How do you choose the cable cross-sectional area (Attached: examples)

         When it comes to cables, everyone is very familiar with it, but when it comes to selecting the cable cross-sectional area, many electrical personnel may be confused, especially some electrical beginners who are confused. We all know that a cable generally refers to a rope-like cablet wisted by several groups of conductors (at least two in each group). Each group of conductors is insulated from each other and soft entwisted around the center. The entire outside is covered with a highly insulating covering. Cables, as electrical energy or signal transmission devices are one of the most heavily used devices in electricity. There are five major types of cables. Among them, power cables are cable products used to transmit and distribute high-power electric energy in the main lines of the power system. They are often used in urban underground power grids, power station leads lines, internal power supply in industrial and mining enterprises, and cross-river and sea under water transmission lines. So, the question is, how should the cable cross-sectional area be correctly selected? Today I will share with you a scientific and simple method for selecting cable cross-sectional area. There are four methods for different occasions. After reading the article, I hope it can give some reference to the major electrical personnel.

 ▶Method1:

        Select according to the long-term allowed able carrying capacity. In order to ensure safety and the service life of the cable, the temperature of the cable after power-on should not exceed the specified long-term allowed operating temperature, which is 70 degrees for PVC insulated cables and 90 degrees for cross-linked polyethylene insulated cables. According to this principle, it is very easy to select a cable is as simple as looking up at able. For example: The transformer capacity of a factory is 2500KV and the power supply is 10KV. If cross-linked polyethylene insulated cables are used to lay them in the bridge, what should be the cross-sectional area of the cables? Step one: Calculate the rated current 2500/10.5/1.732=137A. Step two: Check the cables election manual to find out YJV-8.7/10KV-3X25 carrying capacity is 120AYJV-8.7/10KV-3X35 carrying capacity is 140A.

 ▶Method2:

        Select according to economical current density to simply understand the economic current density, the cross-sectional area of the cable affects the line investment and electric energy loss. In order to save investment, it is hoped that the cable cross-sectional area is smaller; in order to reduce electric energy loss, it is hoped that the cable cross-sectional area is larger. Based on the above considerations, determine areas on able. The cross-sectional area of the cable is called the economic cross-sectional area, and the corresponding current density is called the economic current density. Method: According to the annual operating hours of the equipment, look up the table to obtain the economic current density. Unit: A/mm². For example: The rated current of the equipment is 150A, and the annual operation time is 8,000 hours. What is the cross-sectional area of the copper core cable? According to the above table C-1, it can be seen that for 8000 hours, the economic density is 1.75A/mm² S=150/1.75=85.7. A conclusion: The cable cross-sectional area we can choose according to the cable's pacifications is 95mm². Step 3: Select YJV-8.7/10KV-3X35 cable with a carrying capacity greater than 137A, which can theoretically meet the requirements. Note: This method does not consider the requirements for dynamic stability and thermal stability.

▶Method3:

       Select according to the voltage drop of the power grid. When we use the first and second methods to select the cable cross-sectional area, if the cable is very long, there will be a certain volt age drop during operation and start up. The volt age on the equipment side is lower than a certain range, which will cause the equipment to heat up. According to the requirements of the "Electrician's Manual", the volt age drop of a 400V line cannot be less than 7%, that is, 380VX7%=26.6V. The volt age drops calculation formula (only purely resistive volt age drop sare considered here): ΔU=I×ρ×L/SS=I×ρ×L/ΔUΔU. Volt age drop I is the rated current of the equipment ρconductorresistivity. Sis the cable cross-sectional area Lis the cable length. Example: The rated current of 380V equipment is 150A, using copper core cable (ρofcopper=0.0175Ω.mm²/m), the volt age drop is required to be less than 7%(ΔU=26.6V), the cable length is 600 meters, and the cable cross-sectional area Sis selected How old? According to the formula S=I×ρ×L/ΔU=150×0.0175×600/26.6=59.2 mm². Conclusion: The cable cross-sectional area is selected as70mm².

▶Method 4: 

       Select based on thermal stability coefficient 1. When 0.4KV cables are protected by air switches, general cables can meet the thermal stability requirements, and there is no need to check according to this method 2. For cables above 6KV, after selecting the cable cross-sectional area using the above method, you must check whether it meets the thermal stability requirements according to the following formula. If not, you need to select a larger cross-sectional area.

      Formula: S min=Id×√Ti/C Among them, Ti is the breaking time of the circuit breaker, which is taken as 0.25S, Cis the cable thermal stability coefficient, which is taken as 80, and Id is the three-phase short-circuit current value of the system 3. Give an example: How to choose the cable cross-sectional area when the system short-circuits current is 18KA. S min=18000×√0.25/80=112.5. Conclusion: If the system short-circuits current reaches 18KA, even if the rated current of the equipment is small, the cable cross-sectional area should not be less than120 mm².