Ratings of circuit breakers for current: how to correctly select the machine

Devices for turning off electricity in case of overloads and short circuits are installed at the entrance to any home network.

It is necessary to correctly calculate the ratings of the circuit breakers by current, otherwise their operation will be ineffective: either they will not protect lines and household appliances, or false alarms will often occur.

Circuit Breaker Parameters

To ensure the correct selection of the rating of the shutdown devices, it is necessary to understand the principles of their operation, the conditions and the response time.

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

Key elements and labeling

The design of the switch includes two elements that respond to the excess of a current of a specified range of values:

  • The bimetallic plate under the influence of the passing current heats up and, bending, presses on the pusher, which separates 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 the core, and the latter already acts on the plunger. This is a “short-circuit current protection” that reacts to such an event much faster than a plate.

The types of electrical protection devices have a marking by which their basic parameters can be determined.

Automatic switch marking
Each circuit breaker indicates its main characteristics. This allows not to confuse the device when they are installed in the panel

The type of time-current characteristic depends on the setting range (the magnitude of the current at which the response occurs) of the solenoid. To protect the wiring and appliances in apartments, houses and offices using switches of type "C" or, much less common - "B". There is no particular difference between them in household use.

Type "D" is used in utility rooms or carpenters in the presence of equipment with electric motors, which have large indicators of starting power.

There are two standards for disconnect devices: residential (EN 60898-1 or GOST R 50345) and 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.

According to the rated current, the standard range of automata for use in living conditions contains devices with the following values: 6, 8, 10, 13 (rarely occurs), 16, 20, 25, 32, 40, 50 and 63 A.

Time-current response characteristics

In order to determine the speed of operation of an automaton during an overload, there are special tables for the dependence of the tripping time on the nominal excess ratio, which is equal to the ratio of the existing current to the nominalK = I / In.

Time-current curve
The graph shows the dependence of the range of the response time of type C automata on the ratio of the current intensity to the value that is set for this switch.

A sharp breakage down the graph when the value of the range factor from 5 to 10 units is reached is due to the operation of the electromagnetic release. For switches of type "B" this occurs when the value is from 3 to 5 units, and for type "D" - from 10 to 20.

With K = 1.13, the machine is guaranteed not to disconnect the line for 1 hour, and with K = 1.45 - it is guaranteed to disconnect during the same time. These values ​​are approved in clause 8.6.2.GOST R 50345-2010.

To understand how long the protection will work, for example, whenK= 2, it is necessary to draw a vertical line from this value. As a result, we obtain that according to the above schedule, the disconnection will occur in the range from 12 to 100 seconds. Such a large spread of time is due to the fact that the heating of the plate depends not only on the power of the current passing through it, but also on the parameters of the external environment. The higher the temperature, the faster the automatic fires.

Nominal selection rules

The geometry of the in-house and house electrical networks is individual, so standard solutions for installing switches of a certain nominal value do not exist. The general rules for calculating the permissible parameters of automata are quite complex and depend on many factors. It is necessary to consider them all, otherwise it is possible to create an emergency.

The principle of internal wiring

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.

An example of wiring electricity in the apartment
At the base of the graph, there is an introductory automaton, and immediately after branching, group switches are placed for each individual electrical circuit. This is a proven standard for years.

The functions of the input automaton include total overload monitoring - preventing the amperage from exceeding the allowed value for an object. If this happens, there is a risk of damage to the external wiring. In addition, it is likely that the protection devices outside the apartment, which already belong to the common house property or belong to the local power networks, will be activated.

The functions of the group automat include control of the current intensity along individual lines. They protect against overload the cable in the designated area and the group of electricity consumers connected to it. If during a short circuit such a device does not work, then it is insured by an introductory automat.

Even for apartments with a small number of electrical consumers, it is desirable to run a separate line for lighting. When the automatic circuit breaker of another circuit is turned off, the light will not go out, which will make it possible to eliminate the problem in more comfortable conditions. In almost every panel, the value of the nominal value of the input machine is less than the sum of the group.

