How to choose a circuit breaker for load power

Correct use of protective devices prevents electric shock and the occurrence and development of emergency situations. The correct choice of the machine according to the load power is necessary for the creation of new and modernization of the operated power supply networks.

What are circuit breakers for and how they work

Devices in this category are capable of registering excessive consumption of electrical energy. This happens when there is a short circuit or when high-power or reactive loads are connected. In such situations, the circuit breaker disconnects the 220 (380) V power supply without user intervention.

Two technologies are used in the standard design to carry out the indicated functions. With a rapid increase in current strength above the level determined by the calculation, the solenoid creates a magnetic field that moves the stem. Through a mechanical drive, this unit opens the contact group. The node parameters are calculated taking into account the starting loads in order to exclude false alarms.

The second protection is organized using a well-known phenomenon - heating a conductor by passing a current. The corresponding section of the chain is created from a bimetallic plate. As the temperature rises, it changes shape until the contact breaks. Some models of vending machines provide a special adjustment for adjusting the sensitivity level.

Why is the cable mismatch with the network load dangerous?

Possible problems are easier to understand with a specific example. Initial data:

• standard power supply with alternating voltage U = 220 V;
• the apartment has old aluminum wiring (section 2.5 mm sq.);
• machine amperage - 30 A;
• 6 convectors of 750 W each and one iron of 850 W are connected.

Payment:

• total power (P) of consumers - 5 350 W;
• the current (I) in the circuit is calculated by the formula I = P / U = 5 350/220 = 24.32 A).

The machine will not work in such a situation (30> 24.32A). Such a current will greatly heat the aluminum wire, melt the insulation. The circuit destroyed by a short circuit will have to be restored, which is difficult when installing networks inside building structures. In the worst situation, significant material assets will be destroyed by fire.

According to the "PUE" standards for a conductor with the considered parameters, the load power when connected to a single-phase 220V network should not exceed 4.4 kW. The corresponding current limit is 20 A. The negative development of the situation can be facilitated by "twists", oxides, and other defects in the joints.

Weak link protection

In addition to the cross-section, when choosing suitable cable products, attention is paid to real operating conditions.The normalized values ​​are given for heating up to a temperature not exceeding + 60 ° C. When installing the line on a site near a country house, it is necessary to provide protection against moisture and other adverse external influences.

All parts of the electrical network are carefully checked. The basic rule is reliable protection, taking into account the performance of the site with the worst parameters. It should be borne in mind that copper is designed with the same cross-section for heavy loads compared to aluminum. The purity of the metal is of some importance. As impurities increase, conductivity deteriorates, and unnecessary and dangerous heating losses increase.

Internal wiring device

In such real estate objects, the following standard solutions are used to create a high-quality power supply system:

• an introductory machine must be placed before the counter;
• behind the control device, a general residual current device (RCD) is mounted;
• further equip individual lines with automatic switches (AB).

RCD prevents accidents that provoke leakage currents. In some situations, prevents electric shock. However, complex protective measures are performed using circuit breakers. Be sure to use effective grounding.

As a rule, it is convenient to put several groups in the kitchen to evenly distribute the loads. It is especially recommended to carefully select the distribution of powerful consumers:

• hobs;
• ovens;
• heating boilers, boilers, flow heaters;
• electric convectors, heat guns;
• conditioners.

The wiring diagram has a tree structure. From the central line of the "trunk" make the necessary branches of the "branch" to connect sockets and switches.

How to calculate the rating of a circuit breaker

The main function of AB is to protect electrical wiring. For this reason, first ensure that the rating of the switch in amperes, the cross-section and the material (copper, aluminum) of the cable cores are matched.

Determination of the total power of consumers

How much electricity is needed for the functioning of lamps and other products is noted in the accompanying documentation. The power is indicated on the case. These data can be obtained on the official website of the manufacturer. However, simply adding up the kilowatts is not enough.

Active and nominal component

The simple calculation algorithm shown in the example describes a resistive load situation. It is this component (active power - P) that is indicated in the technical passport of the corresponding product. It is determined by the meter for regular payments of the consumed energy.

However, when connecting a machine tool or other equipment with an electric drive, you will have to take into account the inductive component. They act in the same way if there is a capacitor in the circuit.

Formulas and explanations:

• P = S * cos ϕ;
• Q = S * sin ϕ;
• S = P / cos ϕ;
• ϕ is the angle between the vectors P and S (phase shift).

Reactive component (Q) refers to the cyclic exchange of energy between the power supply and the load. The sum of the vectors P and Q will help determine the final apparent power (S).

Increased starting currents

Turning on a powerful pump (another reactive load) is accompanied by an inrush current and a subsequent oscillatory process with a transition to a normal operating mode. The pulse duration, as a rule, does not exceed 1.5-2 seconds. This duration is insufficient to warm up the bimetallic plate. But this may be enough to move the solenoid stem.

The list contains typical overshoot levels that activate solenoid trip. The time delays before the circuit breaks by the bimetallic plate (sec) are given in parentheses:

• A - 30% (20-30);
• B - 200% (4-5);
• C - 5 times (1.5);
• D - 10 times (0.4).

The corresponding modes were taken into account when creating profile standards.To prevent erroneous shutdown, it is necessary to select the appropriate type of machine.

Demand factor accounting

This correction factor (Кс) is used to take into account loads in real operating conditions: Calculated = S * Кс. Its value (range from 0 to 1) indicates the number of connected consumers. This method is convenient to use when creating office and industrial projects, which implies the use of the same type of equipment: machines, computers, etc.

For household tasks, it is not difficult to draw the correct conclusion based on a simple check. It is difficult to imagine a situation when an air conditioner is simultaneously used in one room for cooling and the air is heated with a fan heater.

