Current Density

The usual life situation - you need to install a new outlet or switch at the dacha. All for this is - a socket, wire, fasteners, a place is chosen for installation. Take it and do it. But no! Forgot to check, but can I use the existing wire? To determine this possibility, it is necessary to use such a concept as the current density, and to find out its permissible value.

What is this indicator and why is it so important? It is believed that under ordinary conditions the electric current passes through the entire volume of the conductor. Simplified - the value of the current flowing through the conductor section is considered its density. To measure it in the SI system, one unit is used, such as ampere per square meter (A / m 2), and it serves to establish the amount of current allowed by the current flowing through the wire.

Whence such requirements? Everything is very simple. Any conductor exerts resistance to a 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, excessive heating of the conductor will follow, and it will melt. In order to avoid this, an acceptable current density is established 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 conductor section is not selected correctly, i.e. It is supposed to use a load that consumes considerable power, or simultaneously include several household appliances (kettle, computer, TV, etc.), then a significant current will cause heating of the wires and they will melt. Or cause a fire.

That's why when choosing the wire section or in any other case, when the connection of the load is planned, it is necessary to take into account the permissible density of electric current. When performing design work for these purposes, engineers use special formulas and tables. However, calculating the cross-section of the wire from the permissible current density gives a result that is quite acceptable from a practical point of view.

For this, it is not at all necessary to use complex formulas and cumbersome tables. One need only take into account one value - the current density in a copper wire should not exceed six to ten amperes per square meter. millimeter. The data given are valid for copper. Since copper wires are currently used, the specified data should be sufficient to determine whether it can be used.

It is necessary to understand these figures as follows. With long-term operation, the current density in the circuit should not exceed 6 A / sq. Mm. Then the wiring will work in a lightweight mode and it can be used for a long time. However, it can be operated in the 10 A / sq. Mode. Mm, but this mode should be short-lived.

How can this be applied in practice? It is necessary to determine the maximum connected load. Let it be, for example, simultaneously turned on a chandelier with five 100 W lamps, a TV, a computer, a refrigerator. The total power does not exceed 5 kW, we divide this figure by the voltage of the network, we divide by 5000 W into 220 V, we obtain roundly 23A. Hence, to ensure short-term transmission of such a current requires a copper wire cross section of 2.5 square meters. Millimeter.

You can approach this issue in a different way. If the socket is written that it is designed for 6A current, then use copper wires cross-section of more than one square. Mm is not necessary, the socket will burn earlier. Just for information - if aluminum is used as a conductive material, then the permissible current density will be 4-6 A / sq. Mm. So, using these figures, you can determine which wire section should be used for the wiring device.

In the above material, the definition of the term "current density" is given. The necessity of its application for choosing the required cross-section of wires is described. Practical recommendations on the use of this parameter at work are given.