How to correctly select and calculate a circuit breaker (simple calculation of a circuit breaker). Calculation of cable cross-section and circuit breaker Single-phase overcurrent circuit breaker

To control electrical wiring in an apartment or house, special protective devices are used that turn off the electricity when the network is overloaded. Characteristics such as load current and network voltage determine the ratings of circuit breakers.

Types of devices

There are several types of devices that can monitor wiring and, if necessary, cut off electrical power. They are:

Miniature (mini-models);
Air (open version);
Enclosed molded case switches;
RCD (Residual Current Devices);
Automatic switches additionally equipped with RCD (differential).

Miniature devices are designed to work in networks with light loads; as a rule, they do not have additional adjustment functions. This model range is represented by circuit breakers with a breaking capacity designed for a misfire current from 4.5 to 15 kA. Therefore, they are used most often in household wiring, since higher current strength is required for production capacities.

Photo - model with a nominal value of 32 A

Models produced by Schneider Electric are very popular. There are machines on sale with ratings from 2 to 125 A, which allows you to select a separate device even for a small group of devices, for example, for connecting lighting or other electrical equipment (sconce, electric kettle, etc.).

If devices with a higher rating are required, say, to control the operation of electrical networks to which powerful consumers are connected, air-type circuit breakers are selected. Their cutoff current rating is an order of magnitude higher than that of miniature models. As a rule, they are produced in a three-pole design, but now many companies, including IEK, produce four-pole models.

Installation of automatic switches is carried out in a special cabinet where DIN rails are installed for their fastening. Distribution cabinets with the appropriate protection class (at least IP55) can be placed in open space (poles, street switchboards, etc.). The waterproof housing, made of refractory materials, ensures the proper level of safety.

The model line of these circuit breakers allows a slight deviation (up to 10%) from the specified characteristics. The biggest advantage of these machines over miniature ones is the ability to customize the operating parameters of the device.

Photo - option for low-voltage networks

For this purpose, special inserts are used, with which you can control the current strength at the contacts. In other words, when installing a calibrated insert on the active contact, it becomes possible to change the parameters of the switch, which in some conditions makes it possible to expand the nominal characteristics. Regardless of the range of action and ratings, circuit breakers have the same size across the entire model range, the only dimension that changes is the width (modularity). It depends on the number of poles (there can be 2 or more).

Automatic switches are mounted in a vertical position, with the exception of devices designed over 5000 A and 6300 A. They can be used for installation in open areas or in special switchboards. The advantage of such devices is the presence of additional contacts and connections, which significantly expands the scope of use and installation possibilities.

Enclosed circuit breakers are manufactured in a cast housing made of refractory material. This makes them completely sealed and suitable for use in extreme conditions. On average, the range of such machines is used with a current of up to 200 Amperes and a voltage of up to 750 Volts. Based on their operating principle, they are divided into the following types:

adjustable;
Thermal;
Electromagnetic.

Depending on the needs, you need to choose the optimal operating principle of the devices. Electromagnetic type devices are considered the most accurate, since they determine the rms value of active currents and are triggered in the event of a short circuit. This allows you to prevent all negative consequences in advance.

Photo - solid-cast IEC

Any of the listed types of devices can be manufactured in one of four standard sizes, with a cut-off current in the range from 25 to 150 A. The design can be two, three and four poles, which allows them to be used when connecting to the power supply network of both residential and production premises.

Electromagnetic machines have proven themselves to be excellent devices that can control the operation of the motors of machine tools or other equipment. A distinctive feature is the ability to withstand current impulses of up to 70,000 Amperes. The rated operating current is indicated on the device body.

Photo - AE series machine gun

RCDs cannot be considered independent devices for protecting networks from overvoltage. It is recommended to use them either in tandem with automatic machines, or immediately buy a switch equipped with an additional protection device (differential automatic devices). At the same time, during installation of wiring, the RCD is installed in front of the machines, and not vice versa. Otherwise, the device may simply burn out due to high short-circuit current pulses.

Video: load switches

Machine denominations (calculation according to the table)

To select the correct ratings for home and industrial circuit breakers, a special table is used:

Current (A)	Network power with 1 phase (kW)	Power of 3-phase network (kW)	Permissible wire cross-sections (mm 2)
-	-	-	copper	aluminum
1	0,2	0,5	1	2,5
2	0,4	1,1	1	2,5
3	0,7	1,6	1	2,5
4	0,9	2,1	1	2,5
5	1,1	2,6	1	2,5
6	1,3	3,2	1	2,5
8	1,7	5,1	1,5	2,5
10	2,2	5,3	1,5	2,5
16	3,5	8,4	1,5	2,5
20	4,4	10,5	2,5	4
25	5,5	13,2	4	6
32	7	16,8	6	10
40	8,8	21,1	10	16
50	11	26,3	10	16
63	13,9	33,2	16	25
80	17,6	52,5	25	35
100	22	65,7	35	50

Calculating the ratings of circuit breakers is also very simple. You need to select a group of devices, for example, it will be a kettle, a lamp, a refrigerator, after which you need to find out their power to determine the rated current. Let's use Ohm's law: I=P/U, Where:

I – current consumed by the equipment (A);
P – equipment power (W);
U – mains voltage (V).

For example, we have a kettle with a power of 1.5 kW (1500 W), a lamp - 100 W, a refrigerator - 300 W; in total, the total value will be 1.9 kW (1900 W), we calculate the rated current: I = 1900/220 = 8.6. The closest automatic device in terms of operating current is 10 A. Naturally, in practice this figure will be higher; modern wiring must be designed for a load current of at least 16 A.

A slight overestimation of the parameters will not cause harm, but an underestimation may result in a short circuit and fire. Experts recommend that when there are a large number of amperes, use not one powerful machine, but several with an average rating - this ensures greater operational reliability.

When assembling an electrical panel or connecting new large household appliances, the home master will definitely encounter such a problem as the need to select circuit breakers. They provide electrical and fire safety, so choosing the right machine is the key to the safety of you, your family and your property.

What is the machine used for?

A machine is installed in the power supply circuit to prevent overheating of the wiring. Any wiring is designed to carry a certain current. If the current passed exceeds this value, the conductor begins to heat up too much. If this situation persists for a sufficient period of time, the wiring begins to melt, resulting in a short circuit. A circuit breaker is installed to prevent this situation.

The second task of the circuit breaker is to turn off the power when a short circuit current (SC) occurs. When a short circuit occurs, the currents in the circuit increase many times over and can reach thousands of amperes. To prevent them from destroying the wiring and damaging the equipment included in the line, the circuit breaker must turn off the power as quickly as possible - as soon as the current exceeds a certain limit.

