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The low-voltage systems in smart buildings typically consist of various information monitoring and communication facilities, such as BA (Building Automation), OA (Office Automation), CA (Communication Automation) and other related systems. Taking BA as an example, in addition to the necessary information monitoring for facilities like power supply, water supply and drainage, air conditioning, elevators, and parking lots within the building itself, there are also several sub-systems such as SA (Security Automation) and FA (Fire Protection Automation), which enable system integration or partial system integration. The grade and specifications of these systems are determined based on the functional requirements of the building’s structure. These systems include various types of cables and wires; some of them are active cables (with power voltages typically at DC12/24/48V and AC220V), while others are passive electrical (optical) cables such as data cables and video coaxial cables. Therefore, when designing the wiring methods and route selections, it is necessary to make distinctions between these different types of cables. The design must not only comply with relevant standards but also take into account factors such as the safety, scalability, cost-effectiveness, and aesthetics of the wiring, as well as ease of maintenance. Cable trays, which serve as carriers for laying various cables, are also subject to the requirements of wiring design and should be implemented in accordance with the aforementioned principles. Due to the parallel intersection of various pipelines within buildings and the limited space available, especially in large office buildings, financial complexes, hotels, and venues, there are numerous information points. In addition to using floor grooves and pipes embedded in walls for cable installation, cable trays are widely utilized in shafts and ceiling fixtures to facilitate wiring in different directions. The various cables belonging to the low-voltage systems are arranged in these trays; the optimal routing and installation methods must be determined based on the requirements of the wiring paths, as well as in consideration of the building structure and the locations designated for pipes related to air conditioning and electrical systems. Passive cables cannot be laid side by side with active cables; if they must be placed in the same tray due to constraints, metal partitions must be used to separate them. Cables drawn out from such trays should avoid crossing each other in a horizontal manner. When cables trays pass through floors, walls, or expansion joints, corresponding openings and positions should be indicated on the architectural drawings, to prevent the need for last-minute drilling during construction, which could damage the structural elements of the building. To prevent interference from electromagnetic radiation (EMC), the sealing of the cable tray should be considered in its design. I. Cable tray structure: Cable trays are available in various structures such as trough type, tray type, and ladder type, and they consist of brackets, support arms, and installation accessories. (As shown in Figure 1), when selecting, it is necessary to ensure that all components of the cable tray meet the requirements for standardization, versatility, and integration as part of a standardized set. The cable trays inside buildings can be designed as independent units or mounted on various structural elements and conduit supports. They should feature a simple structure, an attractive appearance, flexible configuration, and ease of maintenance. All components must be galvanized. For cable trays installed outdoors in buildings, those located near the sea or in areas prone to corrosion require materials with properties such as corrosion resistance, moisture resistance, good adhesion, and high impact strength. To reduce weight, aluminum alloy cables and fiberglass cable trays can also be used. Their dimensional specifications and load-bearing characteristics are roughly similar to those of steel cable trays. Due to the different specific gravities of aluminum and steel (Al=2.7, Fe=7.86), the weight ratio between aluminum and steel is approximately 1:3. Based on the market prices of these two materials, the cost of aluminum alloy cable trays is 1.5 to 2.0 times higher than that of galvanized steel cable trays of similar type. Aluminum alloy cable trays offer advantages such as aesthetic appeal, light weight, and easy installation. In recent years, they have been employed in various construction projects. II. Load on cable trays and load characteristics 1. Load on cable trays The loads on cable trays are divided into permanent loads, dynamic loads, and additional loads. The static load refers to the type of cables, the number of cables, and the outer diameter and weight per unit length of each cable, all of which are listed and calculated separately according to the different routes along which the cables are laid. Dynamic load refers to the weight of construction and maintenance workers during the installation and maintenance of cable trays. For lightweight cable trays, dynamic loads are generally not taken into account; that is, standing (or walking) on the tray is not permitted. If it is necessary to account for people standing on it, the span should be reduced accordingly. Additional loads applied outdoors refer only to those caused by ice and snow, wind, and electromagnetic forces; they are related to the natural meteorological conditions of the location where installation takes place as well as the properties of the charged bodies. These loads must be calculated taking various conditions into account during the design process. 