The usual life situation - you need to install a new outlet or switch in the country. There is everything for this - a socket, wire, fasteners, a place for installation is selected. Take and do. But no! Forgot to check whether it is possible to use the existing wire? To determine this possibility, it is necessary to use such a concept as current density and find out its allowable value.
What is this indicator and why is it so important? It is believed that under normal conditions, an electric current passes through the entire volume of the conductor. Simplified - the value of the current flowing through the cross section of the conductor is considered to be its density. For its measurement in the SI system, a unit such as amperes per square meter (A / sq.m) is used, and it serves to establish the current value that is acceptable when the current flows through the wire.
Where do such requirements come from? Everything is very simple. Any conductor resists the flowing current, as a result of which its part is “lost”. This “lost” current causes the conductor to heat up. If the losses are large, then an excessive heating of the conductor will follow, and it will melt. To avoid this, an allowable current density is set for each type of device, compliance with the requirements of which will ensure reliable operation of the wiring.
Now it should be clear that if the cross section of the conductor is chosen incorrectly, i.e. If you intend to use a load that consumes significant power, or simultaneously turn on several household appliances (kettle, computer, TV, etc.), then a significant current flow will cause the wires to heat up and they will melt. Or cause a fire.
That is why, when choosing the cross-section of wires or in any other case, when it is planned to connect the load, it is necessary to take into account the permissible electric current density. When carrying out design work for these purposes, engineers apply special formulas and tables. However, the calculation of the cross section of the wire according to the permissible current density gives a quite acceptable result from a practical point of view.
For this, it is not necessary to use complex formulas and bulky tables. It is only necessary to take into account one value - the current density in the copper wire should not exceed six to ten amperes per square. millimeter. The data presented are valid specifically for copper. Since copper wires are mainly used now, the indicated data should be enough to determine the possibility of its application.
Understand these numbers as follows. During prolonged use, the current density in the circuit should not exceed 6 A / sq. mm Then the wiring will work in light mode and it can be used for a long time. However, it can be operated in 10 A / sq. mm, but this mode should be short-lived.
How to put it into practice? It is necessary to determine the maximum connected load. Let it be, for example, a chandelier with five light bulbs of 100 W each, a TV, a computer, a refrigerator, is simultaneously turned on. The total power does not exceed 5 kW, divide this indicator by the voltage of the network, divide 5000 W by 220V, we get a rounded 23A. So, to ensure the passage of such current in a short-term mode, a copper wire with a cross section of 2.5 sq. millimeter.
You can approach this issue in a different way. If the outlet says that it is designed for a current of 6A, then use copper wires with a cross section of more than one square. mm is not necessary, the outlet will burn earlier. Just for information - if aluminum is used as the conductive material, then the permissible current density will be 4-6 A / sq. mm So, using the figures given, you can determine which section of the wire should be used for the wiring device.
The above material defines the concept of “current density”. The need for its application to select the desired wire section is described. Practical recommendations for using this parameter during operation are given.