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. We look forward to your questions and any comments on these questions and our answers. Please send your comments firstname.lastname@example.org. We are especially interested in photos of cable tray installations.
Question 1: 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 2: Is it common practice to use cable trays in the vertical position? Do they maintain their integrity during a 25 or 30 year life of a plant? Is the percent fill of a vertical tray the same as a horizontal cable tray?
(1) It is common practice to use cable trays in the vertical position. I have many photos of such installations. There is no problem. Cables must be fastened securely, see NEC 318-8(b).
(2) Yes, they do maintain their integrity. I have inspected installations that are over 40 years old. The only aging problem could be the tie wraps, especially for single conductor cables.
(3) There are no differences between the cable fill requirements for vertical and horizontal cable tray installations.
Question 3: 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 4: We have a customer who would like to install the majority of cable tray in his new industrial facility in what I call an “Edge-Wise” orientation. That is, each cable tray rung would point in a vertical direction as opposed to the usual horizontal direction.
The local electrical inspector has stated that he has no issues with this as long as the manufacturer’s specifications have guidelines in how to install it this way. I have searched and can find no indication in any vendor’s literature that acknowledges the possibility that cable tray would ever be installed in this orientation.
Answer: There is no NEC or other limitation on cable trays that would prevent the “Edge-Wise” orientation. The CTI needs to develop guidelines for this installation. This type of installation minimizes dust accumulation in dust locations and could be advantageous in other situations.
Question 5: 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 318-6(b) 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 6: It appears that the NEC doesn’t address the maximum allowable fill area for a solid bottom, channel cable tray. It does however, address ventilated channel cable tray (Article 318-9 (e)). What is your opinion regarding the maximum fill area for solid bottom channel, given that multiconductor or signal cables only are installed?
Answer: The CTI has submitted a proposal to amend the 2002NEC to provide this information.
Question 7: 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.
Question 8: 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 318-5(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 318-5(f) provides the criteria for cables rated over 600 volts. The types of cables usually used in cable trays are type TC (article 340), PLTC (article 725), ITC (article 727), MC (article 334) and Communication Cables (800-52 (d)), MI (article 330). 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.