Cooper Bussmann CT02MAN - Manual (Page 20)

high amperage short circuit if a low resistance
metallic path (metallic cable tray or metallic raceway)
is present
[See information under Section 392.5(F)
Nonmetallic Cable Trays] .
392.4. Uses Not Permitted.
This is the only place in the NEC ® where all the
various types of cable tray have limitations on their
place of use. No cable trays can be used in
hoistways or where subject to severe physical
damage. The designer must identify the zones of
installation where a cable tray might be subjected to
severe physical damage. Usually such areas are
limited and provisions can be made to protect the
cable tray by relocating it to a more desirable
location or as a last resort to provide protection
using the appropriate structural members.
The second sentence of Section 392.4 states that
cable tray shall not be used in environmental air
spaces except to support the wiring methods
recognized for use in such spaces. This is not a
restriction on cable tray as long as it is used as a
support for the appropriate cable types.
Metallic cable trays may support cable types
approved for installation in Ducts or Plenums Used
for Environmental Air as per Section 300.22(B) and
the cable types approved for installation in Other
Space Used for Environmental Air as per Section
300.22(C).
The second sentence of Section 300.22(C)(1) is as
follows:
Other types of cables and conductors shall
be installed in electrical metallic tubing,
flexible metallic tubing, intermediate metal
conduit, rigid metal conduit without an
overall nonmetallic covering, flexible metal
conduit, or, where accessible, surface metal
raceway or metal wireway with metal covers
or solid bottom metal cable tray with solid
metal covers.
Reprinted with permission from NFPA 70-1999, the National Electrical Code®,
Copyright© 1998, National Fire 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.
This part of Section 300.22(C) is confusing. The
statement as underlined in the above paragraph
leads some to assume, for installations in Other
Spaces Used for Environmental Air, that the types
of insulated single conductors which are installed in
raceway installations may also be installed in solid
bottom metal cable trays with metal covers. This is
not so. Only the appropriate multiconductor cable
types as per Section 392.3(A) may be installed in
solid bottom cable trays.
Cable tray may be used to support data process
wiring systems in air handling areas below raised
floors as per Sections 300.22(D) and 800.52(D).
392.5. Construction Specifications. (A)
Strength and Rigidity.
The designer must properly select a structurally
satisfactory cable tray for their installation. This
selection is based on the cable tray's strength, the
cable tray loading and the spacing of the supports.
The NEMA Metallic Cable Tray Systems Standard
Publication VE-1 contains the cable tray selection
information and it is duplicated in B-Line's Cable
Tray Systems Catalog.
The NEMA Standard provides for a static load
safety factor of 1.5. A number (Span in Feet - the
distance between supports) and letter (Load in lbs/ft)
designation is used to properly identify the cable
tray class on drawings, in specifications, in quotation
requisitions, and in purchase requisitions to
guarantee that the cable tray with the proper
characteristics will be received and installed. The
designer must specify the cable tray type, the
material of construction, section lengths, minimum
bend radius, width, rung spacing (for a ladder type
cable tray), and the total loading per foot for the
cables on a maximum support spacing
(See page 52
for cable tray specifications checklist) . For many
installations, the cable trays must be selected so that
they are capable of supporting specific concentrated
loads, the weight of any equipment or materials
attached to the cable tray, ice and snow loading,
and for some installations the impact of wind
loading and/or earthquakes must be considered.
Most cable trays are utilized as continuous beams
with distributed and concentrated loads. Cable trays
can be subjected to static loads like cable loads and
dynamic loads such as wind, snow, ice, and even
earthquakes. The total normal and abnormal loading
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Cooper B-Line, Inc Cable Tray Manual