1.         PURPOSE

 

1.1       This engineering standard defines the criteria for sizing, designing, specifying, installing and supporting of cable-tray systems.

 

 

2.         scope

 

2.1       This standard applies to all cable-tray installations. While directed towards Air Products’ owned and operated facilities, it shall be considered the minimum requirements for any facility design. For sale of equipment, the electrical designer shall verify these requirements with the customer representatives before proceeding with facility design.

 

 

3.         related documents

 

3.1       Air Products Engineering Documents

 

4EL64159A                                      Design Philosophy Voltage Separation

STD-G309A                                      Grounding Cable Trays

STD-G310A                                      Grounding Conduit to Cable Tray

STD-P331A, Sheets 1 through 4      Cable Tray Supports

 

3.2       National Fire Protection Association (NFPA)

 

70     National Electrical Code (NEC)

 

3.3       NEMA Standards Publication

 

VE2      Cable Tray Installation Guidelines

 

4.         DEFINITIONS

 

4.1       Cable Tray is a unit or assembly of units or sections and associated fittings made of metal or other noncombustible materials that form a rigid structural system used to support electrical cables. Cable-tray systems include ladder type, troughs, channels, solid-bottom trays, and other similar structures.

 

4.2       Zero-tangent fittings:  When referring to cable-tray fittings, a tangent is a straight portion of tray at the end of a curve to accommodate a flat splice plate. Most tray manufactures have tangent fittings. Zero tangent fittings are supplied with special curved splice plates to allow installation on the curved section of the fitting.

 

 

 

5. APPROVED MANUFACTURERS

 

5.1       All manufacturers are approved for use only if the product is UL listed and manufactured per the requirements of the NEC.

 

5.2       The Air Products Microstation tray modeling library is based on dimensions obtained from TJ Cope in the 1990s. The cell library selections are based on NEMA 20A ladder trays with 133 mm (5 ¼ in) rail height, 38 mm (1 ½ in) outside flange and zero tangent fittings. Caution must be used when using tray systems from other manufactures with straight tangents on the fittings to prevent a design that will be fitting bound when installed. As of this document revision date only TJ Cope and MP Husky offer zero tangent fittings as standard.

 

 

6. SELECTION OF CABLE TRAY AS THE PRIMARY WIRING SYSTEM

 

6.1       Cable tray is the preferred wiring distribution system for low voltage power and instrumentation. Cable tray allows for greater flexibility in both the initial design and future cabling requirements.

 

6.2       The use of cable tray for medium voltage power cables shall be avoided where possible. It has been proven to be more cost effective and an easier installation using underground conduit for these systems.

 

6.3       Cable tray shall not be used in substations unless there is no other method of installation. Cable tray installation in existing substations requires approval by GSS Electrical Engineering.

 

6.4       NEC Article 500 limits the use of cable-tray systems in areas classified Class I, Division I, hazardous areas to the use of type MI mineral-insulated (metallic sheathed) cable or type MC metal-clad cable when permitted. Air Products facilities usually have limited Division I areas. The use of RMC conduit should be used in place of cable tray.

 

 

7. CABLE-TRAY SPECIFICATIONS

 

7.1       Cable tray system designs shall normally be aluminum ladder-type NEMA 20A tray with 225 mm (9 in) rung spacing with and outside flange. A 100mm (4 in) loading depth has been found to be a good selection for most installations. Zero tangent fitting should be specified when the design requires it; see paragraph 5.2.

 

7.2       Solid-bottom-type trays with covers may be required for low-level-signal cables to safeguard from electro-magnetic interference (EMI) when adequate separation from power cables is not possible.

 

7.3       This standard identifies requirements for cable-tray systems based on the NEC and NEMA standards for the United States. Designs in Canada must use manufacturer’s approved by CSA and the installation must be per the Canadian code.

 

7.4       Cable trays and associated hardware shall be aluminum. Aluminum is preferred because of low initial cost, resistance to corrosive atmospheres, short delivery time, and light weight for ease of installation. Aluminum cable tray also provides a higher fault current capacity than steel tray.

 

7.4.1   The use of galvanized steel cable tray is acceptable for most applications; however, it is considered to be higher in initial cost, has a longer delivery time, and is heavier than aluminum tray. When galvanized steel cable tray is used, it shall be specified as hot-dipped galvanized after fabrication.

