This issue of the Cablegram presents questions and CTI answers to these questions that have been asked by interested persons and organizations concerning the application of cable tray systems. We believe you will find the answers useful, that they will assist you in applying Cable Tray Systems, and we look forward to more questions. Fax (314) 872-8686.

Question 1: What types of cables can be installed in Cable Tray systems?

Answer:The types of cables permitted by the 1996 NEC are indicated in Section 318-3, uses permitted, (a) Wiring Methods. They include:

  • Power and Control Tray Cable (Type TC) – NEC Article 340
  • Power Limited Tray Cable (Type PLTC) – NEC Sections 725-61 and 725-71
  • Instrument Tray Cable (Type ITC) – NEC Article 727
  • Optical Fiber Cables – Article 770
  • Fire Alarm Circuit Conductors – Article 760
  • Communication Cables – Article 800
  • Mineral Insulated (MI)Cable – Article 333
  • Metal Clad (MC) Cable – Article 334

and other cables, including those specially approved for installation in cable trays. Medium voltage (type MV) and single conductor cables in sizes 1/0 and larger are permitted with some restrictions in industrial establishes where qualified persons service the installation.

Question 2: Can a person walk on an installed Cable Tray System?

Answer:No; walking on cable trays is not to be permitted. It violates the new version of NEMA standard VE-2, manufacturers marking and recommendations, and the intent of the NFPA70 Electrical Safety in Employee Work Practices. Walking on electrical equipment, conduits, cables or other electrical systems should also be avoided. In addition to the fall hazard, there is the risk of damage to equipment and possible contact with conductors.

Question 3: What are the rules for installation clearances for the telecommunication cables in cable trays?

Answer:The 1996 NEC in sections 318-6(i) and (j) indicates that there shall be sufficient space maintained around cable trays to allow adequate access for installing and maintaining the cables and that cable trays shall be exposed and accessible. Adequate room should be provided around the cable tray to allow for the set-up of cable pulling equipment and to provide easy access for the installation of or removal of cables. Where cable trays are installed one above another, allow 12 to 18 inches between cable trays and the ceiling. This is a guide for installation.

Question 4: Does the NEC apply to telecommunication cabling installations?

Answer: Yes, in the following articles: 645 Information Technology Equipment 725 Class 1, Class 2, and Class 3, Remote-Control, Signaling, and Power-Limited Circuits 770 Optical Fiber Cables and Raceways 800 Communication Circuits 810 Radio and Television Equipment 820 Community Antenna Television and Radio Distribution Systems The sections of these articles that may apply depend on the installation; location; cable selection and equipment. There are other NFPA standards that may apply which include: NFPA 75 Protection of Electronic Computer/Data Processing Equipment NFPA 780 Installation of Lightning Protection Systems

Question 5: Is it necessary to provide tie-down cables installed in a cable tray?

Answer:Yes; cables are tied down in cable trays to keep the cables in the cable tray, to maintain spacing between cables, or to segregate or confine certain types of cables to specific locations. The last two items can also be accomplished with a solid fixed barrier. The NEC in section 318-8(b) indicates that in other than horizontal runs, cables shall be securely fastened to transverse members of the cable trays.

For vertical installations, the cables may hang away from the cable tray if not tied down. Although this section of the NEC does not require cable tiedown in horizontal, it may be necessary to meet other requirements. For instance, it may be necessary and appropriate to space power cables at least a diameter apart to approximate the free air amperage rating of a cable. In hazardous dust locations (class II, division 2), it is required to space type MC and TC cables at least the larger cable diameter apart and arrange the cables in a single layer.

Multiconductor power cables, 4/0 and larger, rated 2,000 volts or less, are required to be installed in a single layer by the NEC [Section 318-9(a)(1)]. Tying down these cables is one way to insure this requirement.

Where single conductor cables are installed it is highly desirable to tie the cables down to keep them in the tray.

There are other situations where tying down the cables is important. The selection of the type of cable tie is also very important. For further information, see CTI Technical Bulletin No. 5, Tie Down Practices for Multiconductor Cables in Cable Trays.

Question 6: Are Cable Trays listed?

Answer:Metallic cable trays are not required to be listed because they are a support system. Metal cable trays can be U.L. classified with regard to suitability for use as an Equipment Grounding Conductor. Compliance with other appropriate NEC cable articles is required. CTI recommends compliance with National Electrical Manufacturers, NEMA, Standards Publications Nos. VE1 and VE2, and the manufacturer’s recommendations.

Question 7: Are there cable fill requirements for cable trays?

Answer:Yes — NEC Sections 318-9, 10, 11 and 12, and Tables 318-9, 318-9(e) and 318-10, describe the fill in terms of area and cable diameters.The key issue is ampacity. The ampacity criteria in article 318 is based on not exceeding these fill values. The number and type of conductors that can be installed in a cable tray is also limited by the weight of the cables and other load factors for the cable tray for a given load rated cable tray. See NEMA VE-1 and manufacturer’s data. Size the width of cable tray and the load rating for expansion and additions. Adding six inches to the width of a tray increases its price by approximately 10%.

Question 8: Can high voltage cables be installed in cable trays?

Answer:Yes — NEC permits type MC (Article 334) and type MV (Article 326) in industrial establishments where qualified persons will service the installation. Multiconductor cables rated over 600 volts shall be separated from lower voltage cables by a separate cable tray or a solid fixed barrier. Type MC cables can be mixed with lower voltage cables. See NEC 318-6(f).