Calculation and installation cases of wires and cables
The measured load current of work area 2# living variable load is 330A. The existing cable is a 120mm four-core copper core cable, and the safe current carrying capacity is 260A after checking the table. The cable will be overloaded and there is a hidden danger of insecurity. In order to ensure the normal power supply, the work area intends to split the current with another cable to ensure normal power supply. (The cables mentioned below refer to 1KV, VV type armored polyethylene four-core copper-core cables).
If you look at 330A-260A=70A according to the safe current carrying capacity. We only need to combine a cable with a current carrying capacity of 70A to theoretically guarantee safe operation (ideally).
According to the calculation method of cable impedance, the impedance values of 16mm, 25 mm, 35 mm, 50 mm, 70 mm, 95 mm, 120 mm are shown in Table 2:
Table 2, four-core copper core cable impedance
According to the above table, we can calculate the impedance mode of the cable. In the case of ignoring the contact resistance of the parallel cable, the parallel cable is understood as two impedances in parallel, and the current distribution value is calculated.
Of course, when two cables are laid in parallel, the safe current carrying capacity of the cables will change. When the two are used together, the safe current-carrying capacity should be 0.92 times of the original current-carrying capacity.
At this time, the safe current carrying capacity of the 120mm copper core cable is 239A. 25mm is 86A and 35mm is 109A. According to the principle of Iux≥Izmax, it can be operated safely by incorporating a 35mm cable.
When merging a 35mm four-core copper cable into the 2# living area:
︱Z35︱=0.534Ω
The load current is 330A, then I120=︱Z16︱/(︱Z16︱+︱Z120︱)*330
Get I120=253.19A; I16=76.81A
It is not difficult to see that the 120mm cable is still running at overload current.
If you look at 330A-260A=70A according to the safe current carrying capacity. We only need to combine a cable with a current carrying capacity of 70A to theoretically guarantee safe operation (ideally).
According to the calculation method of cable impedance, the impedance values of 16mm, 25 mm, 35 mm, 50 mm, 70 mm, 95 mm, 120 mm are shown in Table 2:
|
Nominal cross-sectional area
(mm) |
Resistance
(Ω/km) (at 20℃) |
Reactance (Ω/km)
|
|
16
|
1.15
|
0.068
|
|
25
|
0.75
|
0.066
|
|
35
|
0.53
|
0.064
|
|
50
|
0.37
|
0.063
|
|
70
|
0.26
|
0.081
|
|
95
|
0.19
|
0.06
|
|
120
|
0.15
|
0.06
|
According to the above table, we can calculate the impedance mode of the cable. In the case of ignoring the contact resistance of the parallel cable, the parallel cable is understood as two impedances in parallel, and the current distribution value is calculated.
Of course, when two cables are laid in parallel, the safe current carrying capacity of the cables will change. When the two are used together, the safe current-carrying capacity should be 0.92 times of the original current-carrying capacity.
At this time, the safe current carrying capacity of the 120mm copper core cable is 239A. 25mm is 86A and 35mm is 109A. According to the principle of Iux≥Izmax, it can be operated safely by incorporating a 35mm cable.
When merging a 35mm four-core copper cable into the 2# living area:
︱Z35︱=0.534Ω
The load current is 330A, then I120=︱Z16︱/(︱Z16︱+︱Z120︱)*330
Get I120=253.19A; I16=76.81A
It is not difficult to see that the 120mm cable is still running at overload current.
