Cooper Bussmann CT02MAN - Manual (Page 9)

• The higher the elevation of the wiring system,
the more important the number of components
required to complete the installation. Many
additional man-hours will be required just moving
the components needed for the conduit system up
to the work location.
• Conduit wiring systems require pull boxes or
splice boxes when there is the equivalent of more
than 360 degrees of bends in a run. For large
conductors, pull or junction boxes may be required
more often to facilitate the conductor’s installation.
Cable tray wiring systems do not require pull boxes
or splice boxes.
• Penetrating a masonry wall with cable tray
requires a smaller hole and limited repair work.
• More supports are normally required for rigid
steel conduit due to the requirements of NEC ®
Table 344.30(B)(2).
• Concentric conduit bends for direction changes
in conduit banks are very labor intensive and difficult
to make. However if they are not used, the
installation will be unattractive. The time required to
make a concentric bend is increased by a factor of
3-6 over that of a single shot bend. This time
consuming practice is eliminated when cable tray
wiring systems are used.
• Conductor pulling is more complicated and time
consuming for conduit wiring systems than for cable
tray wiring systems. Normally, single conductor wire
pulls for conduit wiring systems require multiple reel
setups. For conduit wiring systems, it is necessary to
pull from termination equipment enclosure to
termination equipment enclosure. Tray cables being
installed in cable trays do not have to be pulled into
the termination equipment enclosures. Tray cable
may be pulled from near the first termination
enclosure along the cable tray route to near the
second termination enclosure. Then, the tray cable
is inserted into the equipment enclosures for
termination. For projects with significant numbers of
large conductors terminating in switchgear, this may
be a very desirable feature that can save hours of an
electrician's time. Unnecessary power outages can
be eliminated since tray cable pulls may be made
without de-energizing the equipment. For conduit
installations, the equipment will have to be de-
ener gized for rubber safety blanketing to be
installed, otherwise the conductor pulls might have
to be made on a weekend or on a holiday at
premium labor costs to avoid shutting down
production or data processing operations during
normal working hours.
• Conductor insulation damage is common in
conduits since jamming can occur when pulling the
conductors. Jamming is the wedging of conductors
in a conduit when three conductors lay side by side
in a flat plane. This may occur when pulling around
bends or when the conductors twist. Ninety-two
percent of all conductor failures are the result of the
conductor’s insulation being damaged during the
conductor’s installation. Many common
combinations of conductors and conduits fall into
critical jam ratio values. Critical jam ratio (J.R.=
Conduit ID/Conductor OD) values range from 2.8
to 3.2. The J. R. for 3 single conductor
THHN/THWN insulated 350 kcmil conductors in a
2
1 / 2 inch conduit would be 3.0 (2.469 inches/
0.816 inches). If conductor insulation damage
occurs, additional costs and time are required for
replacing the conductors. This cannot occur in a
cable tray wiring system.
• Smaller electrician crews may be used to install
the equivalent wiring capacity in cable tray. This
allows for manpower leveling, the peak and average
crew would be almost the same number, and the
electrician experience level required is lower for
cable tray installations.
• Since the work is completed faster there is less
work space conflict with the other construction
disciplines. This is especially true if installations are
elevated and if significant amounts of piping are
being installed on the project.
MAINTENANCE SAVINGS
• One of the most important features of cable
tray is that tray cable can easily be installed in
existing trays if there is space available. Cable tray
wiring systems allow wiring additions or
modifications to be made quickly with minimum
disruption to operations. Any conceivable change
that is required in a wiring system can be done at
lower cost and in less time for a cable tray wiring
system than for a conduit wiring system.
7
Cable Tray Manual Cooper B-Line, Inc