To select the machine according to the load power, it is necessary to calculate the load current, and select the value of the circuit breaker greater than or equal to the value obtained. The value of the current, expressed in amperes in a single-phase network of 220 V., usually exceeds the value of the load power, expressed in kilowatts, by 5 times, i.e. if the power of the electrical receiver (washing machine, light bulb, refrigerator) is 1.2 kW, then the current that will flow in the wire or cable is 6.0 A (1.2 kW * 5 = 6.0 A). Based on 380 V., in three-phase networks, everything is the same, only the current value exceeds the load power by 2 times.
Power factor
this is a dimensionless physical quantity that characterizes the consumer of alternating electric current from the point of view of the presence of a reactive component in the load. The power factor indicates how much the phase is shifted alternating current, flowing through the load, relative to the voltage applied to it.
Numerically, the power factor is cosine of this phase shift or cos φ
We take the cosine phi from table 6.12 of the normative document SP 31-110-2003 "Design and installation of electrical installations of residential and public buildings"
Table 1. The value of Cos φ depending on the type of power receiver
Let's take our power receiver with a power of 1.2 kW. as a household single-phase refrigerator for 220V, we will take cos φ from the table 0.75 as a motor from 1 to 4 kW.
Let's calculate the current I = 1200 W / 220V * 0.75 = 4.09 A.
Now the most correct way to determine the current of the electrical receiver- take the current value from the nameplate, passport or instruction manual. A nameplate with characteristics is on almost all electrical appliances.
EKF circuit breakers
The total current in the line (for example, a socket network) is determined by summing the current of all electrical receivers. According to the calculated current, we select the nearest rating of the automatic machine in a big way. In our example, for a current of 4.09A, this will be a 6A machine.
It is very important to note that choosing a circuit breaker only according to the load power is a gross violation of fire safety requirements and can lead to ignition of the cable or wire insulation and, as a result, to a fire. When choosing, it is also necessary to take into account the cross section of the wire or cable.
According to the load power, it is more correct to choose the conductor cross section. Selection requirements are set out in the main normative document for electricians called PUE (Electrical Installation Rules), or rather in chapter 1.3. In our case, for a home electrical network, it is enough to calculate the load current, as indicated above, and in the table below select the cross section of the conductor, provided that the obtained value is lower than the long-term permissible current corresponding to its cross section.
Selection of the machine according to the cable section
Let's consider the problem of choosing circuit breakers for home wiring in more detail, taking into account fire safety requirements. The necessary requirements are set out in chapter 3.1 "Protection of electrical networks up to 1 kV", since the network voltage in private houses, apartments, cottages is 220 or 380V.
Calculation of the cross section of cable and wire cores Voltage 220V.
- single-phase network is used mainly for sockets and lighting.
380V. - these are mainly distribution networks - power lines passing through the streets, from which houses are connected by a branch.
According to the requirements of the above chapter, the internal networks of residential and public buildings must be protected from short-circuit currents and overload. To meet these requirements, protection devices called automatic switches (automatic devices) were invented.
Automatic switch "automatic"
it is a mechanical switching device capable of making, carrying currents in normal circuit conditions, as well as making, carrying for a specified time and automatically breaking currents in specified abnormal circuit conditions, such as short-circuit and overload currents.
Short circuit (short circuit)
electrical connection of two points of an electrical circuit with different potential values, which is not provided for by the design of the device and disrupts its normal operation. A short circuit can occur as a result of a violation of the insulation of current-carrying elements or mechanical contact of uninsulated elements. Also, a short circuit is a condition when the load resistance is less than the internal resistance of the power source.
Overload current
- exceeding the rated value of continuous permissible current and causing overheating of the conductor. Protection against short circuit currents and overheating is necessary for fire safety, to prevent ignition of wires and cables, and as a result of a fire in the house.
Continuously admissible current of a cable or wire
- the amount of current that constantly flows through the conductor, and does not cause excessive heating.
The value of continuous current for conductors different section and the material is presented below. The table is a combined and simplified version applicable for household power supply networks, tables No. 1.3.6 and 1.3.7 of the PUE.
The choice of the machine for the short-circuit current
The selection of the circuit breaker for protection against short circuit (short circuit) is carried out on the basis of the calculated value of the short circuit current at the end of the line. The calculation is relatively complicated, the value depends on the power of the transformer substation, the cross section of the conductor and the length of the conductor, etc.