 

7.4.2   Cable trays exposed to highly corrosive conditions shall be either fiberglass or PVC coated.

 

7.4.3   Wire basket tray may be used in office and control buildings for instrumentation and communication cables.

 

 

7.5       Splice plates and other hardware for indoor and outdoor cable trays shall be the manufacturer’s standard used for the tray system in non-corrosive environments. Hardware shall be stainless steel.

 

7.6       Cable tray covers shall be provided when cables might be exposed to damage from objects falling from overhead (that is, when platforms of column boxes are both directly above and adjacent to cable tray) and when personnel protection is required to avoid touching of cables, particularly low-voltage and medium-voltage power cables.

 

7.6.1   Site-specific factors or environmental conditions might also require the application of tray covers to protect cables from exposure to moisture, sunlight, severe corrosive atmospheres prevalent in a chemical plant environment, electromagnetic (EMI) and radio frequency interference, or other similar hazards. These situations shall be reviewed on a project-specific basis to determine the applicability of tray covers.

 

7.6.2   Cable tray covers shall be flanged louvered for all power tray systems. Flanged flat covers may be used on instrumentation and communications tray systems where required.

 

7.6.3   The use of tray covers may require de-rating of power cables. See the NEC for specific requirements.

 

 

8. CABLE TRAY DESIGN CRITERIA

 

8.1       Maximum Fill Requirements

 

8.1.1   All cable trays shall be sized per the NEC maximum fill requirements and criteria including all the following additions:

 

• Fill calculations shall include all required cables for future equipment defined in the initial project scope.

 

• Fill calculations shall include a minimum of 20% additional capacity for future unplanned additions.

 

8.1.2   Cable installation details shall be shown on the contract drawings for the following installations:

 

• Trays that contain requirements for both single layer and stacked cables. The detail to include the dimensional information.

 

• Trays that contain single layer cables requiring minimum separation between adjacent cables to maintain the designed amperage rating. The detail to include dimensional information.

 

8.1.3   The NEC requires that when single conductor cable sized #1/0 AWG through #4/0 AWG is installed in ladder-type cable tray, the allowable rung spacing shall not exceed 225 mm (9 in). Generally, Air Products’ design philosophy is to avoid the use of any single-conductor cable installations in cable tray. Single conductor cables used outdoors must be UV rated. The use of single conductor cable in process areas shall be avoided and used by exception only.

 

8.2       Dedicated Trays:

 

            As a minimum, Air Products requires separate cable-tray systems for each of the following:

 

• Medium Voltage Power >600V-35KV
• Low Voltage Power and Control >50V to 600V
• Instrumentation 24VDC instruments, Thermocouples, RTDs
• Communications Computer network, telephone, fiber optics

 

8.2.1   Per the NEC, cable trays containing electrical conductors shall not contain any other pipe, tube, or equal for any service other than electrical with the exception of fiber-optic cables only if their installation complies with NEC 2011 Article 770.

 

 

8.2.2   Cable tray system routings and layouts shall be designed with minimum spacing criteria as defined in Standard 4AEL64159A.

 

 

9. CABLE TRAY SUPPORTS

 

9.1       Cable tray designs shall limit mid-span tray deflection to 25 mm (1 in) per 3 m (10 ft) based on a 2.0 safety factor. In most applications, a cable tray meeting the requirements of NEMA 20A rating, supported at 6 m (20 ft) intervals and cable loading of 22.6 kg per 300 mm (50 lbs per ft), is sufficient to meet these requirements. However, to meet exact design requirements the following must be determined:

 

9.2       Tray Fill Weight:  Paragraph 9.1 uses the assumption of 22.6 kg per 300 mm (50 lbs per ft) of cable weight. This value can be exceeded when installing wide trays with large power conductors. Geographic location of the tray system must also be taken in consideration for snow, ice and wind loading. NEMA class 20C tray with 225 mm (9 in) or 300 mm (12 in) rung spacing shall be used on all tray systems for large (4/0 AWG and larger) low and medium voltage power cables.

 

9.3       Tray Rigidity:  For pipe racks, building steel, or tee-structure mountings for which support spacing is determined by others, tray rigidity shall be selected from the manufacturer’s data based on the appropriate span length, required loading depth/width, and total cable weight per unit length.

