Cooper Bussmann CT02MAN - Manual (Page 21)

for the cable tray is determined by adding all the
applicable component loads. The cable load + the
concentrated static loads + ice load (if applicable) +
snow load (if applicable) + wind load (if applicable) +
any other logical special condition loads that might
exist. This total load is used in the selection of the
cable tray.
The following is an explanation of the
‘ h i s t o r i c a l ’ N E M A c a b l e t r a y l o a d
classifications found in NEMA VE-1.
There used to be four cable tray support span
categories, 8, 12, 16, and 20 feet, which are
coupled with one of three load designations, "A" for
50 lbs/ft, "B" for 75 lbs/ft, and "C" for 100 lbs/ft.
For example, a NEMA class designation of 20B
identifies a cable tray that is to be supported at a
maximum of every 20 feet and can support a static
load of up to 75 lbs/linear foot.
The cable load per foot is easy to calculate using
the cable manufacturer's literature. If the cable tray
has space available for future cable additions, a
cable tray has to be specified that is capable of
supporting the final future load. Although these
historical load designations are still useful in
narrowing down the choices of cable trays, NEMA
has recently changed the VE-1 document. NEMA
VE-1 now requires the marking on the cable trays to
indicate the exact rated load on a particular span.
Trays are no longer limited to the four spans and
three loads listed above. Now, for example, a tray
may be rated for 150 lbs/ft on a 30 ft. span. It is
recommended when specifying cable tray, to specify
the required load, support span and straight section
length to best match the installation.
Example of Cable Loading per foot:
10 - 3/C No. 4/0 (2.62 lbs/ft)
Total = 26.20 lbs/ft
3 - 3/C No. 250 kcmil (3.18 lbs/ft)
Total = 9.54 lbs/ft
4 - 3/C No. 500 kcmil (5.87 lbs/ft)
Total = 23.48 lbs/ft
Total Weight of the Cables = 59.22 lbs/ft
These cables would fill a 30 inch wide cable tray
and if a 36 inch wide cable tray were used there
would be space available for future cables
(See pages
47 thru 53 for information on calculating tray width.) . To
calculate the proper cable tray design load for the
36" wide cable tray multiply 59.22 lbs/ft x 36
inches/30 inches = 71.06 lbs/ft. If this cable tray is
installed indoors, a load symbol "B" cable tray would
be adequate. If there were additional loads on the
cable tray or the cable tray were installed outdoors,
it would be necessary to calculate all the additional
potential loads. The potential load most often
ignored is installation loads. The stresses of pulling
large cables through cable trays can produce 3 times
the stress of the cables' static load. If the installation
load is not evaluated the cable tray may be damaged
during installation. A 16C or 20C NEMA Class
should be specified if large cables are to be pulled.
Even though walking on cable tray is not
recommended by cable tray manufacturers and
OSHA regulations, many designers will want to
specify a cable tray which can support a 200 lb.
concentrated load "just in case". A concentrated
static load applied at the midspan of a cable tray is
one of the most stressful conditions a cable tray will
experience. To convert a static concentrated load at
midspan to an equivalent distributed load take twice
the concentrated load and divide it by the support
span [(2 x 200 lbs.)/Span]. The strength of the rung
is also a very important consideration when
specifying a concentrated load. The rung must be
able to withstand the load for any tray width, as well
as additional stresses from cable installation.
Excessive rung deflection can weaken the entire
cable tray system. B-Line uses heavier rungs on
their wider industrial trays as a standard. Most cable
tray manufacturer's rungs are not heavy enough to
withstand concentrated loads at 36" tray widths.
For outdoor installations a cable tray might be
subject to ice, snow, and wind loading. Section 25
of the National Electrical Safety Code (published by
the Institute of Electrical and Electronic Engineers)
contains a weather loading map of the United States
to determine whether the installation is in a light,
medium, or heavy weather load district. NESC Table
250-1 indicates potential ice thicknesses in each
loading district as follows: 0.50 inches for a heavy
loading district, 0.25 inches for a medium loading
district, and no ice for a light loading district. To
calculate the ice load use 57 pounds per cubic foot
for the density of glaze ice. Since tray cables are
circular and the cable tray has an irregular surface
the resulting ice load on a cable tray can be 1.5 to
2.0 times greater than the glaze ice load on a flat
surface.
Snow load is significant for a cable tray that is
completely full of cables or a cable tray that has
covers. The density of snow varies greatly due to its
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Cable Tray Manual Cooper B-Line, Inc