The choice of the cross-section of conductors according to the economic current density

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Section 1 General Rules

Selection of conductors for heating, economic current density and corona conditions. The choice of the cross-section of conductors according to the economic current density

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Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE)

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1.3.25. Conductor cross-sections must be checked against economic current density. Economically expedient section s, mm², is determined from the relation

where I is the rated current per hour of the maximum power system, A;

J_eq - normalized value of economic current density, A/mm², for the specified operating conditions, selected according to the table. 1.3.36.

The cross section obtained as a result of the specified calculation is rounded up to the nearest standard cross section. The rated current is taken for normal operation, i.e., the increase in current in the post-accident and repair modes of the network is not taken into account.

1.3.26. The choice of wire cross-sections of DC and AC power lines with a voltage of 330 kV and above, as well as interconnection lines and powerful rigid and flexible conductors operating with a large number of hours of maximum use, is made on the basis of technical and economic calculations

1.3.27. An increase in the number of lines or circuits in excess of that required by the conditions of reliable power supply in order to meet the economic current density is carried out on the basis of a technical and economic calculation. At the same time, in order to avoid an increase in the number of lines or circuits, a twofold excess of the normalized values \u1.3.36b\uXNUMXbgiven in Table. XNUMX..

Feasibility studies should take into account all investments in an additional line, including equipment and switchgear chambers at both ends of the lines. The feasibility of increasing the line voltage should also be checked.

These guidelines should also be followed when replacing existing wires with larger wires or when laying additional lines to ensure economic current density with increasing load. In these cases, the total cost of all dismantling and installation of line equipment, including the cost of apparatus and materials, should also be taken into account.

Table 1.3.36. Economic current density

Guides	Economic current density, A/mm², with the number of hours of use of the maximum load per year
Guides	more than 1000 to 3000	more than 3000 to 5000	more 5000
Bare wires and busbars:
copper	2,5	2,1	1,8
aluminum	1,3	1,1	1,0
Cables with paper and wires with rubber and PVC insulation with cores:
copper	3,0	2,5	2,0
aluminum	1,6	1,4	1,2
Cables with rubber and plastic insulation with cores:
copper	3,5	3,1	2,7
aluminum	1,9	1,7	1,6

1.3.28. The following are not subject to verification by economic current density:

networks of industrial enterprises and structures with voltage up to 1 kV with the number of hours of use of the maximum load of enterprises up to 4000-5000;
branches to individual electrical receivers with voltage up to 1 kV, as well as lighting networks of industrial enterprises, residential and public buildings;
busbars of electrical installations and busbars within open and closed switchgears of all voltages;
conductors going to resistors, starting rheostats, etc.;
networks of temporary structures, as well as devices with a service life of 3-5 years.

1.3.29. When using the table 1.3.36. it is necessary to be guided by the following (see also 1.3.27.):

1. At the maximum load at night, the economic current density increases by 40%.

2. For 16 mm insulated conductors² and less economic current density increases by 40%.

3. For lines of the same cross section with n branching loads, the economic current density at the beginning of the line can be increased by k_у times, and k_у is determined from the expression

where I₁, The₂, ..., I_n - loads of individual sections of the line;

l₁,₂, ..., l_n - the length of individual sections of the line;

l is the total length of the line.

4. When choosing the cross-sections of conductors for powering n of the same type, mutually redundant electrical receivers (for example, water supply pumps, converter units, etc.), of which t are simultaneously in operation, the economic current density can be increased against the values \u1.3.36b\uXNUMXbgiven in Table XNUMX .XNUMX., in k_n times, where k_n equals:

1.3.30. The cross section of wires of 35 kV overhead lines in rural areas, supplying step-down substations 35/6 - 10 kV with transformers with voltage regulation under load, should be selected according to the economic current density. It is recommended to take the design load when choosing wire cross-sections for the future in 5 years, counting from the year the overhead line was put into operation. For 35 kV overhead lines intended for redundancy in 35 kV networks in rural areas, the minimum continuous current cross-sections of wires should be used, based on the provision of power to consumers of electricity in post-emergency and repair modes.

1.3.31. The choice of economic cross-sections of overhead wires and cores of cable lines with intermediate power take-offs should be made for each of the sections, based on the corresponding rated currents of the sections. At the same time, for neighboring sections it is allowed to take the same wire section corresponding to the economic one for the longest section, if the difference between the values of the economic section for these sections is within one step on the scale of standard sections. The cross sections of wires on branches up to 1 km long are taken the same as on the overhead line from which the branch is made. With a longer branch length, the economic section is determined by the calculated load of this branch.

1.3.32. For power lines with a voltage of 6-20 kV, given in table. 1.3.36. Current density values may be used only when they do not cause voltage deviations at power receivers in excess of permissible limits, taking into account the applied means of voltage regulation and reactive power compensation.

See other articles Section Rules for the installation of electrical installations (PUE).

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