Select the wire according to the allowable value of the current carrying capacity of the wire

The wire cross section of the indoor wiring should be selected according to the allowable current carrying capacity of the wire, the allowable voltage loss value of the line, and the mechanical strength of the wire. Generally, the wire carrying surface is selected according to the allowable current carrying capacity, and then the verification is carried out according to other conditions. If the cross section does not meet the requirements of a certain calibration condition, the conductor should be selected according to the minimum allowable cross section that cannot meet the condition.

Wire allowable ampacity: The allowable ampacity of the wire is also called the safe ampacity or safe current value of the wire. The maximum allowable working temperature of the general wire is 65°C. If the temperature exceeds this temperature, the insulation layer of the wire will be rapidly aged, deteriorated and damaged, and may even cause a fire. The so-called allowable current carrying capacity of the wire is the maximum current value that can be passed for a long time when the working temperature does not exceed 65 °C.

Since the working temperature of the wire is not only related to the current passing through the wire, but also related to the heat dissipation condition of the wire and the ambient temperature, the allowable current carrying capacity of the wire is not a fixed value. When the same wire adopts different laying methods (different laying methods, its heat dissipation conditions are also different) or at different ambient temperatures, its allowable current carrying capacity is also different. Refer to the electrical technical manual for the allowable current-carrying capacity of wires with different laying methods.

The current of the line load can be calculated by the following formula:

Single-phase pure resistance circuit: I=P/U

Single-phase circuit with inductance: I=P/Ucosφ

Three-phase pure resistance circuit: I=P/√3UL

Three-phase circuit with inductance: I=P/√3ULcosφ

The meanings of the parameters in the above formulas are:

P: is the power of the load, in watts (W);

UL: is the voltage of the three-phase power supply, in volts (V);

cosφ: is the power factor.

When selecting according to the allowable current carrying capacity of the wire, the general principle is that the allowable current carrying capacity is not less than the calculated current of the line composite.