From the experience of carrying out calculations and designing electrical networks, the most influencing parameter is the length of the line, in our case, the length of the cable from the shield to the socket or chandelier.
Because in apartments and private houses this length is minimal, then such calculations are usually neglected and circuit breakers with characteristic “C” are chosen, of course you can use “B”, but only for lighting inside an apartment or house, because. such low-power lamps do not cause a high inrush current, and already in the network for kitchen appliances with electric motors, the use of automatic machines with characteristic B is not recommended, because. the machine may trip when the refrigerator or blender is turned on due to a jump in starting current.
The choice of the machine for long-term permissible current (DDT) of the conductor
The choice of a circuit breaker for protection against overload or overheating of the conductor is carried out on the basis of the DDT value for the protected section of the wire or cable. The rating of the machine must be less than or equal to the value of the DDT of the conductor indicated in the table above. This ensures automatic shutdown of the machine when the DDT in the network is exceeded, i.e. part of the wiring from the machine to the last electrical receiver is protected from overheating, and as a result, from the occurrence of a fire.
Circuit breaker selection example
We have a group from the shield to which it is planned to connect a dishwasher -1.6 kW, a coffee maker - 0.6 kW and an electric kettle - 2.0 kW.
We consider the total load and calculate the current.
Load = 0.6+1.6+2.0=4.2 kW. Current \u003d 4.2 * 5 \u003d 21A.
We look at the table above, for the current we calculated, all conductor cross-sections are suitable except for 1.5 mm2 for copper and 1.5 and 2.5 for aluminum.
We choose a copper cable with conductors with a cross section of 2.5mm2, because. it makes no sense to buy a cable with a larger cross section for copper, and aluminum conductors are not recommended for use, and may already be prohibited.
We look at the scale of denominations of produced machines - 0.5; 1.6; 2.5; 1; 2; 3; 4; 5; 6; 8; 10; 13; 16; 20; 25; 32; 40; 50; 63.
The circuit breaker for our network is suitable for 25A, since it is not suitable for 16A because the calculated current (21A.) exceeds the nominal value of the 16A circuit breaker, which will cause it to operate when all three electrical receivers are turned on at once. A 32A automatic machine will not work because it exceeds the DDT of the 25A cable we have chosen, which can cause overheating of the conductor and, as a result, a fire.
Summary table for selecting a circuit breaker for a single-phase 220 V network.
| Rated current of the circuit breaker, A. | Power, kWt. | Current, 1 phase, 220V. | Cross-section of cable conductors, mm2. |
| 16 | 0-2,8 | 0-15,0 | 1,5 |
| 25 | 2,9-4,5 | 15,5-24,1 | 2,5 |
| 32 | 4,6-5,8 | 24,6-31,0 | 4 |
| 40 | 5,9-7,3 | 31,6-39,0 | 6 |
| 50 | 7,4-9,1 | 39,6-48,7 | 10 |
| 63 | 9,2-11,4 | 49,2-61,0 | 16 |
| 80 | 11,5-14,6 | 61,5-78,1 | 25 |
| 100 | 14,7-18,0 | 78,6-96,3 | 35 |
| 125 | 18,1-22,5 | 96,8-120,3 | 50 |
| 160 | 22,6-28,5 | 120,9-152,4 | 70 |
| 200 | 28,6-35,1 | 152,9-187,7 | 95 |
| 250 | 36,1-45,1 | 193,0-241,2 | 120 |
| 315 | 46,1-55,1 | 246,5-294,7 | 185 |
Summary table for selecting a circuit breaker for a three-phase network 380 V.