In order for the protective circuit breaker to properly perform its functions, it is necessary to correctly select the machine according to all parameters. There are not many of them - only three, but you need to deal with each one.

What types of circuit breakers are there?

To protect conductors of a single-phase 220 V network, there are single-pole and double-pole disconnecting devices. To single-pole wires, only one conductor is connected - phase, to double-pole wires, both phase and neutral. Single-pole circuit breakers are installed on 220 V indoor lighting circuits, on socket groups in rooms with normal operating conditions. They are also installed on some types of load in three-phase networks, connecting one of the phases.

For three-phase networks (380 V) there are three and four poles. These circuit breakers (the correct name is a circuit breaker) are installed on a three-phase load (ovens, hobs and other equipment that operate on a 380 V network).

In rooms with high humidity (bathroom, bathhouse, swimming pool, etc.) two-pole circuit breakers are installed. They are also recommended for installation on powerful appliances - washing machines, dishwashers, boilers, ovens, etc.

It’s just that in emergency situations - in the event of a short circuit or insulation breakdown - phase voltage can reach the neutral wire. If a single-pole device is installed on the power line, it will disconnect the phase wire, and the zero with dangerous voltage will remain connected. This means there is still a possibility of electric shock when touched. That is, the choice of machine is simple - single-pole switches are installed on some lines, and double-pole switches on others. The specific amount depends on the network condition.

For a three-phase network, there are three-pole circuit breakers. Such a machine is installed at the entrance and at consumers, to which all three phases are supplied - an electric stove, a three-phase hob, an oven, etc. The remaining consumers are equipped with two-pole circuit breakers. They must disconnect both phase and neutral.

Example of three-phase network wiring - types of circuit breakers

The choice of circuit breaker rating does not depend on the number of wires connected to it.

Deciding on the denomination

Actually, from the functions of the circuit breaker, the rule for determining the rating of the circuit breaker follows: it must operate until the current exceeds the capabilities of the wiring. This means that the current rating of the machine must be less than the maximum current that the wiring can withstand.

Based on this, the algorithm for selecting a circuit breaker is simple:

for a specific area.
See what maximum current this cable can withstand (see the table).
Next, from all the ratings of the circuit breakers, we select the nearest smaller one. The ratings of the machines are tied to the permissible long-term load currents for a particular cable - they have a slightly lower rating (see the table). The list of denominations looks like this: 16 A, 25 A, 32 A, 40 A, 63 A. From this list you choose the appropriate one. There are smaller values, but they are practically not used anymore - we have too many electrical appliances and they have considerable power.

Example

The algorithm is very simple, but it works flawlessly. To make it clearer, let's look at an example. Below is a table that shows the maximum permissible current for conductors that are used at. Recommendations regarding the use of machines are also given there. They are given in the column “Nominal current of the circuit breaker”. It is there that we are looking for denominations - it is slightly less than the maximum allowable, so that the wiring works in normal mode.

Cross section of copper wires	Permissible continuous load current	Maximum load power for single-phase network 220 V	Rated current of circuit breaker	Circuit breaker current limit
1.5 sq. mm	19 A	4.1 kW	10 A	16 A	lighting and alarm
2.5 sq. mm	27 A	5.9 kW	16 A	25 A	socket groups and electric heated floor
4 sq.mm	38 A	8.3 kW	25 A	32 A	air conditioners and water heaters
6 sq.mm	46 A	10.1 kW	32 A	40 A	electric stoves and ovens
10 sq. mm	70 A	15.4 kW	50 A	63 A	opening lines

In the table we find the selected wire cross-section for this line. Suppose we need to lay a cable with a cross-section of 2.5 mm 2 (the most common when laying to medium-power devices). A conductor with this cross-section can withstand a current of 27 A, and the recommended rating of the machine is 16 A.

How will the circuit work then? As long as the current does not exceed 25 A, the machine does not turn off, everything works as normal - the conductor heats up, but not to critical values. When the load current begins to increase and exceeds 25 A, the machine does not turn off for some time - perhaps these are starting currents and they are short-lived. It turns off if the current exceeds 25 A by 13% for a sufficiently long time. In this case, if it reaches 28.25 A. Then the power supply will work and de-energize the branch, since this current already poses a threat to the conductor and its insulation.

Power calculation

Is it possible to choose a machine based on load power? If only one device is connected to the power line (usually large household appliances with high power consumption), then it is permissible to make a calculation based on the power of this equipment. You can also choose an introductory machine based on power, which is installed at the entrance to a house or apartment.

If we are looking for the rating of the input circuit breaker, we need to add up the power of all devices that will be connected to the home network. Then the found total power is substituted into the formula, and the operating current for this load is found.

After we have found the current, select the nominal value. It may be either slightly more or slightly less than the found value. The main thing is that its shutdown current does not exceed the maximum permissible current for this wiring.

When can you use this method? If the wiring is laid with a large margin (this is not bad, by the way). Then, in order to save money, you can automatically install switches that correspond to the load, and not the cross-section of the conductors. But once again we draw your attention to the fact that the long-term permissible current for the load must be greater than the maximum current of the circuit breaker. Only then will the choice of circuit breaker be correct.

Selecting the breaking capacity

The selection of a packager based on the maximum permissible load current is described above. But the network circuit breaker must also turn off when a short circuit (short circuit) occurs in the network. This characteristic is called breaking capacity. It is displayed in thousands of amperes - this is the order currents can reach during a short circuit. Selecting a machine based on its breaking capacity is not very difficult.

This characteristic shows at what maximum value of the short-circuit current the circuit breaker remains operational, that is, it will not only be able to turn off, but will also work after being turned on again. This characteristic depends on many factors and for accurate selection it is necessary to determine the short-circuit currents. But for wiring in a house or apartment, such calculations are done very rarely, and are based on the distance from the transformer substation.

If the substation is located close to the entrance to your house/apartment, take a circuit breaker with a breaking capacity of 10,000 A; for all other city apartments, 6,000 A is enough. If the house is located in a rural area or you are choosing a circuit breaker for a summer residence, it may well be enough and a breaking capacity of 4,500 A. The networks here are usually old and short-circuit currents are not large. And since the price increases significantly with increasing breaking capacity, the principle of reasonable savings can be applied.

Is it possible to install bags with a lower breaking capacity in city apartments? In principle, it is possible, but no one guarantees that after the first short circuit you will not have to change it. He may have time to turn off the network, but will be inoperative. In the worst case scenario, the contacts will melt and the machine will not have time to turn off. Then the wiring will melt and a fire may occur.

