The cable current carrying capacity refers to the amount of current that a cable line passes when it delivers electrical energy. Under thermal stability conditions, the cable carrying capacity when the cable conductor reaches the long-term allowable operating temperature is called the long-term allowable current carrying capacity of the cable.
Safety current-carrying meter and calculation method for copper wire
First, the general copper wire current carrying capacity: the safe current carrying capacity of the wire is determined according to the maximum temperature of the core, cooling conditions, laying conditions.
Generally, the safe current carrying capacity of copper wires is 5~8A/mm2, and the safe current carrying capacity of aluminum wires is 3~5A/mm2.
Generally, the safe current carrying capacity of copper wires is 5~8A/mm2, and the safe current carrying capacity of aluminum wires is 3~5A/mm2. Such as: 2.5mm2BVV copper wire safe current carrying capacity recommended value 2.5 & TImes; 8A / mm2 = 20A4mm2BVV copper wire safe current carrying capacity recommended value 4 & TImes; 8A / mm2 = 32A
2. Calculate the cross-sectional area of ​​the copper conductor: Calculate the upper and lower range of the cross-sectional area S of the selected copper conductor by using the recommended value of the safe current carrying capacity of the copper conductor: 5~8A/mm2: S=“I/(5~8)â€=0.125I ~0.2I(mm2)S-----Cu wire cross-sectional area (mm2) I-----load current (A)
Third, the power calculation general load: (can also be used as electrical appliances, such as lighting, refrigerators, etc.) are divided into two types, one type of resistive load, one is an inductive load. Formula for resistive load: P=UI
For the calculation of the fluorescent lamp load: P = UIcos Ñ„, where the power factor of the fluorescent lamp load is cos Ñ„ = 0.5.
Different inductive loads have different power factors. When calculating household appliances, the power factor cosφ can be taken as 0.8. That is to say, if the total power of a household appliance is 6000 watts, the maximum current is I=P/Ucosф=6000/220&TImes; 0.8=34(A)
However, in general, home appliances cannot be used at the same time, so a common factor is added and the common factor is generally 0.5. Therefore, the above calculation should be rewritten as
I=P&TImes;Common coefficient/Ucosф=6000×0.5/220×0.8=17(A)
In other words, the total current value of this family is 17A. The total brake air switch cannot use 16A, it should be greater than 17A.
Cable current flow comparison table
Estimated vocabulary:
Multiply by 9.5 and multiply by nine.
Thirty-five by three-five, and the two groups are reduced by five.
The conditions have been changed and converted, and the high temperature ninefold copper upgrade.
The number of tubes to be worn is two to three, and eighty-seven percent is full of current.
Note: (1) The current carrying capacity (safety current) of various insulated wires (rubber and plastic insulated wire) is not directly indicated, but is expressed by "multiplied by a certain number of cross-sections", which is obtained by mental arithmetic. It can be seen from Table 5 3 that the multiple decreases as the cross section increases.
"Twenty-five times down to nine, go up one minus one" is said to be 2.5mm' and below various cross-section aluminum core insulated wires, the current carrying capacity is about 9 times the number of sections. For example, a 2.5 mm' wire has a current carrying capacity of 2.5 x 9 = 22.5 (A). The multiple of the current carrying capacity and the number of sections from the wire of 4 mm' and above is the upper row along the line number, and the multiple is successively decreased by 1, that is, 4 × 8, 6 × 7, 10 × 6, 16 × 5, 25 × 4.
"Thirty-five times three-five-five, double-group reduction of five points," said that the 35mm" wire current carrying capacity is 3.5 times the number of sections, that is, 35 × 3.5 = 122.5 (A). For a wire of 50 mm' and above, the multiple relationship between the current carrying capacity and the number of sections becomes a set of two two wire numbers, and the multiple is sequentially decreased by 0.5. That is, the current carrying capacity of the 50, 70 mm' wire is 3 of the number of sections. Double; 95, 120mm" wire current carrying capacity is 2.5 times the number of its cross-sectional area, and so on.
"The conditions have been changed and converted, and the high temperature ninefold copper upgrade." The above-mentioned port is determined by the aluminum core insulated wire and the bright coating at an ambient temperature of 25 ° C. If the aluminum core insulated wire is applied in an area where the ambient temperature is higher than 25 °C for a long time, the current carrying capacity of the wire can be calculated according to the above-mentioned calculation method of the mouth and then 10% off; when the copper wire is insulated, the copper core is used. Its current carrying capacity is slightly larger than that of the same specification aluminum wire. The above-mentioned port method can be used to calculate the current carrying capacity of one wire number than the aluminum wire. For example, the current carrying capacity of a 16mm' copper wire can be calculated as a 25mm2 aluminum wire. Calculate the cable ampacity selection cable (select cable according to current):
The current carrying capacity of the wire is related to the wire cross section. It is also related to the material, type, laying method and ambient temperature of the wire. There are many factors affecting and the calculation is complicated. The current carrying capacity of various conductors can usually be found in the manual. But with the use of mouth and then with some simple mental arithmetic, you can directly calculate, do not have to look up the table. 1. The relationship between the current carrying capacity of the aluminum core insulated wire and the cross section
10 to 5, 100 to 2,
25, 35, four, three worlds.
70, 95, two and a half.
