The January 17,1994 Los Angles, California earthquake and the hurricanes of past years emphasize that the design and installation of structures and systems that may be subjected to these types of natural disasters should be such that the damage is kept to a minimum when a disaster occurs. Even though having been damaged to some degree, it is important that the continuity of service for critical systems be maintained until it is convenient and safe to shut the systems down. Figure #l is an Earthquake Seismic Zone Map of the United States. Figure #2 is Wind Velocity Map for the United States.

It is extremely desirable that the circuit integrity of communication, instrumentation, lighting, power and safety circuits be maintained when critical conditions exist. This is especially true for p[……]

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There are three items which require decisions concerning the tying down of multiconductor cables in cable tray wiring systems. Item #1 is to define under what conditions the multiconductor cables in cable trays are to be tied down. Item #2 is to define the frequency at which the multiconductor cables are to be tied down. Item #3 is to select the ties that have the proper characteristics for the specific installations. In the following material, where the word cable is used it means multiconductor cable.

Item #1- Conditions Requiring Cable Tie Down:

The reasons for tying down cables are to keep them in the cable trays, to maintain the proper spacing between cables, or to confine the cables to specific locations in the cable trays. National Electrical Code Section392.8(B) states that i[……]

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What type of cable tray should be used for the main runs of a cable tray wiring system? The cable tray types to choose from are ladder, ventilated trough, or solid bottom. What are the reasons for selecting a specific type of cable tray?

The engineer or designer should select the type of cable tray that has the features which best serve the project’s requirements.

For a few types of installations, the National Electrical Code (NEC) specifies the cable tray type to be used:
Single conductor cables and Type MV cables must be installed in ladder or ventilated trough cable trays. Single conductor cables and Type MV cables are not allowed to be installed in solid bottom cable trays [1993 NEC Section 318-3(b)]

In Class II, Division 2 Hazardous (Classified) Locations (Dust), the types o[……]

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When selecting a facilities wiring system, the potential for the wiring system to allow moisture to flow into the electrical equipment enclosures should be evaluated. This is true for all wiring requirements: electrical power, instrumentation data, communication data, computer data, alarm signals, etc. A wiring system should provide safe service and it should be maintenance free for many years after its initial installation. The wiring system should not be the source of or a contributor to the electrical system outage problems. Electrical system outages in modern industrial facilities may be very costly. For many continuous processes, an electrical power or control system outage may present critical safety problems for the facility’s personnel and the people in the adjacent community.

I[……]

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All materials expand and contract due to temperature changes. It is important that cable tray installations incorporate features which provide adequate compensation for their thermal contraction and expansion.

1993 NEC Section 300-7 (b) states that “Raceways shall be provided with expansion joints where necessary to compensate for the thermal expansion or contraction.” In 1993 NEC Article 318 there are no requirements for the handling of the thermal contraction and expansion of cable tray. This subject is addressed in the NEMA Standards Publication No. VE 1 “Metallic Cable Tray Systems” Section 6.8.

There are expansion joint splice plates and bonding jumpers available from cable tray manufacturers. A cable tray support should be located within 2 feet of each side of the expansion joi[……]

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Cable tray wiring systems have conductor advantages over conduit wiring systems where the installations involve phase conductors installed in parallel.

For large ampacity circuits, the most practical wiring system installations are those where reasonable size conductors are paralleled for each phase to adequately handle the circuit’s ampere requirements. A more efficient use of conductor material is made by paralleling phase conductors than by using very large single conductors per phase.

A 500 ampere – 480 volt three phase 600 feet long feeder that is installed where it is sometimes exposed to moisture and where the maximum ambient temperature is 110 degrees Fahrenheit is a practical circuit to use as an example. A good conductor choice for such a circuit would be 75 degree Celsius[……]

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