2. Steps for selecting a cable tray: (1) Determine the width of the cable tray, the number of layers, the type and spacing of the support points, as well as the distribution of cables across each layer of the tray. (2) Calculate the uniformly distributed load per layer of cable (kN/m2) to preliminarily determine the model and specifications of the cable tray. (3) Check the strength of the cable tray based on the maximum total uniformly distributed load value of the cables. The verification formula is as follows: Q_use = q1 + q2. Where: q1 – the uniform load on the cable (the maximum value among the uniform loads of each layer) (kN/m2); the uniform load refers to the load from trays, ladder racks, or cable trays ； q2--The uniformly distributed load equivalent to the weight of a person, taken into account during cable installation or maintenance (kN/m2). For calculating the value of q2, the weight of a person is generally assumed to be p=90 kg. The bending moments resulting from concentrated loads and uniformly distributed loads are shown in Figure 2; these values are adjusted so that the maximum bending moments are equal. By setting pι/4 = q2ι2/8, it follows that q2 = 2p/ι. Since P = 90 kg, therefore q2 = 180/ι. Here, P represents the load per person in kg, ι represents the distance between support points (the maximum value is used if the distances vary); q2 represents the equivalent uniformly distributed load per person in kg/m. Based on the initially determined model, specifications, and support point distances for the bridge frame, reference the manufacturer’s catalogues to repeatedly verify these values until the load requirements are met. (4) Deflection: There are currently no clear guidelines regarding how to determine the value of deflection. In areas under heavy loads, it is obvious that efforts should be made to reduce deflection, which means that more steel will be required. Therefore, when performing calculations, it is sufficient to make full use of the maximum allowable stress of the steel and ensure an adequate safety factor; generally, a ratio of maximum deflection to span (distance between support points) of 1/250 to 1/150 is appropriate. III. Expansion and contraction issues of cable trays: Due to changes in ambient temperature, steel cable trays experience thermal expansion and contraction. Outdoor cable trays are greatly affected by temperature. For example, if the maximum ambient temperature is 40°C and the minimum temperature is -20°C, the maximum contraction amount of the cable tray can be calculated using the following formula: Δt = 11.2 × 10-6 × 60° × 1000 mm. From this, it can be concluded that when the temperature difference is 60°C, Δι = 0.672 mm/m; when the temperature difference is 50°C, Δι = 0.560 mm/m; and when the temperature difference is 40°C, Δι = 0.448 mm/m. In engineering design, expansion joints should be considered for straight sections of cable trays, and the spacing between these expansion joints is recommended to be as follows: 50 m when the temperature difference is 40°C; 40 m when the temperature difference is 50°C; and 40 m when the temperature difference is 60°C. IV. Grounding: In accordance with relevant regulations, galvanized cable trays must be properly grounded. (1) The contact resistance of each fixing bolt on a galvanized cable tray shall be less than 0.005Ω; under such conditions, the cable tray can be used as a grounding main wire (powder-coated cable trays are not suitable for use as grounding main wires). The resistance value of each cable tray can be calculated using the formula: r = P·L/S, where P = 15×10⁻⁶/cm (at 20°C) ； L=Length is calculated as 100 mm ； S = cross-sectional area in cm2. (2) The unit resistance value of the ladder frame per tray is shown in Table 2. Table 2: Unit resistance value of the cable tray. (3) The unit resistance value of the cable duct is shown in Table 3. (4) Once the cable tray system is fully installed and connected, the resistance per cable side (or per cable duct) is given by: R = L(r + 1/3r’), where R represents the total resistance of the entire length of that cable side (or cable duct), in mΩ ； r--Resistance per unit length of the ladder edges (mΩ/m); r’--Contact resistance at the bolts used to fix the board directly. V. Requirements for the design and installation of cable trays 1. As a component of wiring projects, cable trays currently lack specific regulatory guidelines; the specifications provided by various manufacturers are not universal. Therefore, during the design and selection process, it is necessary to choose the appropriate cable tray based on the type and quantity of cables used in different low-voltage systems. (1) Determine the direction: Based on the building layout plan, taking into account the arrangement of air conditioning and electrical pipelines, the convenience of maintenance, as well as the density of cable routes, the optimal route for the cable tray is determined. Indoors, it should be installed as much as possible along the walls, columns, beams, and floor slabs of the building. When using a utility tunnel for installation, it should be placed parallel to the pipes on one side of them or above them; care should be taken to ensure that the drop wires and branch wires avoid crossing each other. If no other pipe supports are available, it will be necessary to install (support) columns separately. (2) Load calculation: Calculate the weight per unit length of the cables on the longitudinal section of the main cable tray. (3) Determine the width of the cable tray: Based on the number of cables to be installed, the diameter of the cables, and the spacing between them, determine the model and specifications of the cable tray, as well as the length of the supports, the length and spacing between the pillars, the width of the tray, and the number of layers. (4) Determine the installation method: Decide on the way in which the cable tray will be fixed based on the conditions of the installation site; choose between a suspended, vertical, sidewall, or mixed installation style. Connectors and fasteners are usually supplied as a set. In addition, select the appropriate cover plates according to the structure of the cable tray. (5) Draw the plan and sectional views of the cable tray; space views should also be drawn for specific areas, and a material list should be provided. 