Pipe, temperature, eight or nine fold.
Add half of the bare wire.
Copper wire upgrade count.
It is indicated that the current carrying capacity (A) of various cross-sections is not directly indicated, but is multiplied by a certain multiple of the cross-section. To this end, the nominal cross-section (square mm) of common conductors in China is arranged as follows:
1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185...
(1) The first sentence indicates that the aluminum core insulated wire current carrying capacity (A) can be calculated as a multiple of the cross section. The Arabic numerals in the mouth indicate the wire cross section (square millimeters), and the Chinese character numbers indicate multiples. Arrange the cross section of the mouth and the multiple relationship as follows:
1 to 10 16, 25 35, 50 70, 95 120 or more
〉 〉 〉 〉 〉
Five times four times three times two and a half times two times
Now it is more clear to compare with the mouth. The mouth of the mouth is “10 to 5â€, which means that the cross section is below 10 and the current carrying capacity is five times the cross-sectional value. "100 upper two" (reading two hundred and two) means that the current carrying capacity of 100 or more in cross section is twice the cross-sectional value. Sections 25 and 35 are four and three times the boundary. This is the mouth of "25, 35, four or three circles." The cross-sections 70 and 95 are two and a half times. It can be seen from the above arrangement that, except for 10 or less and 100 or more, the cross section of the middle wire is the same multiple of each of the two specifications.
Calculation of aluminum core current carrying capacityFor example, aluminum core insulated wire, the calculation of the current carrying capacity when the ambient temperature is not more than 25 ° C:
When the cross section is 6 mm 2 , the current carrying capacity is 30 amps;
When the cross section is 150 square millimeters, the current carrying capacity is calculated to be 300 amps;
When the section is 70 square millimeters, the calculated current carrying capacity is 175 amps;
It can also be seen from the above arrangement that the multiple decreases as the cross section increases, and the error is slightly larger at the junction of the fold transition. For example, sections 25 and 35 are four times and three times the boundary, 25 is four times the range, which is calculated as 100 amps by mouth, but 97 amps according to the manual; and 35 is opposite, 105 amps by mouth, but Check the table for 117 amps. However, this has little effect on the use. Of course, if you can "have a number in the chest", when you select the cross section of the wire, 25 does not let it fill 100 amps, and 35 can be slightly more than 105 amps more accurate. Similarly, the 2.5 mm square wire is at the beginning of five times, and the actual is more than five times (up to 20 amps or more). However, in order to reduce the power loss in the wire, the current is usually not used so much. 12 amps.
(2) The last three sentences are the treatment of conditional changes. "Pipe, temperature, eight or nine fold" means: if the pipe is laid (including the laying of the trough plate, that is, the wire is covered with a protective cover layer, it is not clear), after the calculation, another 20% off; if the ambient temperature exceeds 25 ° C, after the calculation, then make a 10 fold, if the pipe is laid, the temperature is more than 25 ° C, then hit a 20% discount and then 10% off, or simply press once to make a 30% discount calculation.
Regarding the ambient temperature, the average maximum temperature of the hottest month in summer is specified. In fact, the temperature is variable. In general, it affects the current carrying of the wire and is not very large. Therefore, discounts are only considered when there are more than 25 °C in some warm workshops or hotter areas. For example, the calculation of the download flow rate of the aluminum insulated wire under different conditions:
When the cross section is 10 square millimeters, the current carrying capacity is 10×5×0.8â•40 amps; if it is high temperature, the current carrying capacity is 10×5×0.9â•45 amps; if it is high temperature, the current carrying capacity is 10 × 5 × 0.7 â• 35 amps.
(3) For the current carrying capacity of bare aluminum wire, the mouth indicates that “naked wire plus half†is calculated and then added half. This means that the same cross-section bare aluminum wire is compared with the aluminum core insulated wire, and the current carrying capacity can be increased by half.
For example, the calculation of the carrying capacity of bare aluminum wire:
When the cross section is 16 square millimeters, the current carrying capacity is 16 x 4 x 1.5 â• 96 amps, and if it is at a high temperature, the current carrying capacity is 16 x 4 x 1.5 x 0.9 = 86.4 amps.
(4) For the current carrying capacity of the copper wire, the port indicates “the copper wire upgrade calculationâ€, that is, the cross-sectional arrangement order of the copper wire is increased by one step, and then calculated according to the corresponding aluminum wire condition.
For example, the bare copper wire with a cross-section of 35 square millimeters has an ambient temperature of 25 ° C, and the current carrying capacity is calculated as: 50 × 3 × 1.5 = 225 amps by upgrading to a bare aluminum wire of 50 square millimeters.
For the cable, it is not described in the mouth. Generally, the high-voltage cable directly buried in the ground can be directly calculated by using the relevant multiple in the first sentence. For example, a 35 mm square high-voltage armored aluminum core cable is buried at a current carrying capacity of 35 × 3 = 105 amps. 95 square millimeters is about 95 x 2.5 ≈ 238 amps.
The zero-line cross section in the three-phase four-wire system is usually selected as about 1/2 of the phase line cross section. Of course, it must not be less than the minimum cross-section allowed by mechanical strength requirements. In a single-phase line, since the load currents passing through the neutral line and the phase line are the same, the zero line cross section should be the same as the phase line cross section.
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