1. Select the cable cross-sectional area according to the long-term allowable current carrying capacity
In order to ensure safety and the service life of the cable, the cable temperature after power-on should not exceed the specified long-term allowable operating temperature, which is 70 degrees for polyvinyl chloride insulated cables and 90 degrees for cross-linked polyethylene insulated cables. According to this principle, it is very simple to select the cable by simply looking up the table.
The selection of cable cross-sectional area should be determined based on the maximum power demand of the house and the maximum current carrying capacity that the wires and cables can withstand. People generally calculate residential power consumption according to the architectural design specifications of 40-50W per square meter. Therefore, before decorating the house, you must first plan the total power consumption of the house, and then choose the cross-section of the wire. The required current carrying capacity should be calculated according to the following formula:
I=w/uk
(Maximum current carrying capacity required by the line=total household power/household rated voltage*overvoltage safety factor)
2. Select the cable cross-sectional area according to the economic current density
Simply understand the economic current density. The cross-sectional area of the cable affects the investment and power loss of the line. In order to save investment, the cable cross-sectional area is expected to be smaller; in order to reduce power loss, the cable cross-sectional area is expected to 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.
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3. Select the cable cross-sectional area according to the grid voltage drop
When we use the first and second methods to select the cable cross-sectional area, if the cable is very long, a certain voltage drop will be generated during operation and startup, and the voltage 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 voltage drop of a 400V line cannot be less than 7%, that is, 380VX7%=26.6V. The voltage drop calculation formula (here only pure resistive voltage drop is considered):
U=I×ρ×L/S S=I×ρ×L/U
U--voltage drop; I--equipment rated current; ρ--conductor resistivity; S--cable cross-sectional area; L--cable length
4. Select the cable cross-sectional area according to the thermal stability coefficient (that is, select the cable cross-sectional area according to the short-circuit current)
1. For 0.4KV cables protected by air switches, general cables can meet the thermal stability requirements and do not need to be verified by this method.
2. For cables above 6KV, after selecting the cable cross-sectional area using the above method, it is necessary to verify whether it meets the thermal stability requirements according to the following formula. If not, a larger cross-sectional area needs to be selected.
Formula: Smin=Id×√Ti/C
Ti{{0}}circuit breaker breaking time, 0.25S; C--cable thermal stability coefficient, 80; Id--system three-phase short-circuit current value