| Rated current automatic switch, a. | Power, kWt. | Current, 1 phase 220V. | Cross section cable, mm2. |
| 16 | 0-7,9 | 0-15 | 1,5 |
| 25 | 8,3-12,7 | 15,8-24,1 | 2,5 |
| 32 | 13,1-16,3 | 24,9-31,0 | 4 |
| 40 | 16,7-20,3 | 31,8-38,6 | 6 |
| 50 | 20,7-25,5 | 39,4-48,5 | 10 |
| 63 | 25,9-32,3 | 49,2-61,4 | 16 |
| 80 | 32,7-40,3 | 62,2-76,6 | 25 |
| 100 | 40,7-50,3 | 77,4-95,6 | 35 |
| 125 | 50,7-64,7 | 96,4-123,0 | 50 |
| 160 | 65,1-81,1 | 123,8-124,2 | 70 |
| 200 | 81,5-102,7 | 155,0-195,3 | 95 |
| 250 | 103,1-127,9 | 196,0-243,2 | 120 |
| 315 | 128,3-163,1 | 244,0-310,1 | 185 |
| 400 | 163,5-207,1 | 310,9-393,8 | 2х95* |
| 500 | 207,5-259,1 | 394,5-492,7 | 2х120* |
| 630 | 260,1-327,1 | 494,6-622,0 | 2х185* |
| 800 | 328,1-416,1 | 623,9-791,2 | 3х150* |
* - double cable, two cables connected in parallel, for example 2 cables VVGng 5x120
Results
When choosing an automatic machine, it is necessary to take into account not only the load power, but also the cross section and material of the conductor.
For networks with small areas protected from short-circuit currents, circuit breakers with characteristic "C" can be used.
The nominal value of the machine must be less than or equal for a long time admissible current conductor.
The choice of protective circuit breakers is made not only during the installation of a new electrical network, but also when upgrading the electrical panel, as well as when additional powerful devices are included in the circuit, increasing the load to a level that old emergency shutdown devices cannot cope with. And in this article we will talk about how to properly select the machine for power, what should be taken into account during this process and what are its features.
Failure to understand the importance of this task can lead to very serious problems. Indeed, often users do not bother themselves by choosing a circuit breaker by power, and take the first device they come across in the store, using one of two principles - “cheaper” or “more powerful”. This approach, associated with the inability or unwillingness to calculate the total power of devices connected to the power grid, and in accordance with it, select a circuit breaker, often causes the failure of expensive equipment in the event of a short circuit or even a fire.
What are circuit breakers and how do they work?
Modern AB have two degrees of protection: thermal and electromagnetic. This allows you to protect the line from damage as a result of a long excess of the flowing current of the rated value, as well as a short circuit.
The main element of the thermal release is a plate of two metals, which is called bimetallic. If a current is applied to it for a sufficiently long time increased power, it becomes flexible and, acting on the disconnecting element, causes the machine to operate.
The presence of an electromagnetic release is due to the breaking capacity of the circuit breaker when the circuit is exposed to short-circuit overcurrents, which it cannot withstand.
The electromagnetic type release is a solenoid with a core, which, when a high power current passes through it, instantly shifts towards the disconnecting element, turning off the protective device and de-energizing the network.
This makes it possible to protect the wire and devices from an electron flow, the value of which is much higher than that calculated for a cable of a specific section.
Why is cable mismatch with network load dangerous?
The correct selection of the circuit breaker by power is a very important task. An incorrectly selected device will not protect the line from a sudden increase in current.
But it is equally important to choose the right electrical cable according to the cross section. Otherwise, if the total power exceeds the nominal value that the conductor can withstand, this will lead to a significant increase in the temperature of the latter. As a result, the insulating layer will begin to melt, which can lead to a fire.
In order to more clearly imagine what the mismatch of the wiring cross-section of the total power of the devices connected to the network threatens, consider the following example.
The new owners, having bought an apartment in an old house, install several modern household appliances in it, giving a total load on the circuit equal to 5 kW. The current equivalent in this case will be about 23 A. In accordance with this, a 25 A circuit breaker is included in the circuit. It would seem that the choice of the machine in terms of power was made correctly, and the network is ready for operation. But some time after turning on the appliances, smoke appears in the house with a characteristic smell of burnt insulation, and after a while a flame appears. At the same time, the circuit breaker will not disconnect the network from the power supply - after all, the current rating does not exceed the allowable one.
If the owner is not nearby at this moment, the molten insulation will cause short circuit, which, finally, will provoke the operation of the machine, but the flame from the wiring may already have spread throughout the house.
The reason is that although the power calculation of the machine was done correctly, the wiring cable with a cross section of 1.5 mm² was rated for 19 A and could not withstand the existing load.
So that you do not have to take up a calculator and independently calculate the cross section of the electrical wiring using formulas, we present a typical table in which it is easy to find the desired value.