Type of electromagnetic release

The machine must operate when the current rises above a certain level. But short-term overloads periodically occur in the network. They are usually associated with inrush currents. For example, such overloads can be observed when turning on the refrigerator compressor, washing machine motor, etc. The circuit breaker should not turn off during such temporary and short-term overloads, because they have a certain delay for operation.

But if the current has increased not because of an overload but because of a short circuit, then during the time that the circuit breaker “wait”, its contacts will melt. This is what an electromagnetic automatic release is for. It operates at a certain current value, which can no longer be an overload. This indicator is also called cut-off current, since in this case the circuit breaker cuts off the line from the power supply. The magnitude of the operating current can be different and is displayed by letters that appear in front of the numbers indicating the rating of the machine.

There are three most popular types:

What characteristics should you choose? In this case, the choice of a circuit breaker is also based on the distance of your household from the substation and the state of the electrical networks; the choice of a circuit breaker is carried out using simple rules:

With the letter “B” on the body they are suitable for dachas, houses in villages and towns that receive power supply through air ducts. They can also be installed in apartments of old houses in which the internal electrical network has not been reconstructed. These circuit breakers are not always on sale; they cost a little more than category C, but can be delivered to order.
Bags with a “C” on the body are the most widely used option. They are installed in networks with normal condition, suitable for apartments in new buildings or after major renovations, in private houses near the substation.
Class D is installed in enterprises and workshops with equipment with high starting currents.

That is, in essence, the choice of a circuit breaker in this case is simple - type C is suitable for most cases. It is available in stores in a large assortment.

Which manufacturers should you trust?

And finally, let’s pay attention to the manufacturers. The choice of a circuit breaker cannot be considered complete if you have not thought about which brand of circuit breakers you will buy. You definitely shouldn’t take on unknown companies - electrical engineering is not a field where you can conduct experiments. Learn more about choosing a manufacturer in the video.

Everyone knows that electricity is not something to joke about. Incorrect calculation of the power supply circuit can lead to at least two unpleasant consequences. The first is when, when several energy-intensive electrical appliances (for example, a washing machine, electric kettle and iron) are turned on, the circuit breakers trip and the network is de-energized. Unpleasant, but not fatal. The second is when, when you turn on the same devices, the automatic devices will not work, and the electrical wiring will begin to melt and smoke. And this is already a mortal danger: there is only one step to a fire. That is why choosing a machine based on load power is of paramount importance.

Automatic single-pole switch Schneider BA63 1P 25A C for 25 amperes.

A little theory

It is known from the physics course that there is a relationship between electrical power, current strength and voltage in the electrical network. In a simplified form, this relationship is expressed by the following formula for a single-phase network:

where W is the current power in watts (W);

I - current strength in amperes (A);

V – voltage in volts (V).

In this case, we will be interested in the current strength, since the circuit breaker and electrical wiring characteristics are often selected based on this parameter. For convenience, we transform the above formula into the expression:

As an example, let’s calculate the current strength for the load that the energy-intensive consumers mentioned above provide to the power grid. Their total power will be about 6 kW, and at a voltage of 220 V we get the current in the circuit:

I = 6000 W / 220 V = 27.3 A

For a three-phase connection diagram, formula (2) will take the following form:

I = W / 1.73V (3)

This change is caused by the fact that with an equal load and uniform distribution of power across phases, the current in a three-phase network will be three times less. Thus, with the same total power of 6 kW, but at a voltage of 380 V, the current in the circuit will be equal to:

I \u003d 6000 W / (1.73 x 380 V) \u003d 9.1 A

Having received this indicator, you can begin to select a circuit breaker that provides network overload protection.

Selection of circuit breaker rating for current and load power

To select a suitable machine, it is convenient to calculate the current per kilowatt of load power and draw up the corresponding table. Applying formula (2) and a power factor of 0.95 for a voltage of 220 V, we obtain:

1000 W / (220 V x 0.95) = 4.78 A

Considering that the voltage in our electrical networks often does not reach the required 220 V, it is quite correct to take the value of 5 A per 1 kW of power. Then the table of current versus load will look like this in Table 1:

This table gives an approximate estimate of the strength of alternating current flowing through a single-phase electrical network when household electrical appliances are turned on. It should be remembered that this refers to peak power consumption, not average. This information can be found in the documentation supplied with the electrical product. In practice, it is more convenient to use the table of maximum loads, which takes into account the fact that machines are produced with a certain current rating (Table 2):

Wiring diagram	Current ratings of automatic machines
	10 A	16 A	20 A	25 A	32 A	40 A	50 A	63 A
Single phase, 220 V	2.2 kW	3.5 kW	4.4 kW	5.5 kW	7.0 kW	8.8 kW	11 kW	14 kW
Three-phase, 380 V	6.6 kW	10,6	13,2	16,5	21,0	26,4	33,1	41,6

For example, if you need to find out how many amperes a machine needs for a power of 15 kW with a three-phase current, then we look in the table for the nearest larger value - it is 16.5 kW, which corresponds to a machine with a capacity of 25 amperes.

In reality, there are limitations on the allocated power. In particular, in modern urban apartment buildings with an electric stove, the allocated power is from 10 to 12 kilowatts, and a 50 A automatic machine is installed at the entrance. It is reasonable to divide this power into groups, taking into account the fact that the most energy-intensive appliances are concentrated in the kitchen and bathroom. Each group is equipped with its own machine, which eliminates the complete loss of power to the apartment in the event of an overload on one of the lines.

In particular, under an electric stove (or hob) it is advisable to make a separate input and install a 32 or 40 ampere machine (depending on the power of the stove and oven), as well as a power outlet with the corresponding rated current. You should not connect other consumers to this group. Both the washing machine and the air conditioner should have a separate line - a 25 A machine will be sufficient for them.

The question of how many sockets can be connected to one machine can be answered in one phrase: as many as you like. The sockets themselves do not consume electricity, that is, they do not create a load on the network. You just need to make sure that the total power of simultaneously switched on electrical appliances corresponds to the cross-section of the wire and the power of the machine, which will be discussed below.

For a private house or cottage, the input machine is selected depending on the allocated power. Not all owners are able to obtain the desired number of kilowatts, especially in regions with limited power grid capabilities. But in any case, as for city apartments, the principle of dividing consumers into separate groups remains.

