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There are some designers, engineers, and
inspectors that do not think that cable tray is a
mechanical support system just as strut is a
mechanical support system. Cable tray is not a
raceway in the NEC
®
but some designers, engineers,
and inspectors attempt to apply the requirements for
raceway wiring systems to cable tray wiring systems
even when they are not applicable. Cable tray wiring
systems have been used by American industry for
over 35 years with outstanding safety and continuity
of service records. The safety service record of cable
tray wiring systems in industrial facilities has been
significantly better than those of conduit wiring
systems. There have been industrial fires and
explosions that have occurred as a direct result of the
wiring system being a conduit wiring system. In these
cases, cable tray wiring systems would not have
provided the fires and explosions that the conduit
systems did by providing as explosion gas flow path
to the ignition source even though the conduit
systems contained seals.
The most significant part of this section is that the
metallic cable tray system must have electrical
continuity over its entire length and that the support
for the cables must be maintained. These
requirements can be adequately met even though
there will be installation conditions where the cable
tray is mechanically discontinuous, such as at a
firewall penetration, at an expansion gap in a long
straight cable tray run, where there is a change in
elevation of a few feet between two horizontal cable
tray sections of the same run, or where the cables
drop from an overhead cable tray to enter
equipment. In all these cases, adequate bonding
jumpers must be used to bridge the mechanical
discontinuity.
Control Cable Entering Pushbutton and
Power Cable Entering Motor Terminal Box
from 6 Inch Channel Cable Tray System
(Bottom entries provide drip loops to prevent moisture
flow into enclosures.)
Cables Exiting 480 Volt Outdoor
Switchgear and Entering Cable Tray System
(Cable fittings with clamping glands are required to prevent
moisture flow into equipment due to the cable's overhead
entry into the switchgear enclosure).
Cables Entering and Exiting Motor Control
Centers from Cable Tray Systems.
392.6. Installation. (B) Completed Before
Installation.
This means that the final cable tray system must
be in place before the cables are installed. It does
not mean that the cable tray must be 100%
mechanically continuous. The electrical bonding of
the metallic cable tray system must be complete
before any of the circuits in the cable tray system
are energized whether the cable tray system is being
utilized as the equipment grounding conductor in
qualifying installations or if the bonding is being
done to satisfy the requirements of Section 250.96.
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Cooper B-Line, Inc Cable Tray Manual