Weak link protection
So, we made sure that the calculation of the circuit breaker should be made based not only on the total power of the devices included in the circuit (regardless of their number), but also on the cross section of the wires. If this indicator is not the same along the electric line, then we select the section with the smallest cross section and calculate the machine based on this value.
The requirements of the PUE state that the selected circuit breaker must provide protection for the weakest section of the electrical circuit, or have a current rating that will correspond to a similar parameter of the installations connected to the network. This also means that wires must be used for the connection, the cross section of which will withstand the total power of the connected devices.
How to select the wire cross section and circuit breaker rating - in the following video:
If the negligent owner ignores this rule, then in the event of an emergency due to insufficient protection of the weakest section of the wiring, he should not blame the selected device and scold the manufacturer - only he will be responsible for the situation.
How to calculate the circuit breaker rating?
Let's assume that we have taken into account all of the above and selected a new cable that meets modern requirements and has the desired cross section. Now the electrical wiring is guaranteed to withstand the load from the included household appliances, even if there are a lot of them. Now we proceed directly to the choice of the circuit breaker according to the current rating. We recall the school physics course and determine the calculated load current by substituting the appropriate values into the formula: I = P / U.
Here I is the value of the rated current, P is the total power of the installations included in the circuit (taking into account all consumers of electricity, including light bulbs), and U is the mains voltage.
To simplify the choice of a circuit breaker and save you from having to take on a calculator, we present a table that shows the ratings of AB, which are included in single-phase and three-phase networks, and the corresponding total load powers.
This table will make it easy to determine how many kilowatts of load correspond to which rated current of the protective device. As we can see, a 25 Amp machine in a network with a single-phase connection and a voltage of 220 V corresponds to a power of 5.5 kW, for a 32 Amp AB in a similar network - 7.0 kW (in the table this value is highlighted in red). At the same time, for an electrical network with a three-phase delta connection and a rated voltage of 380 V, a 10 Amp machine corresponds to a total load power of 11.4 kW.
Clearly about the selection of circuit breakers on the video:
Conclusion
In the presented material, we talked about why electric circuit protection devices are needed and how they work. In addition, given the information provided and the tabular data given, you will not have any difficulty with the question of how to choose a circuit breaker.
Surely many of us wondered why circuit breakers so quickly replaced obsolete fuses from electrical circuits? The activity of their implementation is justified by a number of very convincing arguments, among which is the opportunity to buy this type of protection, which ideally corresponds to the time-current data of specific types of electrical equipment.
Doubt what kind of machine you need and do not know how to choose it correctly? We will help you find the right solution - the article discusses the classification of these devices. As well as important characteristics that you should pay close attention to when choosing a circuit breaker.
To make it easier for you to deal with the machines, the material of the article is supplemented with visual photos and useful video recommendations from experts.
The machine almost instantly turns off the line entrusted to it, which eliminates damage to the wiring and equipment powered by the network. After the shutdown has been completed, the branch can be immediately restarted without replacing the safety device.
If you have knowledge or experience in electrical work, please share it with our readers. Leave your comments about the choice of a circuit breaker and the nuances of its installation in the comments below.
Instruction
A properly selected circuit breaker should operate in the event of a wiring short circuit or at a load that is significantly higher than the calculated one. At the same time, it should not turn off if, for example, you turned on the washing machine at the same time. That is why select the circuit breaker specifically for your wiring and load.
Purchase circuit breakers only in specialized stores, this will allow you to buy a quality product with a high degree of probability. Remember that the rated current of the circuit breaker should not be higher than the maximum current load allowed for your network. In other words, the machine should work, and the wiring should not burn out.
The calculation of the parameters of the circuit breaker is carried out as follows. Let's assume that you have turned on a 2 kW electric kettle. We divide the power, in watts, by 220, we get a current of 9.1 A. This figure can be rounded up to 10 so that there is some margin in the calculations. This means that the circuit breaker must withstand a current of 10 amperes. Now calculate the total power of all electrical appliances you turn on at the same time and calculate, according to the above diagram, the current strength. Let's say you get 30 amperes - which means that the circuit breaker must be designed for this current.