Introductory machine for a private home

Selection of circuit breaker rating based on wire cross-section

Having determined the rating of the machine based on the power of the “suspended” load, you need to make sure that the electrical wiring can withstand the appropriate current. As a guide, you can use the table below, compiled for a copper wire and a single-phase circuit (Table 3):

As you can see, all three indicators (power, current and wire cross-section) are interconnected, so the rating of the machine can, in principle, be selected according to any of them. At the same time, it is necessary to make sure that all parameters fit together and, if necessary, make appropriate adjustments.

In any case, remember the following:

Installing an overly powerful circuit breaker can lead to the fact that before it operates, electrical equipment that is not protected by its own fuse will fail.
A machine with a low number of amperes can become a source of nervous stress, cutting off power to the house or individual rooms when the electric kettle, iron or vacuum cleaner is turned on.

The circuit breaker is designed to protect the electrical wiring in your apartment, to which consumers in the form of electrical appliances (TVs, kettles, etc.) are connected. In this case, the total power of consumers should not exceed the power of the machine itself. Therefore, it is necessary to correctly select the machine according to the load power, in order to avoid overloading the wiring, which can lead to overheating and subsequent ignition.

Wires must match the load

It often happens that a new electric meter and automatic machines are installed in an old house, but the wiring remains the same. A lot of household appliances are bought, the power is summed up and an automatic machine is selected for it, which regularly holds the load of all switched on electrical appliances.

Everything seems to be correct, but suddenly the wire insulation begins to emit a characteristic odor and smoke, a flame appears, and the protection does not work. This can happen if the wiring parameters are not designed for such current.

Let's say the cross-section of the old cable core is 1.5mm², with a maximum permissible current limit of 19A. We assume that several electrical appliances were connected to it at the same time, making up a total load of 5 kW, which in current equivalent is approximately 22.7 A; it corresponds to a 25 A circuit breaker.

The wire will heat up, but this machine will remain on all the time until the insulation melts, which will lead to a short circuit, and the fire can already flare up in full swing.

Power consumption calculation

In everyday life, you often have to deal with calculating power consumption, for example, to check the permissible load on the wiring before connecting a resource-intensive electrical consumer (air conditioner, boiler, electric stove, etc.).

Also in such a calculation there is a need when choosing circuit breakers for the switchboard through which the apartment is connected to the power supply.

In such cases, it is not necessary to calculate the power by current and voltage; it is enough to sum up the consumed energy of all devices that can be turned on at the same time.

referring to the technical documentation of the device;
by looking at this value on the rear panel sticker;
using the table, which shows the average value of power consumption for household appliances.

When calculating, it should be taken into account that the starting power of some electrical appliances may differ significantly from the nominal one.

For household devices, this parameter is almost never indicated in the technical documentation, therefore, it is necessary to refer to the corresponding table, which contains the average values of the starting power parameters for various devices (it is advisable to choose the maximum value).

Table of power consumption / current strength of household electrical appliances

electrical appliance	Power consumption, W	Current strength, A
Washing machine	2000 – 2500	9,0 – 11,4
Jacuzzi	2000 – 2500	9,0 – 11,4
Electric floor heating	800 – 1400	3,6 – 6,4
Stationary electric stove	4500 – 8500	20,5 – 38,6
microwave	900 – 1300	4,1 – 5,9
Dishwasher	2000 – 2500	9,0 – 11,4
Freezers, refrigerators	140 – 300	0,6 – 1,4
Meat grinder with electric drive	1100 – 1200	5,0 – 5,5
Electric kettle	1850 – 2000	8,4 – 9,0
Electric coffee maker	630 – 1200	3,0 – 5,5
Juicer	240 – 360	1,1 – 1,6
Toaster	640 – 1100	2,9 – 5,0
Mixer	250 – 400	1,1 – 1,8
hair dryer	400 – 1600	1,8 – 7,3
Iron	900 –1700	4,1 – 7,7
Vacuum cleaner	680 – 1400	3,1 – 6,4
Fan	250 – 400	1,0 – 1,8
TV	125 – 180	0,6 – 0,8
Radio equipment	70 – 100	0,3 – 0,5
Lighting devices	20 – 100	0,1 – 0,4

Before laying a power cable from the distribution panel to a group of consumers, it is necessary to calculate the power of electrical appliances when they operate simultaneously. The cross-section of any branch is selected depending on the type of metal of the wiring: copper or aluminum.

Wire manufacturers provide similar reference materials to their products. If they are missing, then they are guided by the data from the reference book “Rules for the Construction of Electrical Equipment”.

However, consumers often play it safe and choose not the minimum acceptable cross-section, but one step larger. So, for example, when purchasing a copper cable for a 5 kW line, choose a core cross-section of 6 mm2, when according to the table a value of 4 mm2 is sufficient.

This can be justified for the following reasons: Longer service life of a thick cable, which is rarely subjected to the maximum permissible load for its cross-section. Re-doing it is not an easy and expensive job, especially if the premises have been renovated.

The bandwidth reserve allows you to seamlessly connect new electrical appliances to the network branch. So, you can add an additional freezer to the kitchen or move the washing machine there from the bathroom. The start of operation of devices containing electric motors produces strong starting currents.

In this case, a voltage drop is observed, which is expressed not only in the blinking of the lighting lamps, but can also lead to breakdown of the electronic part of the computer, air conditioner or washing machine. The thicker the cable, the smaller the voltage surge will be.

Unfortunately, there are many cables on the market that are not made according to GOST, but according to the requirements of various specifications. Often the cross-section of their cores does not meet the requirements or they are made of conductive material with greater resistance than required. Therefore, the actual maximum power at which permissible heating of the cable occurs is less than in the standard tables. We will take this into account when choosing a machine for the cable cross section.

How to Protect the Weakest Link in Wiring

Therefore, before choosing a machine according to the load being protected, you need to make sure that the wiring will withstand this load.

According to PUE 3.1.4, the machine must protect the weakest section of the electrical circuit from overloads, or be selected with a rated current corresponding to the currents of the connected electrical installations, which again implies their connection with conductors with the required cross section.

If you ignore this rule, you should not blame the incorrectly calculated machine and curse its manufacturer if a weak link in the wiring causes a fire.

Indoor wiring device

Internal electrical networks have a branched structure in the form of a "tree" - a graph without cycles. This improves the stability of the system in the event of an emergency and simplifies the work to eliminate it. It is also much easier to distribute the load, connect energy-intensive devices and change the wiring configuration.

The functions of the introductory machine include the control of a general overload - preventing the current from exceeding the permitted value for the object. If this happens, there is a risk of damage to the external wiring.

In addition, it is likely that protective devices will operate outside the apartment, which already belongs to the common property or belongs to local power networks. The functions of group automata include the control of the current strength on individual lines.