 

9.4       Support Spacing:  For wall-supported brackets or other support systems for which support spacing is determined by the electrical designer, the spacing shall be selected from the manufacturer’s data based on the appropriate span length, required loading depth/width, and total cable weight per unit length.

 

9.5       Expansion Connectors:  Cable tray expansion connectors shall be provided at building expansion joints and in straight runs of outdoor trays at intervals as required by the tray manufacture or NEMA VE2, not exceeding 30 m (100 ft). A bonding jumper shall be installed at each expansion joint to bridge this sliding expansion joint connection and provide a solid and continuous ground return path. See STD-G309A.

 

9.6       Pulling Tension:  The same limitations of pulling tensions and sidewall pressures apply for cable in trays as for cable in conduits. Attention must be given to the length of runs, the number of turns, and the size of sheaves used for pulling cable around these turns to accommodate changes in direction. Sufficient clearance shall be provided for the installation and maintenance of cable.

 

9.7       Cable-Tray Support:  Cable trays shall be fastened to support steel by using guides that allow for longitudinal movement.

 

9.7.1   Whenever possible, supports and hangers shall be designed to permit vertical adjustment along with horizontal adjustment. This shall be accomplished by the use of channel framing, beam clamps, and threaded hanger rods.

 

9.7.2   In general, Civil/Structural Design will design major tray supports; however, Electrical Design needs to identify and communicate the support requirements.

 

9.7.3   Horizontal- and vertical-tray supports shall provide an adequate bearing surface for the tray and shall have provisions for hold-down clamps or fasteners. Vertical-tray supports shall provide secure means, other than friction, for fastening cable trays to supports.

 

9.7.4   Supports shall be located so that connectors between horizontal straight sections of cable-tray runs fall between the support point and the quarter point of the span. Unspliced straight sections shall be used on all simple spans and on end spans of continuous span arrangements.

 

9.7.5   Supports for horizontal tray fittings shall be placed within 600 mm (2 ft) of each fitting extremity and as required for the fitting itself. See standard STD-P331A and NEMA VE2 for detail. The support requirements are often described in manufacturer’s WEB sites or catalogs.

 

9.7.6   Vertical-tray elbows at the top of runs shall be supported at each end. Vertical, cable-tray elbows at the bottom of runs shall be supported at the top of the elbow and within 600 mm (2 ft) of the lower extremity of the elbow.

 

9.7.7   Vertical straight lengths shall be supported at intervals not exceeding 6 m (20 ft) on center. Sloping trays shall be supported at intervals not exceeding those for horizontal trays of the same design for the same installation.

 

9.7.8   When supplemental “tee” posts are installed, the location of the support posts shall be reviewed to ensure that the installation does not block future access to equipment for maintenance or removal.

 

9.7.9   Cable clamps or straps shall be provided to limit the movement of conductors within the tray. Strapping shall be installed at minimum intervals of 1800 mm (6 ft) for horizontal runs and 900 mm (3 ft) for vertical runs. Power cables may require closer spacing of the clamps or straps to prevent excessive cable movement during a fault.

 

 

10. BUILDING PENETRATIONS

 

10.1    Local building and fire codes need to be reviewed for penetration sleeves and fire stop requirements when cable tray penetrates building walls and floors. All openings shall have a weather tight metal frame and sleeve as required.

 

10.2    When cable trays enter buildings and enclosures, they shall be sloped away from the building to prevent the entrance of water by following the cables. After all cables are installed, the opening through which cable tray enters buildings or enclosures shall be sealed using an approved sealing method or fire stop material as required.

 

 

11. CABLE-TRAY GROUNDING

 

11.1    Cable trays shall be grounded at each end and at intervals not exceeding 15 m (50 ft). See STD-G310A for details.

 

11.2    Expansion fittings, flexible connectors, hinged connectors and non-continuous tray runs shall have a ground bonding strap to insure continuity of the cable tray ground system. See STD-G309A.

 

11.3    Whenever wires and cables enter cable tray from a conduit system, the conduit must be bonded to the cable-tray system. See STD-G310A for details.

 

11.4    Where bare ground wires are required in tray systems they shall be bonded to each tray segment and the equipment ground bars or ground system at each end.