Actual Cable Section ≠ Nominal Section? This Article Answers For You!

Many industry colleagues do not know what is the nominal section of the cable?

In fact, in the cable industry, the cable section integral can be divided into three categories:

1. Nominal sectional area of cable

2. Design sectional area of cable

3. Actual cross-sectional area of cable

Let's discuss the relationship between these three cross-sectional areas, which answers the question of what the nominal cross-section of the cable is.

What is the design cross-sectional area of the cable?

The cable cross-sectional area is a parameter of the cable specification. We know that the cable specification includes the number of cores, cross-sectional size and voltage of the cable. The cable cross-sectional area refers to the cross-sectional area of the cable conductor. Under the same material, the cable cross-sectional area determines the current carrying capacity of the cable, that is, how much current it can carry. The nominal cross-sectional area, design cross-sectional area and actual cross-sectional area of cables are described below.

Design cross-sectional area of cable (English Name: Design cross-sectional area), GB50411-2007 code for acceptance of construction quality of building energy conservation engineering, 12.2.2: "the cross-section of cable and wire selected for low-voltage distribution system shall not be lower than the design value, and its cross-section and electrical resistance of each core conductor shall be witnessed, sampled and submitted for inspection when entering the site". The design value mentioned here refers to the design cross-sectional area of cable. The variable directly related to the current carrying capacity I of the wire and cable is the conductor resistance R, not the actual section of the conductor. When we examine the design section or nominal section of wire and cable, we examine its resistance value rather than measuring its actual section value.

Requirements for conductor cross-sectional area in wire and cable product standards?

At present, the product standards of each major standard system of wire and cable adopt nominal cross-sectional area or specification code to distinguish conductors with different cross-sections. 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: mm2) as the code of conductor specifications of different sections, while American standards (UL758, UL62, etc.) use AWG specification as the code. Among them, the assessment of specifications and codes of different sections is required by the conductor resistance value, rather than the actual measurement of the conductor cross-sectional area (Note: UL standard allows the measurement of the actual cross-sectional area, but the conductor resistance is used as the arbitration method).

In the national standards GB / t5023-2008, GB / t5013-2008 and GB / t12706-2008, it is mainly stipulated that the cable conductor meets the requirements of GB / t3956. The main assessment includes:

1. The maximum diameter of single wire of soft conductor and the minimum number of single wires in hard conductor shall meet the requirements of GB / t3956;

2. The resistance of conductor at 20 ℃ shall meet the requirements of GB / t3956.

In GB / t3956, there are four types of conductors: type 1 (solid conductor), type 2 (stranded conductor), type 5 (flexible conductor) and type 6 (conductor softer than type 5). Among them, the first and second conductors are used in fixed cables (such as laying wires, power cables, etc.). The 5th and 6th conductors are used in flexible cables and cords (such as power lines, rubber sheathed cables, etc.) and can also be used for fixed laying.

Among the above four conductors, the maximum conductor resistance at 20 ℃ assessed by conductor specifications with different nominal cross-sectional areas is shown in the table. It can be seen that there is a one-to-one correspondence between the conductor specifications of each nominal cross-sectional area and their maximum conductor resistance. Therefore, for wire and cable product standards, as long as the conductor can meet the requirements of the maximum resistance corresponding to the claimed nominal cross-sectional area, and the diameter or number of single wires of the conductor meet the standard requirements, it can be judged as qualified. At the same time, the reference index to measure the quality of each model is the maximum resistance value of the corresponding nominal cross-sectional area. As long as this index meets the requirements, it indicates that the product is qualified. If the cross-sectional area is too small, the resistance value will be unqualified. In fact, the cross-sectional area is also assessed accordingly, which is only a reference, not absolute.