They protect the cable in a dedicated area and the group of electricity consumers connected to it from overload. If during a short circuit such a device does not work, then it is insured by an introductory machine. Even for apartments with a small number of electrical consumers, it is advisable to run a separate line for lighting.

When you turn off the circuit breaker of another circuit, the light will not go out, which will allow you to eliminate the problem in more comfortable conditions. In almost every panel, the nominal value of the input machine is less than the amount on the group ones.

Operating principle of circuit breaker

In the event of a short circuit, the circuit breaker operates almost instantly thanks to the electromagnetic release. At a certain excess of the rated current value, the heating bimetallic plate will turn off the voltage after some time, which can be found out from the current characteristic time graph.

This safety device protects the wiring from short circuits and overcurrents exceeding the calculated value for a given wire cross-section, which can heat the conductors to the melting point and cause the insulation to ignite.

To prevent this from happening, you need not only to choose the right protective switch that matches the power of the connected devices, but also to check whether the existing network can withstand such loads.

Types of devices

There are several types of devices that can monitor wiring and, if necessary, cut off electrical power.

miniature (mini-models);
air (open version);
enclosed molded case switches;
RCD (Residual Current Devices);
automatic switches additionally equipped with RCD (differential).

Miniature devices are designed to work in networks with light loads; as a rule, they do not have additional adjustment functions. This model range is represented by machines with a breaking capacity designed for misfire current from 4.5 to 15A.

Therefore, they are used most often in household wiring, since higher current strength is required for production capacities.

As a rule, they are produced in a three-pole design, but now many companies, including IEK, produce four-pole models.

The waterproof housing, made of refractory materials, ensures the proper level of safety.

Regardless of the range of action and ratings, circuit breakers have the same size across the entire model range, the only dimension that changes is the width (modularity). It depends on the number of poles (there can be 2 or more).

Automatic switches are mounted in a vertical position, with the exception of devices designed over 5000A and 6300A. They can be used for installation in open areas or in special switchboards.

The advantage of such devices is the presence of additional contacts and connections, which significantly expands the scope of use and installation possibilities.

Enclosed circuit breakers are manufactured in a cast housing made of refractory material. This makes them completely sealed and suitable for use in extreme conditions.

On average, the range of such machines is used with a current of up to 200 Amperes and a voltage of up to 750 Volts.

adjustable;
thermal;
electromagnetic.

The rated operating current is indicated on the device body. RCDs cannot be considered independent devices for protecting networks from overvoltage. It is recommended to use them either in tandem with automatic machines, or immediately buy a switch equipped with an additional protection device (differential automatic devices).

At the same time, during installation of wiring, the RCD is installed in front of the machines, and not vice versa. Otherwise, the device may simply burn out due to high short-circuit current pulses.

Circuit breaker parameters

To ensure the correct selection of trip device ratings, an understanding of their operating principles, conditions and response times is necessary.

The operating parameters of circuit breakers are standardized by Russian and international regulatory documents.

Basic elements and markings

The bimetallic plate, under the influence of the passing current, heats up and, bending, presses on the pusher, which disconnects the contacts. This is "thermal protection" against overload.
The solenoid, under the influence of a strong current in the winding, generates a magnetic field that presses on the core, which then acts on the pusher. This is a "current protection" against short circuit, which reacts to such an event much faster than the plate.

Types of electrical protection devices have markings that can be used to determine their main parameters.

The type of time-current characteristic depends on the setting range (the magnitude of the current at which operation occurs) of the solenoid. To protect wiring and devices in apartments, houses and offices, type “C” or, much less common, “B” switches are used. There is no particular difference between them for everyday use.

Type “D” is used in utility rooms or carpentry in the presence of equipment with electric motors that have high starting power. There are two standards for disconnect devices: residential (EN 60898-1 or GOST R 50345) and a more stringent industrial (EN 60947-2 or GOST R 50030.2).

They differ slightly and machines of both standards can be used for residential premises. In terms of rated current, the standard range of automatic machines for domestic use contains devices with the following values: 6, 8, 10, 13 (rare), 16, 20, 25, 32, 40, 50 and 63 A.

Current ratings of circuit breakers

To select the correct ratings for home and industrial circuit breakers, a special table is used:

Circuit breaker current rating (A)	Power in 1-phase network (kW)	Power in 3-phase network (kW)	Permissible wire cross-section (mm 2)
			copper	aluminum
1	0,2	0,5	1	2,5
2	0,4	1,1	1	2,5
3	0,7	1,6	1	2,5
4	0,9	2,1	1	2,5
5	1,1	2,6	1	2,5
6	1,3	3,2	1	2,5
8	1,7	5,1	1,5	2,5
10	2,2	5,3	1,5	2,5
16	3,5	8,4	1,5	2,5
20	4,4	10,5	2,5	4
25	5,5	13,2	4	6
32	7	16,8	6	10
40	8,8	21,1	10	16
50	11	26,3	10	16
63	13,9	33,2	16	25
80	17,6	52,5	25	35
100	22	65,7	35	50

I – current consumed by the equipment (A);
P – equipment power (W);
U – mains voltage (V).

For example, our kettle has a power of 1.5 kW (1500 W), a lamp – 100 W, a refrigerator – 300 W; in total, the total value will be 1.9 kW (1900 W), we calculate the rated current: I = 1900/220 = 8.6. The closest automatic device in terms of operating current is 10A. Naturally, in practice this figure will be higher; modern wiring must be designed for a load current of at least 16A.

For example, consider a 16 ampere machine, how many kilowatts can it withstand. From the table above, we see that the power in a single-phase network is 3.5 kW. Machines with such ratings are placed in separate groups that can withstand a modern oil heater (Max 2.5 kW) or an electric kettle (Max 2.0 kW), but not both of these electrical appliances at the same time.

Rules for choosing denomination

The geometry of intra-apartment and house electrical networks is individual, so there are no standard solutions for installing switches of a certain rating.

The general rules for calculating the permissible parameters of machines are quite complex and depend on many factors. It is necessary to take them all into account, otherwise an emergency situation may be created.

Selection of machine by power

Let’s say right away that there are several ways. The simplest is to calculate the machine’s power using one of the online calculators. But no matter which one you choose, first of all you need to determine the total load on the network. How to calculate this indicator? To do this, you will have to deal with all the household appliances that are installed on the power supply section.

It is more convenient to calculate the machine by power, rather than select the machine by current. In order not to be unfounded, we will give an example of a network into which a large number of household appliances are usually connected. It's a kitchen.

