Cooper Bussmann CT02MAN - Manual (Page 13)

The words "and other similar structures." were
incorporated in Section 392.1 for future types of
cable tray that might be developed, such as center
supported type cable tray. All the technical
infor mation developed by the 1973 N E C
®
Technical Subcommittee on Cable Tray for Article
318 - Cable Trays was based on cable trays with
side rails and this technical information is still the
basis for the 2002 NEC
® Article 392 - Cable Trays.
The standard lengths for cable trays are 10, 12,
20 and 24 feet (consult B-Line for the availability of
nonstandard cable tray lengths). Selecting a cable
tray length is based on several criteria. Some of
these criteria include the required load that the cable
tray must support, the distance between the cable
tray supports, and ease of handling and installation.
One industry standar d that is str ongly
recommended is that only one cable splice be
placed between support spans and, for long
span trays, that they ideally be place at
1 / 4 -span.
This automatically limits the length of tray you
choose, as the tray must be longer than or equal to
the support span you have selected. Matching the
tray length to your support span can help ensure
that your splice locations are controlled.
Cable trays can be organized into 4 categories:
Short Span, Intermediate Span, Long Span, and
Extra-Long Span.
Short Span trays, typically used for non-industrial
indoor installations, are usually supported every 6 to
8 feet, while Intermediate Span trays are typically
supported every 10 to 12 feet. A 10 or 12 foot
cable tray is usually used for both of these types of
installations. To keep from allowing two splices to
occur between supports, a 12 foot tray should be
used for any support span greater than 10 feet, up
to 12 feet. Placing the cable tray splices at
1 /
4
-span
is not critical in a short or inter mediate span
application given that most trays have sufficiently
strong splice plates.
In an indoor industrial installation 10 or 12 foot
tray sections may be easier to handle and install as
you may have piping or ducting to maneuver
around. However, using 20 foot instead of 12 foot
straight sections may provide labor savings during
installation by reducing the number of splice joints.
If this is done, the selected tray system should meet
the loading requirements for the support span you
are using. If you are interested in supporting 100
lbs/ft and you are buying 20 foot tray sections while
supporting it every 12 feet, it isn’t necessary to
specify a NEMA 20C tray (100 lbs/ft on a 20 foot
span). A NEMA 20A tray (50 lbs/ft on a 20 foot
span) will support over 130 lbs/ft when supported
on a 12 ft span with a safety factor of 1.5.
Specifying a 20C tray is not an economical use of
product. If you desire to use 20 foot sections of
cable tray, it makes more sense to increase your
support span up to 20 feet. This not only saves
labor by decreasing the number of splices, but also
by decreasing the number of supports that must be
installed.
Long Span trays are typically supported anywhere
from 14 to 20 foot intervals with 20 feet being the
most popular. In long span situations, the placement
of the splice locations at 1/4-span becomes much
more important. Matching the tray length to your
support span can help control your splice locations.
Extra-Long Span trays are supported on spans
exceeding 20 feet. Some outdoor cable tray
installations may have to span anywhere from 20 to
30 feet to cross roads or to reduce the number of
expensive outdoor supports. The distance between
supports affects the tray strength exponentially;
therefore the strength of the cable tray system
selected should be designed around the specific
support span chosen for that run.
[See Section 392.5(A) on page 18 for additional
information on cable tray strength and rigidity.]
B-Line has many cataloged fittings and accessory
items for ladder, ventilated trough, ventilated
channel, and solid bottom cable trays which
eliminate the need for the costly field fabrication of
such items. When properly selected and installed,
these factory fabricated fittings and accessories
improve the appearance of the cable tray system in
addition to reducing labor costs.
11
Cable Tray Manual Cooper B-Line, Inc
Center Supported Cable Tray
(B-Line’s Cent-R-Rail System)