The total power of electrical appliances

The maximum load on the circuit occurs when all electrical appliances are switched on at the same time. Therefore, usually, the total power is calculated by simple addition. However, in some cases, this figure will be less.

For some lines, the simultaneous operation of all electrical appliances connected to it is unlikely and sometimes impossible. In houses, sometimes they specifically set limits on the operation of powerful devices. To do this, remember to prevent their simultaneous inclusion or use a limited number of outlets.

Office space with electrical appliances
The probability of simultaneous operation of all office equipment, lighting and auxiliary equipment (kettles, refrigerators, fans, heaters, etc.) is very low, therefore, when calculating the maximum power, the correction factor is used

In the electrification of office buildings, an empirical coefficient of simultaneity is often used for calculations, the value of which is taken in the range from 0.6 to 0.8. Maximum load is calculated by multiplying the sum of the power of all appliances by a factor.

When calculating, there is one subtlety - it is necessary to take into account the difference between nominal (full) power and consumed (active), which are related by a factor (cos (f)).This means that the device requires a current of power equal to that consumed divided by this factor:

Ip= I / cos (f)


  • Ip- nominal current strength, which is used in load calculations;
  • I is the current consumed by the device;
  • cos (f) <= 1.

Usually, the rated current immediately or through an indication of the value of cos (f) is indicated in the technical data sheet of the electrical device. For example, the coefficient value for luminescent light sources is 0.9; for LED-lamps - about 0.6; for ordinary incandescent bulbs - 1. If the documentation is lost, but the power consumption of household appliances is known, then for a guarantee take cos (f) = 0.75.

Power Factor Table
The recommended power factor values ​​listed in the table can be used in the calculation of electrical loads when there is no data on the rated current.

Choice of cross-section

Before laying the power cable from the switchboard to the group of consumers, it is necessary to calculate the power of electrical appliances when they are working simultaneously.

The cross section of any branch is chosen according to the calculation tables depending on the type of metal of the wiring: copper or aluminum. Wire manufacturers accompany manufactured products with similar referencematerials. If they are absent, then they are guided by data from the reference book “Rules for the Design of Electrical Equipment”.

Table for selecting the cross section of copper wires
The reference table presented in PUE allows you to select the required cross-section from the standard range for different operating conditions of copper cable

However, often consumers are reinsured and choose not the minimum permissible cross-section, but a larger step. For example, when buying a copper cable for a 5 kW line, choose a section of 6 mm cores2when according to the table 4 mm is enough2. This is justified for the following reasons:

  • Longer operation of the thick cable, which is rarely subjected to the maximum permissible for its cross section load. Re-laying the wiring is not an easy and costly job, especially if repairs have been made in the room.
  • Reserve bandwidth allows you to seamlessly connect to the network branch new appliances. So, in the kitchen, you can add an additional freezer or move the washing machine from the bathroom.
  • Getting started devices containing electric motors, gives a strong starting currents.In this case, there is a voltage drop, which is expressed not only in the flashing of the lighting lamps, but can also lead to a breakdown of the electronic part of the computer, air conditioner or washing machine. The thicker the cable, the less the power surge.

Unfortunately, there are many cables on the market that are not made in accordance with GOST, but in accordance with the requirements of various technical specifications. Often the cross section of their veins does not meet the requirements or they are made of conductive material with greater resistance than expected. Therefore, the actual maximum power at which the permissible heating of the cable occurs is less than in the normative tables.

Difference of cables made in accordance with GOST and TU
This photo shows the differences between cables made in accordance with GOST (left) and according to specifications (right). The difference in the cross section of the cores and the tightness of the insulation material is obvious.

Calculation of the switch rating for cable protection

The automatic machine installed in the panel should ensure that the line is disconnected when the power output of the current is outside the range allowed for the electric cable. Therefore, for the switch, it is necessary to calculate the maximum allowable rating.