Refrigerator with power consumption of 500 W.
Microwave oven – 1 kW.
Electric kettle – 1.5 kW.
Hood – 100 W.

This is almost a standard set, which can be a little larger or a little smaller. Adding up all these indicators, we get the total power of the site, which is equal to 3.1 kW. And now here are the methods for determining the load and the choice of machine itself.

To increase safety, electrical wiring in the apartment should be divided into several lines. These are separate machines for lighting, kitchen sockets, and other sockets. High-power household appliances with increased danger (electric water heaters, washing machines, electric stoves) must be turned on through an RCD.

The RCD will respond in time to a current leak and turn off the load. To choose the right machine, it is important to consider three main parameters; - rated current, switching capacity of short-circuit current interruption and class of circuit breakers.

The calculated rated current of the machine is the maximum current that is designed for long-term operation of the machine. When the current is higher than the rated one, the contacts of the machine are disconnected. The class of machines means a short-term value of the starting current when the machine has not yet triggered.

The starting current is many times greater than the rated current value. All classes of machines have different starting current levels.

class B, where the starting current can be 3 to 5 times greater than the rated current;
class C has a 5-10 times excess of the nominal current;
class D with possible excess current of the rated value from 10 to 50 times.

In houses and apartments, class C is used. The switching capacity determines the magnitude of the short circuit current when the machine is instantly turned off. We use circuit breakers with a switching capacity of 4500 amperes; foreign circuit breakers have a short-circuit current. 6000 amp. You can use both types of machines, Russian and foreign.

Tabular method

How to choose a machine by power table. This is the easiest option for choosing the right circuit breaker. To do this, you will need a table in which you can select a machine (single- or three-phase) based on the total indicator.

Selection of machines by power and connection:

Connection type	Single phase	Single-phase input	Three-phase delta	Three-phase star
Machine polarity	Single-pole circuit breaker	Two-pole machine	Three-pole machine	Four-pole circuit breaker
Supply voltage	220 volt	220 volt	380 Volt	220 volt
Automatic 1A	0.2 kW	0.2 kW	1.1 kW	0.7 kW
Automatic 2A	0.4 kW	0.4 kW	2.3 kW	1.3 kW
Automatic 3A	0.7 kW	0.7 kW	3.4 kW	2.0 kW
Automatic 6A	1.3 kW	1.3 kW	6.8 kW	4.0 kW
Automatic 10A	2.2 kW	2.2 kW	11.4 kW	6.6 kW
Automatic 16A	3.5 kW	3.5 kW	18.2 kW	10.6 kW
Automatic 20A	4.4 kW	4.4 kW	22.8 kW	13.2 kW
Automatic 25A	5.5 kW	5.5 kW	28.5 kW	16.5 kW
Automatic 32A	7.0 kW	7.0 kW	36.5 kW	21.1 kW
Automatic 40A	8.8 kW	8.8 kW	45.6 kW	26.4 kW
Automatic 50A	11 kW	11 kW	57 kW	33 kW
Automatic 63A	13.9 kW	13.9 kW	71.8 kW	41.6 kW

Everything is quite simple here. Most importantly, you need to understand that the calculated total power may not be the same as in the table. Therefore, the calculated indicator will have to be increased to the tabular one.

From our example it can be seen that the power consumption of the site is 3.1 kW. There is no such indicator in the table, so we take the nearest larger one. And this is 3.5 kW, which corresponds to a 16-amp machine.

As we can see from the table, the calculation of a machine with a power of 380 differs from the calculation of a machine with a power of 220.

Graphic method

This is practically the same as the tabular one. Only instead of a table, a graph is used here. They are also freely available on the Internet. As an example, we give one of these.

On the graph, circuit breakers with current load indicator are located horizontally, and the power consumption of the network section is located vertically.

To determine the power of the circuit breaker, you must first find the calculated power consumption on the vertical axis, and then draw a horizontal line from it to the green column that determines the rated current of the machine.

You can do this yourself with our example, which shows that our calculation and selection was done correctly. That is, this power corresponds to a machine with a load of 16A.

Nuances of choice

Today it is necessary to take into account the fact that the number of convenient household appliances is limited, and every person tries to acquire new devices, thereby making their life easier.

This means that by increasing the number of equipment, we increase the load on the network. Therefore, experts recommend using a multiplying factor when calculating the power of the machine.

Let's go back to our example. Imagine that the owner of the apartment purchased a 1.5 kW coffee machine. Accordingly, the total power indicator will be equal to 4.6 kW. Of course, this is more power than the circuit breaker we selected (16A). And if all the devices are turned on at the same time (plus the coffee machine), the machine will immediately reset and disconnect the circuit.

It is difficult to predict exactly what additional household appliances can be installed. Therefore, the simplest option is to increase the total calculated indicator by 50%. That is, use a multiplying factor of 1.5. Let's go back to our example again, where the end result will be like this:

3.1x1.5=4.65 kW. Let's return to one of the methods for determining the current load, in which it will be shown that for such an indicator you will need a 25 ampere machine.

For some cases, a reduction factor can be used. For example, there is not enough sockets for all devices to work simultaneously. This could be one socket for an electric kettle and a coffee machine. That is, it is not possible to turn on these two devices at the same time.

When it comes to increasing the current load on a network section, it is necessary to change not only the circuit breaker, but also check whether the electrical wiring can withstand the load, for which the cross-section of the laid wires is considered. If the cross-section does not meet the standards, then it is better to change the wiring.

Calculation of the machine according to the cross-section of the electrical wiring

To select a machine, you can use the table. The current selected for the cross-section of the electrical wiring is reduced to the lower current value of the machine to reduce the load on the electrical wiring.

Load power depending on rated current
circuit breaker and cable section

Cable cross-section, sq.mm		Rated current of the machine, A	Power of 1-phase load at 220V, kW	Power of 3-phase load at 380V, kW
Copper	Aluminum
1	2.5	6	1.3	3.2
1.5	2.5	10	2.2	5.3
1.5	2.5	16	3.5	8.4
2.5	4	20	4.4	10.5
4	6	25	5.5	13.2
6	10	32	7	16.8
10	16	40	8.8	21.1
10	16	50	11	26.3
16	25	63	13.9	33.2

For sockets, the machines take a current of 16 amperes, since the sockets are designed for a current of 16 amperes; for lighting, the optimal version of the machine is 10 amperes. If you do not know the cross-section of the electrical wiring, then it is easy to calculate it using the formula:

S – wire cross-section in mm²;
D is the diameter of the wire without insulation in mm.

