General principles for selecting wires and cables

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Abstract: When selecting wires and cables, it is generally important to pay attention to the selection of wire and cable models and specifications (conductor cross-section).

Keywords: wire and cable selection

1、 General principles for selecting wires and cables

When selecting wires and cables, it is generally important to pay attention to the selection of wire and cable models and specifications (conductor cross-section).

1. Selection of wire and cable models

When selecting wires and cables, consideration should be given to their intended use, installation conditions, and safety; For example,

According to different purposes, power cables, overhead insulated cables, control cables, etc. can be selected;

According to different laying conditions, general plastic insulated cables, steel tape armored cables, steel wire armored cables, anti-corrosion cables, etc. can be selected;

According to safety requirements, non combustible cables, flame-retardant cables, halogen-free flame-retardant cables, fire-resistant cables, etc. can be selected.

2. Selection of wire and cable specifications

When determining the specifications for the use of wires and cables (conductor cross-section), selection criteria such as heat generation, voltage loss, economic current density, and mechanical strength should generally be considered.

Based on experience, low-voltage power lines generally have a large load current, so the section is selected according to the heating conditions first, and then the voltage loss and mechanical strength are verified; Low voltage lighting lines, due to their high requirements for voltage levels, can first select the cross-section according to the allowable voltage loss conditions, and then verify the heating conditions and mechanical strength; For high-voltage lines, the cross-section should be selected based on the economic current density, and then the heating conditions and allowable voltage loss should be verified; And the mechanical strength of high-voltage overhead lines can also be calculated. If the user has no experience, they should seek the opinions of relevant professional units or individuals. The selection of general wire and cable specifications is shown in the following table:

Reference Table for Selection of Wire and Cable Specifications

Conductor cross-section

Mm 2

Copper core PVC insulated cable

Overhead laying at an ambient temperature of 25 ℃

227 IEC 01 (BV)

Copper core PVC insulated power cable

Direct burial laying at an ambient temperature of 25 ℃

VV22-0.6/1 (3+1)

Steel cored aluminum stranded wire

Overhead laying at an ambient temperature of 30 ℃

Explanation: 1. The current carrying capacity of aluminum core wires of the same specification is about 0.7 times that of copper core wires. Choosing aluminum core wires can be one size larger than copper core wires. Cross linked polyethylene insulation can be selected in a smaller size, while fire-resistant wires and cables should be selected in a larger size.

2. The capacity calculation in this table is based on three-phase 380V and Cos φ=0.85. If single-phase 220V Cosφ=0.85， The capacity should be multiplied by 1/3.

When the ambient temperature is high or when using exposed methods, the safe current carrying capacity will decrease. In this case, larger specifications should be selected; When used for frequent starting of motors, 2-3 larger specifications should be selected.

4. The PVC insulated wires in this table are calculated based on the single overhead laying method. If they are laid through pipes or multiple wires, 2-3 larger specifications should be selected.

The data above 5 is for reference only. The final design and determination of the cable model and specifications should refer to relevant professional materials or electrician manuals.

2、 Characteristics of the use of wires and cables

The product usage characteristics are detailed in the specific product catalog.

3、 Transportation and storage of wires and cables

During transportation, it is strictly prohibited to throw cables or cable reels containing cables from a height, especially at lower temperatures (usually around 5 ℃ or below). Throwing or dropping cables may cause insulation and sheath cracking.

2. Avoid storing cables in an exposed manner in the open air as much as possible, and cable reels are not allowed to be laid flat.

When lifting packaging, it is strictly prohibited to lift several plates at the same time. On vehicles, ships, and other transportation vehicles, cable reels should be secured using appropriate methods to prevent collisions or tipping over, in order to prevent mechanical damage to the cables.

4. Cables must not come into contact with acids, alkalis, and mineral oils, and must be stored separately from these corrosive substances There shall be no harmful gases that damage insulation or corrode metals in the warehouse where cables are stored.

During the storage period, cables should be rolled regularly (once every three months in summer, and may be extended at discretion in other seasons). When rolling, roll the storage tray upside down to prevent the bottom from getting damp and rotting. When storing, it is important to regularly pay attention to whether the cable head is intact and undamaged.

The storage period of cables is limited to the product's delivery date, generally not exceeding one and a half years, and the longest not exceeding two years.

4、 Installation and construction of wires and cables

The design and construction of wire and cable laying and installation should be carried out in accordance with relevant regulations such as GB 50217-94 "Code for Design of Cables in Power Engineering", and necessary cable accessories (terminals and joints) should be used. The operation quality, safety, and reliability of the power supply system are not only related to the quality of the wires and cables themselves, but also to the construction quality of cable accessories and lines.

Through statistical analysis of line faults, it is found that faults caused by construction, installation, and connection factors are often much more likely than those caused by defects in the wire and cable body. Therefore, in order to correctly select wires, cables, and supporting accessories, in addition to designing and constructing according to regulatory requirements, attention should also be paid to the following aspects:

Cable laying and installation should be carried out by qualified professional units or personnel. Construction and installation that do not comply with relevant regulatory requirements may result in the cable system not functioning properly.

When laying cables manually, the command and control rhythm should be unified, with one person carrying the cable on their shoulder every 1.5 to 3 meters, pulling and slowly releasing it.

When mechanically laying cables, a dedicated cable laying machine is generally used and necessary traction tools are equipped. The traction force is appropriate and evenly controlled to avoid damaging the cables.

Before laying the cable, check the appearance of the cable and whether the head is intact. When laying, pay attention to the rotation direction of the cable reel and do not flatten or scratch the outer sheath of the cable. In winter, do not straighten the cable by hitting it to avoid insulation and sheath cracking.

When laying, the bending radius of the cable should be greater than the specified value. Before and after cable laying and installation, use a 1000V megohmmeter to measure the insulation resistance between each conductor of the cable to ensure it is normal. Adjust the measurement results appropriately based on the cable model, length, and ambient temperature. For small-sized (solid core conductors below 10mm ²) cables, the continuity of the conductors should also be measured.

When laying cables directly, attention should be paid to soil conditions. Generally, the burial depth of cables under buildings should not be less than 0.3 meters. For softer or more complex environments such as farmland, construction sites, or roads, a certain burial depth (0.7-1 meters) should be established to prevent accidental damage to directly buried cables. If necessary, clear signs should be erected.