For PUE, the permissible continuous load, laid in boxes or by air (for example, above the stretch ceiling) of copper cables, is taken from the table above. These values ​​are intended for emergency cases when there is a power overload. Some problems begin when correlating the rated power of the switch to the long-term permissible current, if this is done in accordance with the current GOST R 50571.4.43-2012.

fragment p. 433.1 of GOST R 50571.4.43-2012.
A fragment of clause 433.1 GOST R 50571.4.43-2012 is given. In the formula "2" an inaccuracy was made, and for a correct understanding of the definition of the variable In it is necessary to take into account Appendix "1"

First, the decoding of the variable I is misleading.n, as the nominal power, if you do not pay attention to the Annex "1" to this item GOST. Secondly, in the formula "2" there is a typo: the coefficient of 1.45 is added incorrectly and this fact is ascertained by many experts.

According to clause GOST R 50345-2010 for household switches with nominal values ​​up to 63 A, the conditional time is 1 hour. The set tripping current is equal to the nominal value multiplied by a factor of 1.45.

Thus, according to the first and modified second formulas, the nominal current of the switch should be calculated using the following formula:

In<= IZ/ 1,45


  • In- rated current of the machine;
  • IZ- long permissible cable current.

We will carry out the calculation of the ratings of switches for standard cable sections with single-phase connection with two copper conductors (220 V). To do this, we divide the long-term permissible current (when laying through the air) by a tripping factor of 1.45. Choose an automaton so that its nominal is less than this value:

  • 1.5 mm cross section2: 19 / 1.45 = 13.1. Rating: 13 A;
  • 2.5 mm cross section2: 27 / 1.45 = 18.6. Rating: 16 A;
  • Section 4.0 mm2: 38 / 1.45 = 26.2. Rating: 25 A;
  • Section 6.0 mm2: 50 / 1.45 = 34.5. Rating: 32 A;
  • Section 10.0 mm2: 70 / 1.45 = 48.3. Rating: 40 A;
  • Section 16.0 mm2: 90 / 1.45 = 62.1. Rating: 50 A;
  • Section 25.0 mm2: 115/1 .45 = 79.3. Rating: 63 A.

13A circuit breakers are rarely available commercially, so devices with a rated capacity of 10A are more often used instead.

Table for the selection of sections of aluminum conductors
Aluminum-based cables are now rarely used in the installation of internal wiring. For them, too, there is a table that allows you to select a section for the load

In a similar way for aluminum cables, we calculate the values ​​of the automata:

  • 2.5 mm cross section2: 21 / 1.45 = 14.5. Rating: 10 or 13 A;
  • Section 4.0 mm2: 29 / 1.45 = 20.0. Rating: 16 or 20 A;
  • Section 6.0 mm2: 38 / 1.45 = 26.2. Rating: 25 A;
  • Section 10.0 mm2: 55 / 1.45 = 37.9. Rating: 32 A;
  • Section 16.0 mm2: 70 / 1.45 = 48.3. Rating: 40 A;
  • Section 25.0 mm2: 90 / 1.45 = 62.1. Denomination: 50 A.
  • Section 35.0 mm2: 105 / 1.45 = 72.4. Rating: 63 A.

If the manufacturer of power cables declares a different dependence of the permissible power on the cross-sectional area, then it is necessary to recalculate the value for the switches.

Formulas for dependence of current strength on power
The formulas for power versus power for single-phase and three-phase networks are different. Many people who have devices rated for 380 volts are mistaken at this stage.

Preventing overload from consumers

Sometimes an automat with a nominal power much lower than necessary is installed on the line in order to guarantee the preservation of the efficiency of the electrical cable.

It is advisable to lower the rating of the circuit breaker if the total power of all devices in the circuit is significantly less than the cable can withstand. This happens if, for security reasons, when some of the devices were removed from the line after the wiring.

