor cables and Type MV cables are not allowed to be installed in solid bottom cable trays [1993 NEC Section 318-3(b)] In
Class II, Division 2 Hazardous (Classified) Locations (Dust), the types of cables
that are allowed to be installed in cable trays must be in ladder or ventilated
trough cable trays. Solid bottom cable trays are not allowed to be installed in
Class II, Division 2 locations [1993 NEC Section 02-(b)]. Ladder cable tray is used for about 75 percent of the cable tray wiring system installations. It is the predominate cable tray type due to its many desirable features:
* A ladder cable tray without covers permits the maximum free flow of air across
the cables. This allows the heat produced in the cable's conductors to effectively
dissipate. Under such conditions, the conductor insulation in the cables of a
properly designed cable tray wiring system will not exceed its maximum operating
temperature. The cables will not prematurely age due to excessive operating temperatures. Ventilated Trough Cable Tray The
only reason to select a ventilated trough cable tray over a ladder type cable
tray is aesthetics. No drooping of small cables is visible. The ventilated trough
cable tray does provide more support to the cables than does the ladder cable
tray but this additional support is not significant. It doesn't have any impact
on the cables service record or life. The main reason for selecting solid bottom cable tray (with covers) is the concern of EMI/ RFI shielding protection for very sensitive circuits. A solid bottom steel cable tray with steel covers provides a good degree of shielding if there are no breaks or holes in the completed installation. The solid bottom cable tray system has a disadvantage in that moisture can build up in the cable trays. This can be controlled by drilling 1/4 inch drain holes in the bottom of the cable tray at three foot intervals (at the middle and very near the sides) if the cable tray is not being used for EMI/RFI shielding. Some engineers and designers specify solid bottom cable trays (often with covers) in the belief that all electrical circuits have to be totally enclosed by metal. The cable trays are just supporting cables that are designed for such installations. Cable failures in cable tray runs rarely happen. Cable failures due to cable support problems in cable trays are nonexistent. Cable Tray Wiring Systems Have Many Cost Advantages Cost is usually a major consideration in the selection of a wiring system. This article provides information as to where cable tray wiring system cost savings will occur; however, it is not the intent of this article to state that the selection of a wiring system should be based only on cost. Early in the life of a project, the costs and the features of the applicable wiring methods should be evaluated to provide decision information for the selection of the best possible wiring method or methods for the project. The evaluations should include items that relate to cost, dependability, future changes, maintenance, safety, and space savings. Usually the evaluation will determine if a cable tray wiring system or a conduit wiring system is to be selected as the projects major wiring system. Both large scale and small cable tray wiring systems have been in use for the last 45 years in North America and longer in other parts of the world. Forty-five years of operating experience has proven that cable tray wiring systems are superior to conduit system wiring systems for power, control signal and instrumentation circuits. The following functions must be properly executed to obtain a quality wiring system installation:
1. Select the most desirable wiring method. Depending on the type of circuits and the wiring density, an installed cable tray wiring system may result in a total cost reduction (material + labor) of up to 60 percent compared to the cost of an equivalent conduit wiring system. There is also the potential for cost savings to occur in the design, material procurement, installation and maintenance areas when the wiring system is a cable tray wiring system. Potential Design Cost Savings: 1.Very few projects are completely defined at the start of design. As a project progresses through the design phase, the operating logic and safety requirements are developed and refined. The changes and additions required to meet the projects needs occur all through the design cycle and at times even into the initial construction phase. For projects that are not 100 percent defined before the start of design, the cost of and time used to cope with changes during the engineering and drafting design phases will be substantially less for a cable tray wiring system than for an equivalent conduit system.
