Determination of the cross section of the conductors of wires. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Determination of the cross section of the conductors of wires. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric installation work

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To accurately determine the cross section of wire cores, select switches, sockets, protection devices and electricity metering, you need to know currents flowing through them, the value of which depends on the wiring diagram and the power of consumers.

As an example of calculating currents, we will use the calculated electrical circuit (Fig. 28).

Determining the cross section of wire cores
Rice. 28. Estimated electrical circuit of a residential building

The currents of electricity consumers can be taken from passports and instructions. If these data are not available, they can be calculated by dividing the power of the electrical appliance in watts by the rated supply voltage (usually 220 V). When determining the current of a single-phase electric motor, the resulting value should be multiplied by 2 to take into account the efficiency and real currents consumed from the network.

The calculation is carried out against the flow of energy - from the distant consumer to the shield (in our case, the calculation must be carried out from the wall lamp to the shield). Only the wall lamp current of 51 A flows through the wires of the X52 socket to the X0,1 branch. Next, the currents of the TV (1 A) and the chandelier (0,81 A) are added to it. This means that a total current of 1,91 A flows through the wires introduced into the room. If we carry out the calculation further, it turns out that the largest load in the line is an electric stove with a current of 7,2 A, and a total current flows through the branch wires to sockets X53 and X54 stove and refrigerator 9,2 A. The most loaded wires in the area from the branch to the sockets X53 and X54 to the shield. Through them and through the fuse link of the fuse FU1, the total current of all line loads flows, equal to approximately 12 A.

The calculation of the second line gives the value of the current flowing through the wires of its head section and the fuse insert of the FU2 fuse (6 A). And the total current of both lines passes through the meter - about 45 A.

The currents in the phase and neutral wires are the same in magnitude. The directions of the currents in the wiring sections shown near the phase wires are taken arbitrarily.

When drawing up the scheme, the following was taken into account.

Electrical loads of significant power are placed closer to the input shield so that the wires to their sockets do not have connections and are as short as possible. With a significant removal of a powerful load from the shield, its current will cause a voltage drop in the extended wires of the line. The voltage on other consumers will change when a powerful load is turned on and off, which will manifest itself in the flashing of lighting lamps, a change in the brightness of the TV screen, etc. And with a short line to a powerful consumer, made with wires with a large cross section of cores, such effects will be minimal.

Of the currents of two consumers connected to the X55 socket - a washing machine and an iron - the one that creates large loads for the wiring is taken into account.

The cases of most electric stoves, ovens must be zeroed. To connect them, you need a socket with a protective zeroing contact, which is installed in the kitchen.

The calculation of consumer currents shows that currents of 1, 2 and 11 A will flow through the fuses FU76 and FU6,45. These indicators are important when choosing the rated currents of the fuse links.

It must be borne in mind that the real currents through the fuses and the meter will be less, since all consumers rarely turn on at the same time in the house, however, the choice of wires, protection devices and electricity metering must be carried out in relation to such a case.

Now it's worth considering selection of wire cross section.

The wires of the wiring, when current flows through them, have a temperature greater than the ambient temperature. It does not negatively affect the metal of the wires, but destroys the insulation on them, which, when heated, ages, becomes brittle, cracks and crumbles.

The heating of wire cores is affected by the method of laying wires, their number, placement nearby, insulation material. These factors should be taken into account when choosing the cross section of the wires, which would provide power to consumers without overheating the wires.

Of the two values of the cross section of the conductors of the wires, selected according to the conditions of heating by a long-flowing current and mechanical strength, the largest is selected for installation.

Example of wire size selection

Initial data (currents in the wiring sections according to our calculation scheme): wire - APRF with 2 or 3 aluminum conductors in rubber insulation in a folded aluminum sheath; laying method - on the surface of walls and ceilings.

Selection by heating conditions. The cross section of the wires of the input wires from the insulator on the outer side of the wall of the house to the meter is 2,5 mm (long-term permissible current - 21 A, calculated - 18, 21 A). The cross section of the wires from the shield to the sockets X53, X54 is 2,5 mm (long-term permissible current is 21 A, calculated 11,76 A). The APRF wire is produced with cores of at least 2,5 mm, therefore it is also applicable in all other areas where the currents are less than the currents in the head sections of the first line.

Selection according to the condition of mechanical strength. The smallest allowable section of aluminum conductors of protected wires connected to screw terminals is 2 mm2, the minimum allowable section of aluminum conductors of input wires is 4 mm2.

The cross section of the conductors of the wires entering the building is selected according to the condition of mechanical strength - 4 mm2. The cross section of the wires from the shield to the sockets X53, X54 is 4 mm2. This decision looks irrational, since wires with a conductor cross section of 2,5 mm allow a long current to flow up to 21 A without overheating, and the rated current does not exceed 12 A. But since you have to purchase a wire with a cross section of 4 mm to enter the house, that is, it makes sense and the most loaded section of the wiring from the shield to the electric stove socket should be made with wires of a larger cross section, which will reduce their heating and reduce voltage fluctuations in the network. For all other wiring sections, a wire with a core cross section of 2,5 mm can be used.

Author: Bannikov E.A.

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