The cross-sectional method of calculating a circuit breaker is more preferable, as it protects the electrical wiring in the room.

The formula for calculating power by current and voltage

How to calculate power from current? In AC circuits, power calculations are carried out taking into account the laws of sinusoidal changes in voltage and current. In this regard, the concept of total power (S) was introduced, which includes two components: reactive (Q) and active (P). A graphical description of these quantities can be made through the power triangle.

The active component (P) refers to the power of the payload (the irreversible conversion of electricity into heat, light, etc.). This value is measured in watts (W), at the household level it is customary to calculate in kilowatts (kW), in the industrial sector - megawatts (mW).

The reactive component (Q) describes the capacitive and inductive electrical load in the alternating current circuit, the unit of measurement of this quantity is Var.

In accordance with the graphical representation, the relationships in the power triangle can be described using elementary trigonometric identities, which makes it possible to use the following formulas:

S = √P2+Q2, – for full power;
and Q = U*I*cos⁡ φ, and P = U*I*sin φ – for the reactive and active components.

These calculations are applicable for a single-phase network (for example, household 220 V); to calculate the power of a three-phase network (380 V), you need to add a multiplier to the formulas - √3 (with a symmetrical load) or sum the powers of all phases (if the load is asymmetrical).

To better understand the process of influence of the components of total power, let's consider the “pure” manifestation of the load in active, inductive and capacitive form.

Let's take a hypothetical circuit that uses a "pure" active resistance and an appropriate AC voltage source. A graphical description of the operation of such a circuit is shown in Figure 2, which displays the main parameters for a certain time range (t).

We can see that the voltage and current are synchronized in both phase and frequency, while the power has double the frequency. Note that the direction of this quantity is positive and it is constantly increasing.

As can be seen in Figure 3, the graph of the characteristics of a capacitive load is slightly different from an active one.
The frequency of capacitive power oscillations is twice the frequency of the sinusoidal voltage change. As for the total value of this parameter, during one harmonic period it is equal to zero.

At the same time, no increase in energy (∆W) is observed either. This result indicates that its movement occurs in both directions of the chain. That is, when the voltage increases, charge accumulates in the capacitance. When a negative half-cycle occurs, the accumulated charge is discharged into the circuit circuit.

During the process of energy accumulation in the load capacitance and subsequent discharge, no useful work is performed.

Negative effects of reactive load

In the examples above, options were considered where there was a “pure” reactive load. The factor of influence of active resistance was not taken into account. Under such conditions, the reactive effect is zero, which means it can be ignored. As you understand, in real conditions this is impossible.

Even if hypothetically such a load existed, the resistance of the copper or aluminum conductors of the cable necessary to connect it to the power source cannot be ruled out.

The reactive component can manifest itself in the form of heating of the active components of the circuit, for example, the motor, transformer, connecting wires, power cable, etc. A certain amount of energy is spent on this, which leads to a decrease in basic characteristics.

does not produce any useful work;
causes serious losses and abnormal loads on electrical appliances;
may cause a serious accident.

That is why, when making appropriate calculations for an electrical circuit, one cannot exclude the influence of inductive and capacitive loads and, if necessary, provide for the use of technical systems to compensate for it.

The function of a circuit breaker is to protect the electrical wiring connected downstream of it. The main parameter by which the automatic machines are calculated is the rated current. But the rated current of what, the load or the wire?

Based on the requirements of PUE 3.1.4, the setting currents of circuit breakers that serve to protect individual sections of the network are selected as much as possible less than the rated currents of these sections or according to the rated current of the receiver.

The calculation of the machine in terms of power (according to the rated current of the electrical receiver) is carried out if the wires along the entire length in all sections of the wiring are designed for such a load. That is, the permissible current of the electrical wiring is greater than the rating of the machine.

The time-current characteristic of the machine is also taken into account, but we will talk about it later.

For example, in an area where a wire with a cross-section of 1 square meter is used. mm, the load value is 10 kW. We select the machine according to the rated load current - we set the machine to 40 A. What will happen in this case?

The wire will begin to heat up and melt, since it is rated for a rated current of 10-12 amperes, and a current of 40 amperes passes through it. The machine will turn off only when a short circuit occurs. As a result, wiring may fail and even cause a fire.

Therefore, the determining value for choosing the rated current of the machine is the cross section of the conductive wire. The load size is taken into account only after selecting the wire cross-section. The rated current indicated on the machine must be less than the maximum current allowed for a wire of a given section.

Thus, the choice of the machine is made according to the minimum cross-section of the wire that is used in the wiring.

For example, the permissible current for a copper wire with a cross-section of 1.5 kW. mm, is 19 amperes. This means that for this wire we select the closest value of the rated current of the machine to the smaller side, which is 16 amperes.

If you choose a machine with a value of 25 amperes, the wiring will heat up, since the wire of this cross-section is not designed for such a current. In order to correctly calculate the circuit breaker, it is necessary, first of all, to take into account the cross-section of the wire.

Calculation of the machine by current We calculate the entire power of the loads on the machine. We add the power of all electricity consumers, and using the following formula: I = P/U we obtain the calculated current of the machine. P is the total power of all electricity consumers U is the network voltage. We round the calculated value of the received current up.

When can you reduce the rated power of a machine?

Sometimes a machine is installed on the line with a rated power significantly lower than what is necessary to ensure that the electrical cable remains operational. It is advisable to reduce the rating of the switch if the total power of all devices in the circuit is significantly less than the cable can withstand.

This happens if, for safety reasons, when some of the devices were removed from the line after installation of the wiring. Then reducing the rated power of the machine is justified from the position of its faster response to emerging overloads.

For example, when an electric motor bearing jams, the current in the winding increases sharply, but not to short circuit values. If the machine reacts quickly, then the winding will not have time to melt, which will save the engine from an expensive rewinding procedure.

They also use a value less than the calculated value due to strict restrictions on each circuit. For example, for a single-phase network, a 32 A switch is installed at the entrance to an apartment with an electric stove, which gives 32 * 1.13 * 220 = 8.0 kW of permissible power. Let, when performing in the apartment, 3 lines were organized with the installation of group automatic machines with a rating of 25 A.

Let's assume that there is a slow increase in load on one of the lines. When the power consumption reaches a value equal to the guaranteed tripping of the group switch, only (32 - 25) * 1.45 * 220 = 2.2 kW will remain for the remaining two sections. This is very little relative to total consumption.

With this scheme, the input machine will turn off more often than devices on the lines. Therefore, in order to maintain the principle of selectivity, it is necessary to install switches with a rating of 20 or 16 amperes in the areas. Then, with the same imbalance in power consumption, the other two links will account for a total of 3.8 or 5.1 kW, which is acceptable.