It only takes a few minutes of design time to change the width of a cable tray
to gain significant additional cable fill capacity. For an additional cost of
less than 10 percent of the basic cable tray cost, 6 inches of additional cable
tray width can be obtained. This extra 6 inches will accommodate large numbers
of small diameter analog and/or digital signal cables. Where banks of conduits
are involved, any change in wiring capacity requirements during the late stages
of engineering and drafting design are very costly and time consuming. Significant
conduit system additions or revisions are usually required to provide exit and/or
entry points in the conduit runs for the circuit additions made late in the design
phase. Cable tray's unique feature that allows a cable to enter or exit a cable
tray anywhere along the cable tray's route provides for the easy accommodation
of cable additions. No raceway wiring system has this unique feature. Potential Material Procurement Costs Savings:
1. There are fewer different components in a cable tray wiring system than in
a conduit wiring system. Fewer different components means savings due to fewer
components to specify, order, receive, store and distribute. Potential Installation Cost Savings:
1. The installation of a cable tray wiring system requires fewer man-hours than
an equivalent conduit wiring system. This is where the major cost savings are
obtained for the cable tray wiring system. Smaller sized electrician crews may
be used to install a cable tray wiring system as compared to an equivalent conduit
wiring system. This allows for manpower leveling, the peak and the average crew
size would be almost the same number. The electrician experience level required
for cable tray can be lower than that for a conduit wiring system as fewer electrician
with conduit bending skills are required.
Concentric conduit bends for direction changes in conduit banks are very labor
intensive and costly. However if they are not used, the installation will not
be very attractive. The time required to make a concentric bend is increased by
a factor of three to six over that of a single shot conduit bend. This labor intensive
practice is eliminated when cable tray wiring system are used. Potential Maintenance Cost Savings: 1. An article in the October 1991 EC&M magazine, "Cable Pulling for Conduit Wiring Systems," stated that 92 percent of the insulated conductors that fail do so due to the fact that they were damaged during installation. The failures of the insulated conductors may create unnecessary safety conditions and significant cost problems. Why not select a wiring method where during the past 45 years its conductor failures due to installation damage have been almost non-existent? Cable tray with quality cables is that wiring method.
Conductor insulation failures in cable tray wiring systems are rare. The reason
for this that the tray cables are rarely damaged during the installation. Many
of the conduit conductors that fail do so due to the fact that they have been
damaged when they were pulled into the conduits. Excessive forces imposed on the
conductor's insulation system during the conductor installation process can be
very destructive. For some critical combinations of conductors and sizes of conduit,
jamming of the conductors in the conduit can occur during the conductor installation.
This may result in conductor insulation damage. Critical jam ratio (J.R. = Conduit
ID/Conductor OD) values range from 2.8 to 3.2. The 1996 NEC Chapter 9 Table 1.
Fine Print Note is an alert for this serious problem. In the July 1995 EC&M magazine, "Protecting Life Safety Circuits In High Rise Buildings" the section titled "Protecting signal and communication wiring" states the following: "Results of Steiner Tunnel testing performed by various cable manufacturers actually indicates that conduits tend to act as heat sinks, thereby decreasing the time required to damage insulation to cause conductor failures." This is a big negative for conduit systems. Cable tray wiring systems have significant cost savings advantages over conduit wiring systems. They also have convenience, dependability and safety advantages over conduit wiring systems. Cable
Tray Grounding: Power, Instrumentation, and Telecommunications Grounding has always been a controversial topic. But, with the growth of digital high frequency systems the issues are more complex. Grounding means connected to earth or a conducting body that acts in place of earth. Some international standards refer to grounding as earthing. Bonding is the interconnection of metal parts to establish electrical continuity. These definitions are NEC terminology and apply to power system grounding. The purpose of grounding is:
* Fire Protection Power
