Cable application

Question: Can mechanical utility piping or tubing containing water or compressed air be installed in cable trays with electrical cables?

Answer: No. Cable trays are a support system for electrical cables, power, signal, and communication and optical fiber cables. NEC section 300.8 does not permit any tube, pipe, or equal for water, air gas, drainage, steam, or any service other than electrical in raceways or cable trays containing electrical conductors.

Question: I am in the process of establishing guidelines for raised floors in communications facilities and plan to mandate that all cabling under raised floors be installed on an appropriate type cable tray. Are you aware of any industry standard that may mandate the use of cable trays under raised floors, particularly, power and signal cables?

Answer: We are not aware of such industry standard, but cable trays offer significant advantages for this type of installation and in other computer, telecommunications, and power installations. The telecommunications industry is a very strong cable tray user.

Question: We are using ladder type cable trays at many of our facilities for telecommunications wiring. Do you have any information available for recommended installation clearances for this type of cable tray?

Answer: The NEC does not have a specific installation clearance, but indicates in section 392.6(H) that cable trays should be exposed and accessible. Telecommunications standard TIA/EIA-569 recommends a minimum of 12-inch access headroom above the cable tray.

Question: Are there required code grounding practices regarding cable tray used only for telephone cable? A contractor has just installed a new phone system at my location and he utilized cable trays in the switch room. I did not see any deliberate attempt to ground the system. Our existing cable tray system is heavy bonded and grounded. If this is a code violation, could you refer me to the publication?

Answer: Low energy systems may not be required to be grounded for shock or arcing, ut should be grounded for noise, lightening protection and electromagnetic interference. See CTI Technical Bulletin No. 15.and NEMA VE -2 section 4.7.

Question: Are there any requirements for separation and segregation of various types of cables (i.e. Power, instrumentation, signal, telecommunications, etc.) in cable tray systems?

Answer: Yes, there are NEC rules. Instrumentation, signal, and telecommunications cabling should be separated from power cabling. There are NEC requirements, but also for noise and electromagnetic pick-up from adjacent power cables. This can be accomplished by a separate cable tray system or by a divider within a cable tray.

NEC section 392.6(E)indicates that multiconductor cables rated 600 volts or less are permitted in the same cable tray, however, separation of power and control cables is necessary as indicated in other sections of the NEC and for cross-talk noise reasons. NEC section 392.6(F) provides the criteria for cables rated over 600 volts. The types of cables usually used in cable trays are type TC (article336), PLTC (article 725), ITC (article 727), MC (article 336) and Communication Cables (800-52 (d)), MI (article 332). Fire Alarm Systems (article 760), Emergency Systems (article 700), Optical Fiber Cables (article 770) and Intrinsic Safety (section 504-30). The requirements in these sections are complex. We will discuss them in detail and the general noise problem in the next CableGram.

The requirements for cables that have an outer metal armor are less than for plastic jacketed cables. The general rule is separate communication, control, signal, and instrumentation cabling from power cabling. Power cabling includes 460-volt motor power, 120-volt power, and lightening circuits. Note 120-volt circuits can generate noise. Generally, a separation of two inches is minimum, but the individual circuit and cable are the determining factors in separate requirements.

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

Answer: The types of cables permitted by the 2005 NEC are indicated in Section 392.3 uses permitted, (a) Wiring Methods. They include:

  • Power and Control Tray Cable (Type TC) – NEC Article 336
  • Power Limited Tray Cable (Type PLTC) – NEC Sections 725-61?and 725.82(E) 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 332
  • Metal Clad (MC) Cable – Article 330

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 Establishments where qualified persons service the installation.


Source: Cable Tray Institute (CTI)