1. Select Material and Finish
The most suitable material and finish for your application will depend on cost, the potential for corrosion, and electrical considerations. SAN offers cable tray systems fabricated from corrosion-resistant steel, stainless steel and aluminum alloys along with corrosion-resistant finishes, including zinc, PVC and epoxy.
2. Select the Tray Class
NEMA standard VE-1 defines 12 load classes. The classes are designated by a number (8,12, 16, and 20), specifying maximum span in feet and a letter (A, B, and C), specifying the maximum load (A = 50 lbs./ft., B = 75 lbs./ft., and C = 100 lbs./ft.). The load rating must include the weight of the cables plus any applicable wind or snow loads. The load capacity available for cable is therefore reduced for outdoor applications. Costs vary between different load classes. Since labor and coupling costs are similar for a given length of tray, the heavier classes are more cost effective on a load length basis. The designer should therefore specify the lightest class of tray compatible with the weight requirements of the cable tray.
3. Select the Tray Type
Cable tray is available with three styles of bottom:
Ladder Cable Tray is a prefabricated structure consisting of two longitudinal siderails connected by individual transverse members.
Ventilated Cable Tray is a prefabricated structure consisting of a ventilated bottom within integral or separate longitudinal siderails, with no openings exceeding 4 in. in a longitudinal direction.
Solid Bottom Cable Tray is a prefabricated structure without openings in the bottom. Ladder tray is most often used because of its cost effectiveness. The designer has a choice of four nominal rung spacings: 6, 9, 12, and 18 inches. The greatest rung spacing compatible with an adequate cable bearing surface area should be selected. Heavy power cables often require greater cable bearing area due to the possibility of creep in the jacket material of the cable. If this is a concern, consult the cable manufacturer. This condition may require the use of ventilated tray, which also offers additional mechanical protection for the cables.
systems under certain conditions. The designer should verify these before specifying the type of tray to be used. Electromagnetic shielded tray may be used in areas where control or data cables need to be protected from RFI interference. For more information, see the “Electromagnetic Shielded” section of this manual.
4. Select the Tray Size
The width or height of a cable tray is a function of the number, size, spacing and weight of the cables in the tray. Available nominal widths are 6, 9, 12, 18, 24, 30, 36 and 42 inches. When specifying width, it is important to remember that the load rating does not change as the width increases. Even with six times the volume, a 36 in. wide tray cannot hold any more weight than a 6 in. wide tray. If the load rating of the tray permits, cable can be piled deeper in the tray. Most tray classes are available in a nominal 3d, 4, 5, 6 and 7 inch height. Cable ties or other spacing devices may be used to maintain the required air space between cables.
5. Select the Fittings
Fittings are used to change the size or direction of the cable tray. The most important decision to be made in fitting design concerns radius. The radius of the bend, whether horizontal or vertical, can be 12, 24, 36 or 48 in., or even greater on a custom basis. The selection requires a compromise with the considerations being available space, minimum bending radius of cables, ease of cable pulling, and cost. The typical radius is 24 in. Fittings are also available for 30°, 45°, 60°, and 90° angles. When a standard angle will not work, field fittings or adjustable elbows can be used. It may be necessary to add supports to the tray at these points.
6. Consider Deflection
Deflection of the cable tray affects the appearance of an installation, but it is not a structural issue. In the case of non-metallic cable tray, deflection may be affected by elevated temperatures.
NEMA Load Test.
The NEMA load test is a simple beam, uniformly distributed load test. This type of test was initially selected because:
- It was easiest to test.
- It represents the worst case beam condition compared to continuous or fixed configurations. When consulting the manufacturer’s catalog for deflection information, the designer must verify whether the data shown represents simple or continuous beam deflection. If continuous beam deflection is shown, the calculation factor should be given.
NEMA has one criterion for acceptance under their load test: the ability to support 150% of the rated load.
Simple vs. Continuous Beam Deflection.
Theoretical maximum deflection for a simple beam, uniformly distributed load may be calculated as: .0130 w L4 / E I
Where: w = Load in lbs./ft.
L = Length in inches
E = Modulus of Elasticity
I = Moment of Inertia
The maximum deflection calculation for a continuous beam of two spans with a uniformly distributed load is: .00541 w L4 / E I
A continuous beam of two spans therefore has a theoretical maximum deflection of only 42% of its simple beam deflection. As the number of spans increases, the beam behaves increasingly like a fixed beam, and the maximum deflection continues to decrease. As this occurs, the system’s load carrying capability increases.
Since different bending moments are created in each span, there is no simple factor to approximate deflection as the number of spans increases. It is possible to calculate these deflections at any given point by using second integration of the basic differential equation for beams. Testing shows that the center span of a three-span cable tray run can deflect less than 10% of its simple beam deflection.
Location of Couplers.
The location of the coupler dramatically affects the deflection of a cable tray system under equal loading conditions. Testing indicates that the maximum deflection of the center span of a three-span cable tray run can increase four times if the couplers are moved from one-quarter span to above the supports. This can be a major concern for designers considering modular systems for tray and pipe racks.