System Grounding The purpose of power grounding (Article 250) is to minimize the damage from wiring or equipment ground fault. Cable tray systems are in the path of ground fault currents. Cable tray systems are bonded together through their bolting, connectors splice plates, clamps, and bonding jumpers where there are gaps in the cable tray system. Cable tray systems are not required to be mechanically continuous, but shall be electrically continuous. Cable trays are also bonded to conduit, cable channel or other wiring drops. They must also be bonded back to the power source. All bonding jumpers must be sized (as a minimum) to meet the requirements of equipment grounding conductors. Both side rails of the tray must be bonded together to the next section. Cable trays can be used as the only equipment grounding conductor (EGC), but they must meet certain criteria (only in qualifying facilities, minimum cross-sectional areas, U.L. classified as to suitability, etc., see NEC 318-7). There are other alternatives-use EGC’s in the cable (U.L. listed cable can be supplied with ECG’s in certain conductor sizes) or a separate EGC in the cable tray that bonds the cable tray sections together and can also be used to tap EGC’s to individual drop-outs from the CT. These two alternatives can be used for non-metallic cable trays. Cables with equipment ground conductors within the cable are an accepted practice in industry. They provide a two-point connection from the power source to the load, however, any conduit, cable tray, or raceway must still be bonded back to the power source. Some companies do not accept conduit as an EGC. The EGC system is a critical safety system. Therefore, it is prudent to treat the cable tray system as an equipment grounding conductor in parallel with the ground conductors in the cables or an individual ground conductor. Cable
Tray Grounding-Signal and Communication Circuits Cable tray designs are also available that are EMI/RFI shielded. The tray is totally enclosed and the gaskets and covers are constructed and tested to meet EMI standards for the protection of the sensitive circuits in the cable tray against external electric and magnetic fields. Solid bottom cable trays also provide some degree shielding as do cable tray covers. Steel provides effective shielding at frequencies up to approximately 100 kilohertz however at higher frequencies, in the megahertz range, aluminum or copper shielding is more effective. Cross
Talk The sensitivity of signal systems depends on a number of complex factors. Including electronic circuitry involved, isolation or coupling to ground, filtering, the signal type and logic, type of signal cable (untwisted pair, twisted pair, shielded twisted pair, coaxial cable double-shielded coaxial cable) and characteristic impedance of the circuit and cable. Some systems are quite tolerant to external noise. For instance, 4 to 20MA instrument signal systems and telecommunication circuits do quite well with respect to noise. Some companies and organizations have published their own recommended practices and they should be followed. The national standard that includes separation distances is the Institute of Electrical Electronic Engineers (IEEE) Standard 518, IEEE Guide for the Installation of Electronic Equipment to Minimize Electrical Noise Inputs to External Sources. The cable spacing criteria found in this standard is large, based on industry experience. Many systems work quite well with lesser distances. Much depends on the particular installation. Typical spacing of cables in trays used in various industry standards varies from two inches to four feet. In some situations, two inches is probably adequate. AC
Drives The noise frequency can be as high as 30MHZ. A number of IEEE papers have been presented on this topic. In particular, they provide detailed studies analysis and noise measurements using different types of motor power cable types. The conclusion is that one can manage this concern by proper grounding and power cable selection. At these frequencies, based on tests, the power cable should be shielded with a metal armor or foil either copper or aluminum. These studies and technical papers indicate that:
1. Shielded cable-either type TC or MC should be used Conclusion
* Signal cable Tie Down Practices for Multiconductor Cables in Cable Trays The value of any practice that increases a system's cost and complexity should be justified. Is the practice serving mandatory functions or is the practice encompassing both mandatory and useless functions? The objective should be to eliminate the useless functions and to serve the mandatory functions in the best possible manner at the lowest- possible cost. This is the approach that should be taken when developing standard practices for tying down multiconductor cables in cable trays. In many cable tray wiring systems, the tying down of multiconductor cables is a useless function. There
are three items which require decisions concerning the tying down of multiconductor
cables in cable tray wiring systems. Item #1 is to define under what conditions
the multiconductor cables in cable trays are to be tied down. Item #2 is to define
the frequency at which the multiconductor cables are to be tied down. Item #3
is to select the ties that have the proper characteristics for the specific installations.