Let's consider the possibility of installing a switch with a rating of 20A using the example of a separate line dedicated to the kitchen.

Refrigerator with a rated power of 400 W and a starting current of 1.2 kW;
Two freezers, power 200 W;
Oven, power 3.5 kW;

When operating an electric oven, it is allowed to additionally turn on only one additional device, the most powerful of which is an electric kettle, consuming 2.0 kW.

A twenty-amp machine allows you to pass current for more than an hour with a power of 20 * 220 * 1.13 = 5.0 kW. A guaranteed shutdown in less than one hour will occur with a current flow of 20 * 220 * 1.45 = 6.4 kW.

When the oven and electric kettle are turned on simultaneously, the total power will be 5.5 kW or 1.25 parts of the machine’s nominal value. Since the kettle does not work for long, it will not turn off. If at this moment the refrigerator and both freezers turn on, the power will be 6.3 kW or 1.43 parts of the nominal value.

This value is already close to the guaranteed tripping parameter. However, the likelihood of such a situation occurring is extremely low and the duration of the period will be insignificant, since the operating time of the motors and the kettle is short.

The starting current that occurs when starting the refrigerator, even in the sum of all operating devices, will not be enough to trigger the electromagnetic release. Thus, under the given conditions, a 20 A circuit breaker can be used.

In this article I want to touch on such an important topic as the correct calculation of the cross-section of the electrical wiring cable. The choice of cable cross-section should be taken with all possible seriousness, because the quality and safety of all electrical wiring directly depends on it. If the cable cross-section is too low, the current in the line will exceed the maximum permitted operating current. In this case, the operating current of the electrical wiring is limited by the maximum permissible heating temperature of the wire when current flows through it. When this temperature is exceeded, the insulation begins to overheat and melt, which leads to destruction of the cable. For hidden electrical wiring, the thermal conductivity of the wire is less than for open wiring, the wire is cooled less well and, accordingly, the permissible operating current is less.

You should not skimp on the cable, since if you choose the wrong one, it will have to be replaced, and this is a labor-intensive process that often means the start of a new repair.

Calculation and selection of cable cross-section

The rated current of the circuit breaker is selected greater than or equal to the rated current of the line, and should not exceed the maximum permissible load in the electrical circuit or cable:

I calculation<=I н <=I доп

To provide overcurrent protection, the rated tripping current of the circuit breaker must be 45% less than the maximum load capacity of the electrical circuit or cable:

Itr<=1,45*I доп

where I calculated is the calculated current of the circuit;

I additional – permissible load of an electrical circuit or cable;

I n – rated current of the circuit breaker;

I tr – Thermal release current;

The maximum current that the cable can withstand should be determined from Table 1.3.4. (Rules for electrical installations). Hidden wiring made in a groove under the plaster is equivalent to wiring laid in a pipe.

According to modern electrical safety requirements, wiring in apartments (cottages, offices) must be done with a three-wire copper cable or wire, but in the calculations the grounding conductor (PE) is not taken into account, so we use a column with the parameters of two-core wires:

If you have electrical wiring in your house using aluminum wire, you can use Table 1.3.5. , which indicates the maximum permissible current values for wires and cables with aluminum conductors:

When choosing a wire cross-section, it is necessary to take into account the requirements for its mechanical strength. According to TKP 339-2011, clause 8.4.4, cables and wires with copper conductors should be used in buildings. The smallest permissible cross-sections of current-carrying conductors of wires and cables in electrical wiring according to TKP 121 are given in Table 8.1.

According to this table, the minimum conductor cross-section for power and lighting circuits is 1.5 mm 2. Therefore, if as a result of calculations it turns out that the required cross-section is 1 mm 2, then it is necessary to select a conductor of at least 1.5 mm 2.

What happens if you don’t take into account the thermal release setting when choosing a machine? For convenience, let's look at an example:

Let's take the most common rating of the machine - 16 A, the overload current at which the machine will operate within an hour will be equal to 16 * 1.45 = 23.2 A (a table was presented above, from which it can be seen that the value of the thermal release setting is 1.45 rated current). Accordingly, it is for this current that it is worth selecting the cable cross-section. From table 1.3.4. we select a suitable cross-section: for hidden electrical wiring made of copper - this is at least 2.5 mm 2 (maximum overload current 27 A).

In a similar way, you can carry out calculations for a 10 A machine. The current at which the machine will turn off within an hour will be equal to 10·1.45 = 14.5A. According to the table, this current corresponds to a cable with a cross section of 1.5 mm 2.

Very often, installers neglect this rule and, to protect a line with a cross-section of 2.5 mm2, install a circuit breaker with a rating of 25 A (after all, the line can withstand a current of 25 A for a long time). But they forget that the unswitched current of such a machine is 25 * 1.13 = 28.25 A, and this is already more than the long-term permissible overload current. The current at which the machine will turn off within an hour will be 25*1.45=36.25 A!!! With such a current and for such a time, the cable will overheat and burn out.

Also, do not forget that the majority of the cable market consists of cables manufactured not according to GOST, but according to specifications. It follows from this that their actual cross-section will be underestimated. By purchasing a cable manufactured according to specifications, instead of a cable with a core cross-section of 2.5 mm 2, you can get a cable with an actual core cross-section of less than 2.0 mm 2!
Here is an example of what can happen if the rules for choosing the cross-section of the cable and machine are neglected:

Selecting a circuit breaker

Taking into account all the above factors, to increase the safety, reliability and durability of electrical wiring, it is worth using the following ratios of the cross-section of the cable and the machine protecting this line:

1.5 mm²→ 10 A → 2200 W→ Mainly used for lighting lines.
2.5 mm²→16 A → 3520 W→ used in separate lines for sockets of powerful household appliances (washing machine, dishwasher, etc.) or groups of sockets for household purposes.
- 4 mm²→ 25 A → 5500 W→ for power circuits (powerful electrical appliances, electric heating system, etc.).
6 mm²→ 32 A → 7040 W→ for power circuits (electric stove, electric heating system, etc.).
10 mm²→ 40 A → 8800 W→ for input lines or power circuits;

After selecting the wire cross-section, a check is made for the permissible voltage loss. If the wires are long, the voltage to consumers can reach significantly lower than the nominal voltage. The permissible voltage loss in the wires should not exceed 5% of the rated voltage. If she turns out to be more than permissible, then it is necessary to select a wire of a larger cross-section. In this article, we will not consider testing for voltage loss.