Four methods for selecting cable cross-sectional area

In electrical design and technological transformation, electrical personnel often do not know how to scientifically choose the cable cross-sectional area. Experienced electricians will calculate the current based on the size of the electrical load and easily select the cable cross-sectional area; Some electricians also choose the cable cross-sectional area based on the electrician's mnemonic; What I want to say is that their experience is practical but not scientific. There are also many posts online, often but not comprehensive enough and difficult to understand. Today, I will share with you a scientific and simple method for selecting cable cross-sectional area. There are four methods in different situations.

One Select cable cross-sectional area based on long-term allowable current carrying capacity

1. 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 allowable working temperature. The temperature for PVC insulated cables is 70 degrees, and for cross-linked polyethylene insulated cables it is 90 degrees. According to this principle, it is easy to select the cable by simply checking the table.

2. For example, if a transformer with a capacity of 2500KVa is used for 10KV power supply in a certain factory, and a cross-linked polyethylene insulated cable is laid in the bridge, what is the cross-sectional area of the cable?

Step 1: Calculate the rated current of 2500/10.5/1.732=137A

Step 2: Check the cable selection manual to find out

YJV-8.7/10KV-3X25 with a current carrying capacity of 120A

YJV-8.7/10KV-3X35 has a current carrying capacity of 140A

Step 3: If the cable current carrying capacity of YJV-8.7/10KV-3X35 is greater than 137A, theoretically it can meet the requirements,

Note: This method does not consider the requirements for dynamic stability and thermal stability.

II Select cable cross-sectional area based on economic current density

A simple understanding is that economic current density and cable cross-sectional area affect the investment and energy loss of the line. In order to save investment, it is hoped that the cable cross-sectional area will be smaller; In order to reduce energy loss, it is hoped that the cable cross-sectional area will be larger; Based on the above considerations, a reasonable cable cross-sectional area is determined, which is called the economic cross-sectional area, and the corresponding current density is called the economic current density.

Method 2: According to the annual operating hours of the equipment, check the table to obtain the economic current density. Company: A/mm

For example, with a rated current of 150A and an annual operating time of 8000 hours, what is the appropriate cross-sectional area for copper core cables?

According to Table C-1 above, for 8000 hours, the economic density is selected as 1.75A/mm2

S=150/1.75=85.7A

Conclusion: The cable cross-sectional area that we can choose based on cable specifications is 95mm

III Select cable cross-sectional area based on power grid voltage drop

When we select the cable cross-sectional area using the first and second methods, if the cable is very long, there will be a certain voltage drop during operation and startup, and the voltage on the equipment side will be lower than a certain range, which will cause the equipment to heat up.

According to the requirements of the Electrician Manual, the voltage drop of a 400V line cannot be less than 7%, that is, 380VX7%=26.6V. The voltage drop calculation formula (only considering pure resistive voltage drop here):

∆U=I×ρ×L/S S=I×ρ×L/∆U

∆ U voltage drop I is the rated current of the equipment ρ, conductor resistivity S is the cross-sectional area of the cable L is the length of the cable

Example 2: The rated current of 380V equipment is 150A, using copper core cable (copper ρ=0.0175 Ω. mm/m), requiring voltage drop

Less than 7% (∆ U=26.6V), with a cable length of 600 meters, what is the appropriate cross-sectional area S for the cable?

According to the formula S=I × ρ × L/∆ U=150 × 0.0175 × 600/26.6=59.2mm.

Conclusion: The cable cross-sectional area is selected as 70mm