The movement of charges inside the conductor causes it to heat up. Electric cables work in difficult conditions, they are selected according to the workload and the long-term permissible current. There are methods that allow you to independently calculate the parameters of the cores, wires and sheaths of cords. In practice, they often resort to using the tables recommended by the rules of the PUE - electrical installations.
Content
- Criterias of choice
- Determination of the limiting current
- Table values
Criterias of choice
The design of the power grid begins with the characteristics of the equipment and transmission facilities intended for installation. The main ones are cords and wires. To prevent cable destruction due to long-term operation under load, it is necessary to correctly select its parameters.
The main criteria for assessing the performance of the circuit:
- values of working and long-term permissible current;
- power receivers of electricity;
- mains voltage.
The heating source of the conductors is a kind of friction
There are many reasons for overheating, but more often it occurs due to the wrong choice of conductor cross-section and weak contacts on connecting devices. In the first case, they act according to the following scheme.
When several consumers are in the circuit, their load is summed up with a margin of 30-40% for the correct selection of the cross-section of the transmission lines at each section and at the input from the generating source. According to the installed power and voltage of the network, the long-term permissible current of the cable is determined, the choice of the cross-section of individual cores depends on it. After that, they find out the operating conditions of the network: the ambient temperature and the method of laying - in the ground, in a box or in an open space.
Another factor manifests itself in distribution cabinets, panels, disconnectors and circuit breakers. If the terminals and conductors do not fit completely, heating occurs to the degree of destruction of the device. This reason is eliminated by periodic monitoring of the condition and tightening the connections, using special terminal blocks.
The long-term permissible current load across the cable cross-section is the current value at which the temperature reaches the maximum permissible value. For different brands of products, external conditions and operating mode, the appropriate size of the permissible load is set.
Determination of the limiting current
Correctly selected cross-section of cable conductors excludes overheating from moving electrons at the lowest consumption of non-ferrous metals. Copper cords are used in electrical engineering more often than others, because they have better conductivity.
The permissible current is determined by the formula: I = P / U, where P is the total power of consumers, U is the voltage in the network. For copper, the value of I is 10 A per 1 mm2 of the section, aluminum - 8.
The flow of 10 A through a conductor with an area of 1 mm / sq. Is possible only for a short period - at the time the device is turned on. A load of 12 amperes at the same cross-section will lead to an increase in temperature and melting of the insulation. When installing a hidden electrical network (in a tubular channel or wall), the maximum allowed value is reduced using a factor of 0.8. For reasons of mechanical strength, power wires are selected with a cross section of ≥4 mm / sq.
The concept of continuous allowable current (Idd) for heating means the load on the cable for a long time when the nominal temperature of the conductor is reached. Calculation formula: Idd = √Kdd x S x Tdd / R, where:
- Kdd - heat transfer coefficient;
- S = 3.14 x d x L - cooled surface;
- Тдд - permissible temperature rise;
- R - resistance.
When calculating Idd, the indicators of the hottest environment are used, since at low temperatures the heat transfer efficiency is much higher. For cables laid in the ground at 70-80 cm, take 15 ºС, indoors - up to 25 ºС. Formula calculations are quite complex, therefore, in practice, they use the recommended PUE tables of permissible currents for the cross-sections of wires and cables. The nominal temperature of conductors in rubber, plastic and lead insulation is assumed to be +65 ºС.
Table values
When performing electrical work in everyday life, cords with a cross section of 1.5 are often used; 2,5; 4,0; 6.0 mm / sq. Based on the normative indicators of the rules of the PUE, a table has been formed for the selection of Idd of cable products and protection currents for a single-phase network.
| Types of electrical loads | Lighting and signaling | Sockets for household appliances | Water heaters, air conditioners | Stove, oven | Entrance to the apartment |
| Maximum power at a voltage of 220 V, kW | 4,1 | 5,9 | 8,3 | 10,1 | 15,4 |
| Cross-section of conductors of copper wires and cables, mm2 | 1,5 | 2,5 | 4,0 | 6,0 | 10,0 |
| Amount of current, A | |||||
| -long admissible | 19 | 27 | 38 | 46 | 70 |
| - limit protective | 16 | 20 | 32 | 40 | 63 |
| -nominal safety | 10 | 16 | 25 | 50 | 52 |
The technical parameters of cables are diverse and differ in marking, number of cores, design of insulating sheaths. Overheating of the conductors is excluded by the correct selection of the long-term permissible current strength.