In the following material, where the word cable is used it means multiconductor
cable. The reasons for tying down cables are to keep them in the cable trays, to maintain the proper spacing between cables, or to confine the cables to specific locations in the cable trays. National Electrical Code Section 318-8(b) states that in other than horizontal cable tray runs, the cables shall be fastened securely to transverse members of the cable trays. In horizontal cable tray runs, cables are not required to be tied down. The cable's weight will keep them in the cable trays. In non-horizontal cable tray runs, the cables must be tied down. For a vertical cable tray installation, the cables may hang away from the cable tray if they are not tied down. The more flexible small diameter cables will hang further away from the cable trays than the large diameter cables if they are not tied down. The smaller diameter cables will need to be tied to the cable tray more frequently than the stiff large diameter cables. Cable installations as per 1993 EC Sections 31811(a). Exception #2 and 318-13(a). Exception #2 require that a space equal to the diameter of the larger cable be maintained between adjacent cables for heat dissipation reasons. The way to make sure that this spacing is maintained is to tie down the cables. Type MC or TC cable installations as per 1993 NEC Section 502-4(b) require that a space equal to the diameter of the larger cable be maintained between adjacent cables to reduce the impact of dust build up and heat dissipation problems in Class II, Division 2 Combustible Dust Areas. The way to make sure that this spacing is maintained is to tie down the cables. There are installations where the owner may want the cables tied down to guarantee the separation of low energy signal cables and power cables. This condition may also be obtained by installing a permanent barrier in the cable tray. For installations where a single large cable or several cables are installed in ventilated channel cable trays, it is at times desirable to tie the cables to the horizontal as well as to the non-horizontal ventilated channel cable trays. Then if an abnormal condition occurs, the cables would not be knocked out of the ventilated channel cable trays which are only 1 1/2 inches high. Where Type MI cables are installed that are to have two hour fire resistant ratings, the MI cables must be securely supported every three feet. A desirable installation would be to install the MI cable in steel cable trays and to use stainless steel ties to secure the MI cable to the cable tray every three feet. Where
cables drop from the cable trays to equipment enclosures, it may be desirable
to anchor the cables to the last one, two or three rungs before the drop depending
on the size of the cables. This easily provides distributed secured support for
the cables when the length of cable between the cable tray and the equipment enclosure
is six feet or longer. The NEC doesn't specify any distances between ties for cables in cable tray wiring systems. This is a decision that must be made by those designing and installing the cable tray wiring systems. It is desirable to develop some standards for this activity. A conservative recommendation for non-horizontal cable trays is that the small diameter cables (diameters less than 1 inch) be tied down at approximately 3 foot intervals and that cables 1 inch and larger be tied down at approximately 6 foot intervals. The vertical cable wiring system installations that contain horizontal bends require the cables to be tied down at every or every other rung in the bend and to the first rung before entering the bend and the first rung after exiting the bend. In
horizontal cable trays where cable spacing is to be maintained, the cables should
be tied down at approximately 10 foot intervals. For horizontal ventilated channel
cable trays, there are installations containing a single large cable or several
cables where it is desirable to tie down the cables at approximately 10 foot intervals. Designers should select cable ties that have the proper characteristics for the specific installations. The initial installation of the wrong cable ties may require maintenance expenditures to replace the cable ties. Plastic ties that are not ultraviolet resistant will fail in one to two years if they are installed where they are exposed to the rays of the sun. Where both indoor and outdoor cables are to be tied down on the same project, it is best to have only ultraviolet resistant ties on the project and use them on both the indoor and the outdoor cables. This way it will not be possible to have the incorrect type of ties for the outdoor cables. When selecting cable ties the following must be considered: moisture resistance, ultraviolet resistance, extremely high ambient temperatures, extremely low ambient temperatures, chemical resistance, flammability (UL 94 V-O flammability rating), low smoke characteristics, tensile strength, appropriate lengths (the surplus lengths of the cable ties are cut off so it is possible to use one tie length as standard where many different lengths are required). There are quality plastic ties available that if properly applied have a life span of up to 20 years. There are non-magnetic stainless steel ties as well as the plastic ties. The stainless steel ties are capable of satisfactorily satisfying a wide range of requirements.
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Cable
Manager • Requires NO bending or cutting;
ready–made 4-way, “T” and “L” shaped junctions•
Conforms to the National Electric Code, UL Classified• Acts as a continuous
ground conductor; no grounding splices• Adapts to difficult architectural
designs to accommodate changes in direction and elevation of cable runs•
Cuts installation time up to 50 percent