Having carried out the previous calculations, you have calculated what current the circuit breaker should be rated for. But you also need to know how much current your network can handle, and that depends on the wires you use. For example, if the wiring is made with 2.5 mm aluminum wire (the most common option), it is able to withstand a current of 24 amperes and a load of 5.2 kW. So, in your case, the circuit breaker should be designed for a current slightly less than 24 amperes. If you use a copper wire of this section, then it will withstand a current of 30 amperes and a load of 6.6 kW. If you use a wire of a different size, look up the data on the withstand current and load for it on the Internet.
When installing wiring in the house, foresee its division into several isolated circuits. For example, make lighting circuits and wiring to sockets separate. This will allow you to install a separate circuit breaker for each circuit, which will greatly increase overall safety. Electrical appliances such as electric water heaters, additionally turn on through the RCD - a residual current device that can protect a person from electric shock. The RCD reacts to leakage, therefore, in case of touching the phase wire, it will immediately turn off the current.
In this article, I want to touch on such important topic as the correct calculation of the cross section of the 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 this. With an underestimated cable cross-section, the current in the line will exceed the maximum permitted operating current. At the same time, the operating current of the wiring is limited by the maximum allowable 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 the destruction of the cable. For hidden wiring, the thermal conductivity of the wire is less than for open wiring, the wire is cooled worse and, accordingly, the permissible operating current is less.
You should not save on the cable, because if you choose the wrong one, you will have to replace it, and this is a laborious process, often meaning the start of a new repair.
Calculation and selection of the cable section
The rated current of the circuit breaker is selected greater than or equal to the rated current of the line, and must not exceed the maximum allowable load in the electrical circuit or cable:
I calculation<=I н <=I доп
To provide overcurrent protection, the rated operating current of the circuit breaker must be 45% less than the maximum allowable load for the electrical circuit or cable:
Itr<=1,45*I доп
where I calc is the rated current of the circuit;
I add - permissible load of the electrical circuit or cable;
I n - rated current of the circuit breaker;
I tr - Tripping current of the thermal release;
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 strobe under plaster is equivalent to wiring laid in a pipe.
According to modern electrical safety requirements, wiring in apartments (cottages, offices) must be carried out with a three-wire copper cable or wire, but the ground conductor (PE) is not taken into account in calculations, so we use a column with the parameters of two-wire wires:
In the event that you have electrical wiring in your house with aluminum wire, you can use table 1.3.5. , which indicates the values \u200b\u200bof the maximum allowable 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 allowable sections of the conductive cores of wires and cables in electrical wiring according to TKP 121 are shown in Table 8.1.
According to this table, the minimum conductor cross-section for power and lighting circuits is 1.5 mm2. 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 will happen if you do not take into account the setting of the thermal release when choosing an automatic machine? For convenience, consider an example:
Let's take the most common rating of the machine - 16 A, the overload current at which the machine will work for 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 setting value of the thermal release is 1.45 rated current). Accordingly, it is for this current that the cable section should be selected. From table 1.3.4. we select a suitable section: for hidden electrical wiring made of copper - this is a minimum of 2.5 mm 2 (maximum overload current 27 A).
In a similar way, calculations can be made for a 10 A machine. The current at which the machine turns off within an hour will be 10 1.45 = 14.5 A. 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 the line with a cross section of 2.5 mm 2, they install a circuit breaker with a nominal value of 25A (after all, the line can withstand a current of 25 A for a long time). But at the same time, they forget that the non-switchable current of such an automaton 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 cable products market is dominated by cables produced not according to GOST, but according to TU. It follows from this that their actual cross section will be underestimated. Buying a cable produced 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 rule for choosing the cable section and the machine is neglected:
Circuit breaker selection
Considering all of the above factors, in order to increase the safety, reliability and durability of electrical wiring, it is worth using the following ratios of the cable cross-section and the machine that protects this line:
- 1.5mm²→ 10 A → 2200 W→ mainly used for lighting lines.
- 2.5mm²→16 A → 3520 W→ used in separate lines for sockets of powerful household appliances (washing machine, dishwasher, etc.) or groups of household sockets.
- - 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 the wire sections have been selected, a test is carried out for permissible voltage loss. With a large length of wires, the voltage to consumers can reach significantly lower than the nominal voltage. Permissible voltage loss in the wires must not exceed 5% of the rated voltage. If she will be greater than the allowable, then it is necessary to choose a wire of a larger cross section. In this article, we will not consider a voltage loss test.
