Basic knowledge of wires and cables

Basic knowledge of wires and cables

1mm2What is the maximum current and power that the power cord can withstand? for example2.5How to calculate the size of a square wire to be used in construction projects?

①For1.5、2.5、4、6The10mm2The cross-sectional area of the wire can be multiplied by5Twice.

②For16The25mm2The cross-sectional area of the wire can be multiplied by4Twice.

③For35The50mm2The cross-sectional area of the wire can be multiplied by3Twice.

④For70The95mm2The cross-sectional area of the wire can be multiplied by2.5Twice.

⑤For120、150The185mm2The cross-sectional area of the wire can be multiplied by2Twice.

Operating Temperature30℃Long term continuous90%The current carrying capacity under load is as follows:

1.5square millimeter――18A

2.5square millimeter――26A

4 square millimeter――26A

6 square millimeter――47A

10 square millimeter――66A

16 square millimeter――92A

25square millimeter――120A

35square millimeter――150A

powerP=voltageU×currentI＝220ambush×18An=3960tile

National StandardGB4706.1-1992/1998The specified load current value of the wire (partial)

Copper core wire: cross-sectional area of copper core wire.. Allow long-term current..

2.5 square millimeter(16A～2)

4square millimeter(2～32A)

6square millimeter(32A～40A)

Aluminum core wire: cross-sectional area of aluminum core wire.. Allow long-term current..

2.5 square millimeter(13A～20A)

4square millimeter( 20A～2)

6square millimeter( 2～32A)

1Each computer consumes approximately200～300W(about1～1.)So, then10A computer needs one2.5 Supply power with copper core wires of square millimeters, otherwise a fire may occur.

2, Big3The power consumption of the air conditioner is about3000W(about14A)So, then1A separate air conditioner is needed2.5 Powered by copper core wires of square millimeters.

3The current housing entry line is generally4Square millimeter copper wire, therefore, household appliances that are simultaneously turned on must not exceed2(namely5500tile)Someone replaced the wires inside the house with6Square millimeter copper wire is useless because the wire entering the meter is4Square millimeters.

4Early housing(15年前)The incoming line is usually2.5Aluminum wire of square millimeters, therefore, household appliances that are turned on simultaneously must not exceed13A(namely2800tile).

5The household appliances with relatively high power consumption are: air conditioning(1.2horse)Electric water heater10A, Microwave Oven4A, rice cooker4ADishwasher, dishwasher8AWashing machine with drying function10AElectric water heater4A.

In fires caused by power sources, there are90%It is caused by the heating of the joints, so all joints must be welded. Contact devices that cannot be welded5～10Annual replacement is required(For example, sockets, air switches, etc).

Long term current allowed by national standards

4The square is25-32A

6The square is32-40A

Actually, these are all theoretical safety values,The limit values should be greater than these,2.5What is the maximum power allowed for square copper wire:5500W. 4squared8000W,6square9000WNo problem.

40AThe digital electricity meter is functioning properly9000WNo problem.mechanical12000WIt won't burn either.

Copper core wire and cable current carrying capacity standard cable current carrying capacity port decision:

Estimation mnemonic:

Multiply 2.5 by 9 and subtract one from the top to follow the order.

Thirty five times three and a half, in pairs, subtract five points.

Conditions are subject to change and conversion, with high temperature copper upgraded at a 10% discount.

The number of pipes passing through is 2-34, with a full load flow of 876.

Instructions:

This section's mnemonic applies to various insulated wires(Rubber and plastic insulated wires)The current carrying capacity(safe current)Not directly pointed out, but”Multiply the cross-section by a certain multiple”To express, obtained through mental calculation.

“Multiply 2.5 by 9, subtract one from the top and follow the order”What he said is2．5mm’The current carrying capacity of aluminum core insulated wires with various cross-sections and below is approximately the number of cross-sections9Twice.

as2．5mm’Wire, with a current carrying capacity of 2．5×9＝22．5(A).

from4mm’The multiple relationship between the current carrying capacity and the number of cross-sections of wires above and above is that they are arranged in ascending order according to the wire number, and the multiple decreases graduallylThat is4×8、6×7、10×6、16×5、25×4.

“Fifteen times three and a half, in pairs, subtract five points”What he said is35mm”The current carrying capacity of a wire is the number of cross-sections3.5Double, that is35×3.5＝122.5(A).

50mm’The multiple relationship between the current carrying capacity and the number of cross-sections of wires with a current carrying capacity of two or more becomes a group of two wire numbers, and the multiple decreases sequentially0.5That is50The70mm’The current carrying capacity of a wire is determined by the number of cross-sections3Double;95The120mm”The current carrying capacity of a wire is the number of its cross-sectional areas2.5Multiply, and so on.

“Conditions are subject to change and conversion, with high temperature copper upgraded at a 10% discount”The above mnemonic is aluminum core insulated wire, exposed to ambient temperature25℃It depends on the conditions.

If the aluminum core insulated wire is exposed for a long time in an environment with a temperature higher than 25℃In the region, the current carrying capacity of the wire can be calculated using the above formula, and then discounted by 10%; When using copper core insulated wire instead of aluminum wire, its current carrying capacity is slightly larger than that of aluminum wire of the same specification. The current carrying capacity that is one wire size larger than aluminum wire can be calculated using the above formula. as16mm’The current carrying capacity of copper wire can be determined according to25mm2Aluminum wire calculation.

Cable current carrying capacity selection cable (select cable based on current):

The current carrying capacity of a wire is related to the wire cross-section, as well as the material, model, laying method, and environmental temperature of the wire. There are many influencing factors, and the calculation is also complex. The current carrying capacity of various wires can usually be found in the manual. But by using mnemonics combined with some simple mental arithmetic, it can be calculated directly without looking up tables.

