To ensure safety during the operation of household electrical appliances, it is necessary to correctly calculate the cross-section of the supply cable and wiring. Since the wrongly selected cable cross-section can lead to a fire in the wiring due to a short circuit. This threatens to start a fire in the building. This also applies to the choice of cables for connecting electric motors.

Calculation of the current

The magnitude of the current is calculated by power and is necessary at the design (planning) stage of a dwelling - an apartment, a house.

• The value of this quantity depends on choice of supply cable (wires), through which power consumption devices can be connected to the network.
• Knowing the voltage of the electrical network and the full load of electrical appliances, you can use the formula calculate the current that you need to pass through the conductor(wire, cable). By its size, the cross-sectional area of ​​the veins is chosen.

If the electrical consumers in the apartment or house are known, it is necessary to perform simple calculations in order to correctly mount the power supply circuit.

Similar calculations are performed for production purposes: determining the required cross-sectional area of ​​the cable cores when connecting industrial equipment (various industrial electric motors and mechanisms).

Single-phase network with a voltage of 220 V

The current strength I (in amperes, A) is calculated by the formula:

I = P / U,

where P is the electric full load (must be indicated in the technical passport of the device), W (watt);

U is the voltage of the electrical network, V (volts).

The table below shows load values ​​of typical household electrical appliances and the current consumed by them (for a voltage of 220 V).

The figure shows a diagram of an apartment's power supply device with a single-phase connection to a 220 V network.

As can be seen from the figure, various consumers of electricity are connected through the appropriate machines to the electric meter and then to the general machine, which must be designed for the load of the devices with which the apartment will be equipped. The wire that supplies the power must also satisfy the load of the power consumers.

Following is the table for hidden wiring for a single-phase apartment wiring diagram for wire selection at a voltage of 220 V

As can be seen from the table, the cross-section of the conductors depends, in addition to the load, on the material from which the wire is made.

Three-phase network with a voltage of 380 V

With a three-phase power supply, the current I (in amperes, A) is calculated by the formula:

I = P / 1.73 U,

where P is the power consumption, W;

U - mains voltage, V,

since the voltage with a three-phase power supply scheme is 380 V, the formula will take the form:

I = P / 657.4.

In the case of a 380 V three-phase power supply to the house, the connection diagram will look like this.

The cross-section of the conductors in the supply cable at different loads with a three-phase circuit with a voltage of 380 V for hidden wiring is presented in the table.

To calculate the current in the power supply circuits of a load characterized by a large reactive apparent power, which is typical for the use of power supply in industry:

• electric motors;
• chokes for lighting devices;
• welding transformers;
• induction furnaces.

When calculating, this phenomenon must be taken into account. In powerful devices and equipment, the proportion of reactive load is higher and therefore for such devices in the calculations, the power factor is taken equal to 0.8.

The question of choosing a cable cross-section for installing electrical wiring in a house or apartment is very serious. If this indicator does not correspond to the load in the circuit, then the wire insulation will simply start to overheat, then melt and burn. The end result is a short circuit. The thing is that the load creates a certain current density. And if the cross-section of the cable is small, then the current density in it will be large. Therefore, before buying, it is necessary to calculate the cable cross-section for the load.

Of course, you should not just randomly choose a wire of a larger cross section. This will hit your budget first. With a smaller cross-section, the cable may not withstand the load and will quickly fail. Therefore, the best place to start is how to calculate the load on the cable? And only then, according to this indicator, select the electrical wire itself.

Power calculation

The easiest way is to calculate the total power that the house or apartment will consume. This calculation will be used to select the cross-section of the wire from the power transmission pole to the input machine to the cottage or from the driveway to the apartment to the first junction box. In the same way, wires are calculated for loops or rooms. It is understood that the input cable will have the largest cross-section. And the further from the first junction box, the more this indicator will decrease.

But back to the calculations. So, first of all, it is necessary to determine the total power of consumers. Each of them (household appliances and lighting lamps) has this indicator on the body. If not found, look in the passport or in the instructions.

After that, all powers must be folded. This is the total capacity of a house or apartment. Exactly the same calculation must be done along the contours. But there is one controversial point. Some experts recommend multiplying the total indicator by a reduction factor of 0.8, adhering to the rule that not all devices will be simultaneously included in the circuit. Others, on the contrary, propose to multiply by a multiplying coefficient of 1.2, thereby creating a certain margin for the future, since there is a high probability of additional household appliances appearing in the house or apartment. In our opinion, the second option is optimal.

Cable selection

Now, knowing the total power indicator, you can select the wiring cross-section. The PUE has tables that make it easy to make this choice. Here are some examples for an electrical line powered by 220 volts.

• If the total power is 4 kW, then the wire cross-section will be 1.5 mm².
• Power 6 kW, cross-section 2.5 mm².
• Power 10 kW - cross section 6 mm².

The exact same table is also available for a 380 volt electrical network.

Calculation of the current load

This is the most accurate value for calculating the load current. For this, the formula is used:

I = P / U cos φ, where

• I is the current strength;
• P is the total power;
• U is the voltage in the network (in this case 220 V);
• cos φ is the power factor.

There is a formula for a three-phase electrical network:

I = P / (U cos φ) * √3.

It is according to the indicator of the current strength that the cable cross-section is determined according to the same tables in the PUE. Here are some examples again.

• Current 19 A - cable cross-section 1.5 mm².
• 27 A - 2.5 mm².
• 46 A - 6 mm².