Power consumption calculation

The phase shift formulas given are used to correct inductive and capacitive loads. Resistive ones are taken into account according to passport data without recalculation. The cos ϕ value is taken from the accompanying documentation.

You can calculate the amperage as follows:

• P / U - constant power supplies, resistive loads;
• P / (U * cos ϕ) = P / (220 * cos ϕ) - one phase, ~ 220V, reactive characteristics of the consumer;
• P / (U * √3 * cos ϕ) = P / (380 * 1.7321 * cos ϕ) - three-phase network ~ 380V, inductive (capacitive) parameters of equipment.

You can measure real voltages using a multimeter. The method of performing working operations is given in the official instructions of the manufacturer.

Choice of cross-section

The necessary information about the load capacities is contained in the official documentation of cable manufacturers. It is recommended to choose a larger cross-section from the series in order to exclude overheating and damage during operation. According to the current rules, conductors with an area of ​​1.5 mm or more are suitable for residential premises.

Given the increased power-to-weight ratio of modern real estate objects, the minimum network capacity will not be enough. Experts advise to provide for the connection of additional equipment as part of the subsequent modernization.

Current circuit breaker ratings

The limiting value of the rating is determined by the formula Inom ≤ Ipr / 1.45, where Ipr is the permissible continuous current for a certain wiring. If you plan to install a network, proceed as follows:

• specify the connection diagram for consumers;
• collect technical data, measure voltage;
• according to the presented scheme, they are calculated separately, the currents in individual circuits are summed;
• for each group, it is necessary to select an automatic machine that will withstand the corresponding load;
• determine cable products with a suitable conductor cross-section.

If the nets are installed in gates and covered with plaster, disassembly is too difficult. In this case, the selection of the machine over the section of the cable is used. They start with an assessment of the load capacities of the existing lines. This result is used to evaluate suitable models of protective devices. Further, consumers are divided into groups, taking into account the total capacity (shared use).

The rules for choosing the denomination

For correct conclusions, it is necessary to take into account the peculiarities of the connected equipment. If the calculated total current is 19 amperes, users prefer to buy a 25A device. This solution assumes the possibility of applying additional loads without significant restrictions.

However, in some situations it is better to choose a 20A circuit breaker. This provides a relatively shorter time for power off when the current rises (temperature rise) with a bimetallic disconnector. This precaution will help keep the motor winding intact when the rotor is blocked by a jammed drive.

Different response times are useful to ensure the selective operation of protective equipment. Devices with lower latency are installed on the lines.In an emergency, only the damaged part is disconnected from the electricity. The introductory machine will not have time to shut down. Power supply through other circuits is useful for maintaining lighting, signaling, and other engineering systems in working order.

Selection of the machine by power

Individual work steps are simplified using specialized calculators. Such programs provide information and reference sites for free. But the selection of the machine for power will have to be done on the basis of real equipment.

Typical algorithm:

• specify the initial data on the consumption of individual devices;
• distribute into groups, specify the total values;
• the results obtained are used to select protective equipment.

This parameter denotes the preservation of functionality with a multiple excess of current loads (times):

• B (3-5);
• C (5-10);
• D (40-50).

The requirements for the models of machines are formed taking into account the rating and class of switching capabilities. Protective equipment is chosen with a margin to ensure operability in the event of an emergency.

Tabular way

The reference materials indicate how much load power can be applied if the machines are installed in different power networks. Example for a 2 A model (values ​​in kW):

• 220 V, 1 (2) poles, single-phase connection - 0.4;
• 380 V, 3 pole, "triangle" - 2.3;
• 380 V, 4th pole, "star" - 1.3.

The result must be increased to the closest value in the model range for reliability.

Graphical way

This technique applies similar principles. But the test parameters are presented in a clear graphical form.

Nuances of choice

In any case, the choice of a current (power) circuit breaker is made with a margin. Experts recommend using a multiplying factor of 1.4-1.6. At the same time, the ability of the wiring to withstand the maximum load is checked.

Calculation of the machine for the section of the wiring

For a typical household network, the data can be taken from the following table:

Choosing a breaker by cable size helps to protect the wiring. This technique is recommended by experienced specialists. If the area is unknown, it is calculated using a typical geometric formula, taking into account the measured diameter (D) of the conductor: S = (π * D2) / 4 = 0.785 * D2.

Formula for calculating power by current and voltage

For calculations on these parameters, the definitions of total (S), active (P) and reactive (Q) power are used. The following formulas are suitable for calculating single-phase 220 V networks:

• S = U * I;
• P = U * I * cos ϕ;
• Q = U * I * sin ϕ.

The initial data for the calculation can be taken from the reference books. Measurement results are also used.

Active load

Incandescent lamps and heaters are not reactive. Such loads do not shift the phases of currents and voltages. Power is consumed in full at twice the frequency.

Capacitive load

When a capacitor is connected to an AC network, energy is exchanged in both directions. This process is not accompanied by useful work.

Negative impact of reactive load

In the explanations provided, the ideal situation is considered. However, in reality, each reactive element has a certain electrical resistance. Be aware of the corresponding losses in the connecting wires and other circuit components.

With significant values ​​of the capacitive (inductive) component, the noted problems must be taken into account. In some schemes, in addition to increasing the load capacity of the machines, additional compensation components are used.

What currents are used for calculating machines

The power of the protective device is selected according to the wiring current (calculated or tabular value), taking into account the consumption of the connected load.The rating of the machine is chosen less in order to maintain the integrity of the power line during operation. In different parts of the network, conductors of the corresponding section are installed, guided by the principles of a tree structure.

Intentional reduction of the circuit breaker rating is permissible when connecting loads with reduced power consumption. This option implies the use of lines with a large current margin. This solution better protects the connected equipment from damage.