392.8.Cable installation. (C) Bushed Conduit and Tubing.
For most installations, using a conduit to cable tray clamp for terminating conduit on cable tray is the best method. Where a cable enters a conduit from the cable tray, the conduit must have a bushing to protect the cable jacket from mechanical damage; a box is not required [See Section 300.15(C). Boxes, Conduit Bodies, or Fittings - Where Required. Where cables enter or exit from conduit or tubing that is used to provide cable support or protection against physical damage. A fitting shall be provided on the end(s) of the conduit or tubing to protect the wires or cables from abrasion.]. There are some special installations where the use of conduit knockouts in the cable tray side rail for terminating conduit is appropriate. This would not be a good standard practice because it is costly and labor intensive, and if randomly used may result in damaging and lowering the strength of the cable tray.
Channel to Tray
Channel to
Channel
Cable Channel Branch Circuit
392.8.Cable Installation. (D) Connected in Parallel.
Section 310.4. Conductors in Parallel. States the following:
The paralleled conductors in each phase, neutral or grounded conductor shall:
(1)Be the same length.
(2)Have the same conductor material.
(3)Be the same size in circular mil area.
(4)Have the same insulation type.
(5)Be terminated in the same manner.
Where run in separate raceways or cables, the raceways or cables shall have the same physical characteristics. Conductors of one phase, neutral, or grounded circuit shall not be required to have the same physical characteristics as those of another phase, neutral, or grounded circuit conductor to achieve balance.
A difference between parallel conductors in raceways and those in cable trays is that the conductors in the cable tray are not derated unless there are more than three current carrying conductors in a cable assembly [as per Exception
No.2 of Section 310.15(B)(2)(a) and Section
392.11(A)(1)]. Where the single conductor cables are bundled together as per Section 392.8(D) and if there are neutrals that are carrying currents due to the type of load involved (harmonic currents) it may be prudent to derate the bundled single conductor cables .
To maintain the minimum distance between conductors, the single conductor cables should be securely bound in circuit groups every three or four feet using high strength plastic or stainless steel ties. These circuit groups provide the lowest possible circuit reactance which is a factor in determining the current balance among various circuit groups.
For installations that involve phase conductors of three conductor or single conductor cables installed in parallel, cable tray installations have conductor cost savings advantages over conduit wiring systems. This is because the conductors required for a cable tray wiring system are often a smaller size than those required for a conduit wiring system for the same circuit. No paralleled conductor ampacity adjustment is required for single conductor or three conductor cables in cable trays [See NEC® Section 392.11(A)].
There were changes in the 1993 NEC® and 1996 NEC® for installations where an equipment grounding conductor is included in a multiconductor cable: the equipment grounding conductor must be fully rated per Section 250.122. If multiconductor cables with internal equipment grounding conductors are paralleled, each multiconductor cable must have a fully rated equipment grounding conductor.
Section 250.122 now prohibits the use of standard three conductor cables with standard size EGCs when they are installed in parallel and the EGCs are paralleled. There have been no safety or technical
| Reprinted with permission from NFPA |
| Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National |
| Fire Protection Association, on the referenced subject which is represented only by the standard in its entirety. |
Cable Tray Manual | Cooper |
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