Calculation of cable cross-section based on economic current density

Selecting the cross-section of wires and cables based on economic current density

Today we will consider such a concept as economic current density, we will analyze the requirements of the PUE for choosing the cross-section of wires and cables according to the economic current density, we will learn the physical and economic meaning of the concept of economic current density when choosing the cross-section of cables.

Any consumer wants to receive electricity at the lowest cost. The cost of electricity directly depends on the cross-section of wires and cables of electrical networks. If the cross sections are underestimated, then electricity losses in the networks will increase, and this will lead to an increase in the cost of 1 kWh. With increased cross-sections of supply lines, electricity losses will be less, but the initial investment in network construction will increase.

The cross section that corresponds to the minimum cost of electricity transmission is usually called economic.

It is not so easy to mathematically determine the economic cross-section of the conductors of wires and cables.

According to the PUE, the economic cross-section of wires and cables should be determined depending on the economic current density (Chapter 1.3):

S=Imax/Jek

where Imax is the calculated current per hour of maximum power system, A;

Jek is the normalized value of economic current density, A/mm2, for given operating conditions, selected from the table. Jek depends on the material and the time of use of the maximum load.

The calculated current is taken in normal mode, without taking into account emergency overloads. The resulting section is rounded to a larger standard section.

Table 1.3.36 (PUE) - Economic current density.

According to the rules for electrical installations based on economic current density, the following are not selected:

• 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 voltages up to 1 kV, as well as lighting networks of industrial enterprises, residential and public buildings;
• networks of temporary structures, as well as devices with a service life of 3-5 years.

From my experience, I can say that 0.4 kV networks are not checked for economic current density; it is enough to select based on the heating of the conductors and the permissible voltage drop, as well as perform protection against short-circuit currents.

1.3.29

When using the table. 1.3.36 the following must be followed (see also 1.3.27):

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

2. For insulated conductors with a cross-section of 16 mm or less, the economic current density increases by 40%.

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

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