Cable cross-section evaluation

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Many professionals in the industry are unclear about what the nominal cross-sectional area of a cable is. In fact, in the cable industry, the cable cross-sectional area is divided into three categories: the nominal cross-sectional area of the cable, the designed cross-sectional area of the cable, and the actual cross-sectional area of the cable. Next, we will explore the relationship between these three cross-sectional areas, which also answers the question of what the nominal cross-sectional area of a cable is. First, let’s talk about what the cable cross-sectional area is. The cable cross-sectional area is a parameter that defines the specifications of a cable. We know that cable specifications include the number of conductors, the cross-sectional size, and the voltage. The cable cross-sectional area refers to the cross-sectional area of the cable’s conductors; for materials of the same kind, it is the cross-sectional area that determines the cable’s current-carrying capacity, that is, the maximum current it can carry. Next, we will discuss the nominal cross-sectional area of cables, the designed cross-sectional area of cables, and the actual cross-sectional area of cables respectively. What is the design cross-sectional area of a cable? The design cross-sectional area of a cable (in English: Design cross-sectional area) is referred to in GB50411-2007, the \"Code for Acceptance of Construction Quality of Building Energy Efficiency Projects\", in clause 12.2.2: \"The cross-sectional area of cables and wires selected for low-voltage distribution systems shall not be lower than the designed value; upon arrival at the site, samples of their cross-sectional areas as well as the resistance value of each conductor core must be taken for testing.\" The designed value mentioned here refers to the design cross-sectional area of the cable. The variable that is directly related to the current-carrying capacity I of wires and cables is the conductor resistance R, not the actual cross-sectional area of the conductor. When we evaluate the design cross-section or nominal cross-section of wires and cables, we are assessing their resistance value, rather than measuring their actual cross-sectional area. The requirements regarding the cross-sectional area of conductors in the standards for wire and cable products: At present, the product standards within various standard systems for wires and cables use nominal cross-sectional areas or specification codes to distinguish between conductors of different cross-sections. For example, IEC standards (IEC60227, IEC60245, IEC60502, etc.), European standards (EN50525, etc.), and **Chinese standards (GB/T5013, GB/T5023, GB/T12706, etc.) use the nominal cross-sectional area (in square millimeters) as a code for different conductor specifications, while American standards (UL758, UL62, etc.) use AWG ratings as codes. In this regard, the evaluation for different cross-sectional specification codes is based on the conductor resistance value, rather than an actual measurement of the conductor’s cross-sectional area (Note: The UL standard allows for the measurement of the actual cross-sectional area, but uses conductor resistance as the decisive criterion). In the **standard specifications GB/T5023-2008, GB/T5013-2008, and GB/T12706-2008, it is stipulated that cable conductors must meet the requirements of GB/T3956. The key assessment criteria are as follows: 1) The maximum diameter of individual wires in flexible conductors, as well as the minimum number of wires in rigid conductors, must comply with the requirements of GB/T3956; 2) The resistance of the conductors at 20°C must also meet the requirements of GB/T3956. In GB/T3956, conductors are classified into four types: Type 1 (solid conductor), Type 2 (stranded conductor), Type 5 (soft conductor), and Type 6 (a conductor that is even softer than Type 5). Among them, the first and second types of conductors are used in fixed-cable installations (such as wiring cables, power cables, etc.). The 5th and 6th types of conductors are used in flexible cables and wires (such as power cords, rubber-sheathed cables, etc.), and can also be used for fixed installations. For the above four types of conductors, the maximum conductor resistance values at 20°C for conductor specifications with different nominal cross-sectional areas are shown in the table. It can be seen that there is a one-to-one correspondence between various conductor specifications with nominal cross-sectional areas and their maximum conductor resistance values. Therefore, for wire and cable product standards, as long as the conductor can meet the maximum resistance value requirement corresponding to its claimed nominal cross-sectional area, and the diameter of each conductor strand or the number of strands meets the standard requirements, it is considered qualified. At the same time, the reference indicator for measuring the quality of various models is the maximum resistance value that they should meet corresponding to their nominal cross-sectional area; as long as this requirement is satisfied, it indicates that the product is qualified. If the cross-sectional area is made too small, it will result in an unqualified resistance value. This actually also involves an assessment of the cross-sectional area, but it is for reference only and not absolute.