2. When used in conjunction with power cable trays, the power cables and low-voltage cables should be placed on separate sides, with a partition used to separate them. 3. When low-voltage cables are used together with other low-voltage cables in the same tray, it is necessary to strictly use low-voltage cables for low-voltage systems that have an outer shielding layer, in order to avoid interference between them. 4. Other installation requirements: (1) When the cable tray enters the building from the outside, the slope of the tray outward shall not be less than 1/100. (2) When the cable tray crosses electrical equipment, the clear distance between them shall be not less than 0.5 m. (3) When cable trays in the two groups are laid parallel at the same height, the clear distance between them shall be not less than 0.6 m. (4) On the parallel diagram, draw the route of the cable tray, indicating the coordinates or positioning dimensions and elevation of the starting point, ending point, turning points, branching points, and lifting points of the tray. If an axonometric view of the cable tray layout can be drawn, the material estimation will be more accurate. Straight section: Indicate the total length, number of bridge deck layers, elevation, model, and specifications. Turning points and branch points: Indicate the model and specifications of the turning plates used. Lifting section: Indicate the changes in elevation; it can also be shown using detailed local drawings or sectional views. (5) The spacing, installation method, model specifications, and elevation of the tray support points, such as columns, brackets, or non-standard supports and frames, can be indicated either by a list in plan view, or by showing them in separate sections using different sectional views, line drawings, or detailed drawings. (6) The location and method of cable descent: generally, for a large number of cables, vertical bend plates and vertical lifting frames can be used; for a small number of cables, guide plates or guide tubes can be employed. It is sufficient to indicate the method of descent. (7) The cable tray should be positioned at a height of more than 2.2 meters above the ground; the distance between the top of the tray and the ceiling or other obstacles should be no less than 0.3 meters. The width of the tray should not be less than 0.1 meter, and the filling rate of the cross-section within the tray should not exceed 50%. (8) When cables are installed vertically within a cable tray, they should be fixed to the supports of the tray at their upper ends and every 1.5 meters; when installed horizontally, they should be fixed at both ends, at turns, and every 3 to 5 meters. (9) When installed in a ceiling, the opening of the tray cover should maintain an 80-millimeter vertical clearance, and the utilization rate of the cable tray cross-section should not exceed 50%. (10) The cables laid in the cable tray do not need to be tied; they should remain straight inside the tray and avoid crossing as much as possible. The cables must not extend outside the tray, and they should be secured at the points where they enter or exit the tray, as well as at any bends. Cables laid in vertical troughs should be secured to cable supports every 1.5 meters. (11) When laying wires in horizontal and vertical cable trays as well as vertical trunking, the cables should be tied down. 4 pairs of twisted pair cables are bundled in groups of 24; 25 pairs or more of main cables, optical fibers, and other signal cables should be bundled according to their type, cable diameter, and number of conductors. The spacing between ties should not exceed 1.5 meters; the spacing of the clamps should be even, with appropriate tightness. (12) When the cable tray is installed horizontally, the spacing between supports is generally 1.5–3 m; when installed vertically, the spacing between supports fixed to the building structure should be less than 2 m. (13) When installing metal wire trays, supports or hangers should be provided in the following situations: at the joints of the wire trays ； Spacing 3m ； 0.5m away from both ends of the cable tray ； At the turn. 5. Material statistics: (1) Cable trays: Calculate the total length of cable trays of various models and specifications. After deducting the standard length of each tray to determine the number of trays, add a margin of 1%–2%. (2) Columns: If columns of a uniform specification are used, the number of columns can be determined by dividing the total length of the cable tray by the average spacing between columns, and then adding a margin of 2% to 4%. If the specifications of the columns vary, they need to be counted separately. (3) Brackets: The total length of the cable tray is divided by the average spacing between the brackets, and then 1% to 2% is added as a margin to obtain the total required amount. (4) Other components: The total number is calculated by multiplying the number of main units by a certain ratio (determined by the manufacturer). Special components such as vertical bend plates and turning joint plates need to be counted separately.