Then the reduction of the nominal power of the machine is justified from the standpoint of its more rapid response to the occurring overload. For example, when the motor bearing is jammed, the current in the winding increases sharply, but not to the short circuit values.If the machine responds quickly, the winding will not have time to melt down, which will save the engine from an expensive rewinding procedure.

Also, the nominal is used less than the calculated one for the reasons of severe restrictions on each chain. For example, for a single-phase network, at the entrance to an apartment with an electric stove, a 32 A switch is installed, which gives 32 * 1.13 * 220 = 8.0 kW of permissible power. Suppose that during the layout of the apartment 3 lines were set up with the installation of group automat of 25 A.

Switchboard with a large number of machines
If the number of group machines installed in the switchboard is large, then they must be signed and numbered. Otherwise, you can get confused

Suppose that on one of the lines there is a slow increase in load. When the power consumption reaches a value equal to the guaranteed tripping of the group switch, only the remaining two sections will have (32 - 25) * 1.45 * 220 = 2.2 kW. This is very small relative to total consumption. In this scheme, the distribution panel input automaton will be more often disconnected than devices on the lines.

Therefore, in order to preserve the principle of selectivity, you need to put switches with nominal values ​​of 20 or 16 amperes on the plots.Then, with the same imbalance of power consumption, the other two links will account for a total of 3.8 or 5.1 kW, which is acceptable.

Consider the possibility of installing a switch with a rating of 20A on the example of a separate line dedicated to the kitchen. The following electrical appliances are connected to it and can be simultaneously turned on:

  • Refrigerator with a rated power of 400 W and a starting current of 1.2 kW;
  • Two freezers, 200 W;
  • Oven, 3.5 kW;
  • When operating an electric oven, it is allowed to additionally turn on only one appliance, the most powerful of which is an electric kettle that consumes 2.0 kW.

A twenty-amp machine allows over an hour to pass a current with a power of 20 * 220 * 1.13 = 5.0 kW. Guaranteed shutdown in less than one hour will occur when current is passed in 20 * 220 * 1.45 = 6.4 kW.

Sockets in the kitchen
In the kitchen, a permanent connection to electricity should be at the refrigeration equipment and stove. If there is a risk of exceeding the current, the simultaneous operation of other devices can be eliminated by allocating only two sockets for them.

At the same time turning on the oven and electric kettle, the total power will be 5.5 kW or 1.25 parts of the nominal of the machine. Since the kettle does not last long, it will not turn off.If at this moment the refrigerator and both freezers come into operation, the power will be already 6.3 kW or 1.43 parts of the nominal.

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

The starting current arising at the start of the refrigerator, even in sum with all the working devices, will not be enough to trigger the electromagnetic release. Thus, in the given conditions, it is possible to use an automaton on 20 A.

The only caveat is the possibility of increasing the voltage up to 230 V, which is permitted by regulatory documents. In particular, GOST 29322-2014 (IEC 60038: 2009) defines a standard voltage equal to 230 V with the possibility of using 220 V.

Now, most of the networks supply electricity with a voltage of 220 V. If the current parameter is reduced to the international standard of 230 V, then the ratings can be recalculated in accordance with this value.

Conclusions and useful video on the topic

The device switch. Selection of input automaton depending on the connected power.Power Distribution Rules:

Selecting a switch for cable bandwidth:

The calculation of the rated current of the circuit breaker is a complex task, for the solution of which it is necessary to take into account many conditions. The convenience of service and the safety of the local power grid depends on the installed machine. In case of doubt in the ability to make the right choice, you should contact the experts.

Related news

Ratings of circuit breakers for current: how to correctly select the machine image, picture, imagery

Ratings of circuit breakers for current: how to correctly select the machine 19

Ratings of circuit breakers for current: how to correctly select the machine 89

Ratings of circuit breakers for current: how to correctly select the machine 73

Ratings of circuit breakers for current: how to correctly select the machine 100

Ratings of circuit breakers for current: how to correctly select the machine 2