1. mnemonic rhyme The multiple relationship between current carrying capacity and cross-sectional area of aluminum core insulated wire

10Next five,100On the second,

25、35Four, Three Realms,

70、95Two and a half times.

Tube insertion, temperature, 20% off.

Add half of the bare wire.

Copper wire upgrade calculation.

The mnemonic does not directly indicate the current carrying capacity (A) of various cross-sections, but rather represents it by multiplying the cross-section by a certain multiple.

For this purpose, the nominal cross-sections (in square millimeters) of commonly used wires in China are arranged as follows:

1、1.5、 2.5、 4、 6、 10、 16、 25、 35、 50、 70、 95、 120、 150、185……

(1)*The mnemonic states that the current carrying capacity (A) of aluminum core insulated wire can be calculated as a multiple of the cross-section.

The Arabic numerals in the mnemonic represent the cross-sectional area of the wire (in square millimeters), while the Chinese numerals represent multiples. Arrange the cross-section and multiplication relationship of the mnemonic as follows:

1～10 16、25 35、50 70、95 120above

fivefold quadruple triple Two and a half times double

Now comparing it with the mnemonic will make it clearer“10Next Five”It refers to the cross-section in10Below, the current carrying capacity is five times the cross-sectional value.“100Up two”(Read over two hundred) refers to the cross-section100The above current carrying capacity is twice the cross-sectional value. The cross-section is25and35It's the boundary between four times and three times. This is the mnemonic“25、35The Four Three Realms”And the cross-section70、95It is 2.5 times. From the above arrangement, it can be seen that: except10 The following and100In addition to the above, the cross-sectional area of the middle wire belongs to the same multiple for every two specifications.

For example, for aluminum core insulated wires, the ambient temperature should not exceed25℃Calculation of current carrying capacity:

When the cross-section is6When measured in square millimeters, the calculated current carrying capacity is30An;

When the cross-section is150When measured in square millimeters, the calculated current carrying capacity is300An;

When the cross-section is70When measured in square millimeters, the calculated current carrying capacity is175An;

From the above arrangement, it can also be seen that the multiplier decreases with the increase of the cross-section, and the error is slightly larger at the boundary of multiplier transformation. For example, cross-section25and35At the boundary between four and three times,25It belongs to the four fold range, and according to the mnemonic, it is calculated as100An, but according to the manual97An; But35On the contrary, according to the mnemonic, it is calculated as105An, but the table shows117An. However, this has little impact on usage. Of course, if possible“have a good idea of how things stand”When selecting a wire cross-section,25Don't let it fill up100An,35It can be slightly more than105An is even more accurate. Similarly,2.5The position of the square millimeter wire is at the beginning of five times, but in reality, it is more than five times (the largest can reach) 20However, in order to reduce the energy loss inside the wire, the current usually does not need to be so high, and the manual usually only indicates it12An.

(2)The last three mnemonics are for handling changes in conditions.

“Tube insertion, temperature, 20% off”It refers to: if the wire is laid through a conduit (including the laying of trough plates, etc., that is, the wire is covered with a protective layer and not exposed), after calculation, an additional 20% discount will be applied; If the ambient temperature exceeds25℃After calculation, a 10% discount will be applied. If the pipeline is laid and the temperature exceeds25℃If you give it a 20% discount, then give it a 10% discount, or simply give it a 30% discount at once.

Regarding environmental temperature, according to regulations, it refers to the average highest temperature in the hottest month of summer. In fact, temperature is variable and generally does not have a significant impact on the current carrying capacity of wires. Therefore, only for certain warm workshops or hotter areas exceeding25℃Only consider discounts when there are many.

For example, the calculation of download flow rate for aluminum core insulated wire under different conditions:

When the cross-section is10When passing through a square millimeter tube, the current carrying capacity is10×5×0.8═40An; If it is high temperature, the current carrying capacity is10×5×0.9═45An; If the pipe is penetrated and the temperature is high, the current carrying capacity is10×5×0.7═35An.

(3)The mnemonic for the current carrying capacity of bare aluminum wire states“Bare wire plus half”Add half after calculation. This refers to the fact that the current carrying capacity of bare aluminum wire with the same cross-section can be increased by half compared to aluminum core insulated wire.

For example, the calculation of the current carrying capacity of bare aluminum wire:

When the cross-section is16When measured in square millimeters, the current carrying capacity is16×4×1.5═96An, if at high temperature, the current carrying capacity is16×4×1.5×0.9=86.4An.

(4)The mnemonic for the current carrying capacity of copper wires states“Copper wire upgrade calculation”Raise the cross-sectional arrangement order of copper wires by one level, and then calculate according to the corresponding aluminum wire conditions.

For example, the cross-section is35The ambient temperature for square millimeter bare copper wire is25℃The calculation of current carrying capacity is: upgraded to50Square millimeter bare aluminum wire is obtained50×3×1.5=225An.

For cables, there is no introduction in the mnemonic. Generally, high-voltage cables buried directly can be calculated using the relevant multiples in the mnemonic. for example35The current carrying capacity of the square millimeter high-voltage armored aluminum core cable buried underground is35×3=105An.95The square millimeter is approximately95×2.5≈238An.

The neutral wire section in the three-phase four wire system is usually selected as the phase wire section1/2about. Of course, it must not be smaller than the smallest cross-section allowed according to mechanical strength requirements. In single-phase power lines, since the load current passing through the neutral and phase lines is the same, the cross-section of the neutral line should be the same as that of the phase line.