I've noticed that many industry colleagues are confused about what the nominal cable cross-section is. In fact, in the cable industry, cable cross-sections are categorized into three types: nominal cross-sectional area, designed cross-sectional area, and actual cross-sectional area. Let's explore the relationship between these three types of cross-sectional area, which will answer the question of what the nominal cable cross-sectional area is.
What is the nominal cable cross-sectional area?
First, let's discuss what cable cross-sectional area is. Cable cross-sectional area is a parameter in cable specifications. Cable specifications include the number of cores, cross-sectional dimensions, and voltage. The cross-sectional area of a cable refers to the cross-sectional area of the cable conductor. For the same material, the cross-sectional area determines the current carrying capacity of the cable—how much current it can carry. Let's discuss the nominal cross-sectional area, designed cross-sectional area, and actual cross-sectional area of a cable.
What is the nominal cable cross-sectional area?
The nominal cross-sectional area (GBT 3956-2008) specifies the specific dimensions of a conductor but is not directly measured. Note: Each specific conductor dimension in this standard must meet the maximum resistance requirements.
The nominal cross-sectional area of a cable refers to the value specified in the cable product. It is used to describe the specifications and model numbers of wires and cables. It is merely a code or name for this specification, facilitating documentation and production management during the manufacturing process. It facilitates selection and provides guidance for users and designers regarding production and electrical design, but does not require direct measurement of the actual cross-sectional area. Therefore,
What is the Design Cross-sectional Area of a Cable?
GB50411-2007, "Code for Acceptance of Construction Quality of Energy-Saving Building Projects," stipulates in Section 12.2.2: "The cross-sectional area of cables and wires selected for low-voltage distribution systems shall not be less than the design value. Upon delivery, witnessed sampling and inspection of the cross-sectional area and the resistance of each conductor core shall be conducted." The design value here refers to the cable's design cross-sectional area. The variable directly related to the current carrying capacity (I) of a wire or cable is the conductor resistance R, not the actual cross-sectional area. When evaluating the design or nominal cross-sectional area of a wire or cable, we are evaluating its resistance, not measuring its actual cross-sectional area.
What is the actual cross-sectional area of a cable?
The actual cross-sectional area (ACR) of a cable refers to the geometric cross-sectional area of the conductor. For wire and cable manufacturers, the design cross-sectional area of a conductor with a given nominal cross-section to meet standard requirements does not necessarily mean that the actual cross-sectional area must be greater than or equal to the nominal cross-sectional area. Rather, it means that the designed cross-sectional area (electrical cross-sectional area) under this nominal cross-sectional area must meet standard requirements. In other words, whether the DC resistance meets standard requirements is used as the basis for designing the actual cross-sectional area of the conductor. If a conductor material with high conductivity is selected, the actual cross-sectional area can be smaller; otherwise, the actual cross-sectional area should be larger.
By now, industry colleagues should understand that the nominal cross-sectional area of a cable is not equal to the actual cross-sectional area. As long as the DC resistance of a cable meets standard requirements, it is qualified. Below are the requirements for conductor cross-sectional area in wire and cable product standards.
Requirements for Conductor Cross-sectional Area in Wire and Cable Product Standards
Currently, all major wire and cable standards use nominal cross-sectional area or specification codes to distinguish conductors of different cross-sectional areas. For example, IEC standards (IEC60227, IEC60245, IEC60502, etc.), European standards (EN50525, etc.), and national standards (GB/T5013, GB/T5023, GB/T12706, etc.) use nominal cross-sectional area (unit: square millimeters) as the designation for conductor sizes of varying cross-sections. US standards (UL758, UL62, etc.) use AWG size as the designation. The assessment of these different cross-sectional area designations is based on conductor resistance, rather than actual cross-sectional area measurement. (Note: UL standards allow for measurement of actual cross-sectional area, but use conductor resistance as the arbitration method.)
National standards GB/T5023-2008, GB/T5013-2008, and GB/T12706-2008 primarily stipulate that cable conductors must meet the requirements of GB/T3956. Key assessments include:
1) The maximum diameter of a single wire for soft conductors and the minimum number of single wires for hard conductors must meet the requirements of GB/T3956;
2) The electrical resistance of the conductor at 20°C must meet the requirements of GB/T3956.
GB/T3956 categorizes conductors into four types: Type 1 (solid conductor), Type 2 (stranded conductor), Type 5 (soft conductor), and Type 6 (conductor more flexible than Type 5). Types 1 and 2 are used in cables intended for fixed installation (such as wiring cables and power cables). Types 5 and 6 are used in flexible cables and cords (such as power cords and rubber-sheathed cables), and can also be used for fixed installation.
For the four types of conductors mentioned above, the maximum conductor resistance values at 20°C for conductors with different nominal cross-sectional areas are shown in the table. It can be seen that there is a one-to-one correspondence between each nominal cross-sectional area conductor specification and its maximum conductor resistance value. Therefore, for wire and cable product standards, as long as the conductor meets the maximum resistance value requirements corresponding to the claimed nominal cross-sectional area and the single-wire diameter or number of conductors meets the standard requirements, it can be judged as qualified. At the same time, the reference indicator for measuring the quality of each model is the maximum resistance value it should meet for the corresponding nominal cross-sectional area. As long as this indicator meets the requirements, the product is qualified. If the cross-sectional area is too small, the resistance value will fail to meet the requirements. This actually also involves an assessment of the cross-sectional area, but it is only a reference, not an absolute.