As in the case of determining the cross-section for power, it is also best to multiply the current indicator by a multiplying factor of 1.5.

Odds

There are certain conditions under which the amperage inside the wiring can rise or fall. For example, in open electrical wiring, when wires are laid along walls or ceilings, the amperage will be higher than in a closed circuit. This is directly related to temperature. environment... The more it is, the more current this cable can pass.

Attention! All of the above tables of the PUE are calculated under the condition that the wires are operated at a temperature of + 25C with the temperature of the cables themselves not exceeding + 65C.

That is, it turns out that if several wires are laid in one tray, corrugation or pipe at once, then the temperature inside the wiring will be increased due to the heating of the cables themselves. This leads to the fact that the current carrying capacity is reduced by 10-30 percent. The same applies to open wiring inside heated rooms. Therefore, we can conclude: when calculating the cable cross-section, depending on the load current at elevated temperatures operation, you can choose wires of a smaller area. This is, of course, a good saving. By the way, there are also tables of reducing coefficients in the PUE.

There is one more point that concerns the length of the electrical cable used. The longer the wiring, the greater the voltage loss in the sections. In any calculations, losses equal to 5% are used. That is, this is the maximum. If there are more losses given value, you will have to increase the cable cross-section. By the way, it is not difficult to independently calculate the current losses if you know the resistance of the wiring and the current load. Although the best option is to use the PUE table, in which the dependence of the load torque and losses is established. In this case, the moment of load is the product of the power consumption in kilowatts and the length of the cable itself in meters.

Let us consider an example in which an installed cable 30 mm long in a 220-volt alternating current network withstands a load of 3 kW. In this case, the load moment will be equal to 3 * 30 = 90. We look at the PUE table, where it is shown that this moment corresponds to a loss of 3%. That is, it is less than the nominal 5%. What is acceptable. As mentioned above, if the calculated losses would exceed the five percent barrier, then you would have to purchase and install a larger cable.

Attention! These losses greatly affect lighting with low voltage lamps. Because at 220 volts, 1-2 V is not strongly reflected, but at 12 V it is visible immediately.

Nowadays, aluminum wires are rarely used in wiring. But you need to know that their resistance is 1.7 times higher than that of copper ones. And, therefore, their losses are as many times greater.

As for three-phase networks, here the load moment is six times greater. It depends on the fact that the load itself is distributed over three phases, and this is, accordingly, a throne increase in torque. Plus a double increase due to the symmetrical distribution of power consumption in phases. In this case, in the zero loop, the current must be zero. If the phase distribution is asymmetric, and this leads to an increase in losses, then you will have to calculate the cable cross-section for the loads in each wire separately and select it according to the maximum calculated size.

Conclusion on the topic

As you can see, in order to calculate the cable cross-section for loads, you have to take into account various factors (reducing and increasing). It is not easy to do this on your own, if you are versed in electrics at the level of an amateur or a novice master. Therefore, advice - invite a highly qualified specialist, let him do all the calculations himself and draw up a competent wiring diagram. But the installation can be done with your own hands.

Theory calculation of electrical loads, the basis of which was formed in the 1930s, aimed to determine a set of formulas that give an unambiguous solution for given electrical receivers and graphs (indicators) of electrical loads. In general, practice has shown the limitations of the bottom-up approach, based on the initial data for individual power consumers and their groups. This theory retains its significance when calculating the operating modes of a small number of power consumers with known data, when adding a limited number of graphs, when calculating for 2UR.

In the 1980-1990s. the theory of calculating electrical loads increasingly adheres to non-formalized methods, in particular, the integrated method for calculating electrical loads, the elements of which were included in the "Guidelines for calculating electrical loads of power supply systems" (RTM 36.18.32.0289). Probably, working with information databases on electrical and technological indicators, cluster analysis and the theory of pattern recognition, building probabilistic and cenological distributions for expert and professional assessment can finally solve the problem of calculating electrical loads at all levels of the power supply system and at all stages of making a technical or investment decision. ...

Formalization of the calculation of electrical loads developed over the years in several directions and led to the following methods:

1. empirical (method of demand coefficient, two-term empirical expressions, specific power consumption and specific load densities, technological schedule);
2. ordered diagrams, transformed into a calculation according to the calculated active power factor;
3. actually statistical;
4. probabilistic modeling of load curves.

Demand coefficient method

The demand factor method is the simplest, most widespread, and the calculation of loads began with it. It consists in using the expression (2.20): according to the known (specified) value of Py and the tabular values ​​given in the reference literature (see examples in Table 2.1):

The Kc value is taken to be the same for electrical receivers of the same group (operating in one mode), regardless of the number and power of individual receivers. Physical sense is the fraction of the sum of the nominal capacities of electrical receivers, statistically reflecting the maximum practically expected and encountered mode of simultaneous operation and loading of some indefinite combination (implementation) of installed receivers.

The given reference data for Ks and Kp correspond to the maximum value, and not the mathematical expectation. Summing maximum values, not averages, inevitably overestimates the load. If we consider any EP group of the modern electrical economy (and not the 1930-1960s), then the conventionality of the concept of “homogeneous group” becomes obvious. Differences in the value of the coefficient - 1:10 (up to 1: 100 and higher) - are inevitable and are explained by the cenological properties of the electrical economy.

Table 2.2 shows the LGS values ​​characterizing the pumps as a group. When researching KQ4 deeper, for example only for raw water pumps, there may also be a scatter of 1:10.

It is more correct to learn to evaluate Kc as a whole for the consumer (site, department, workshop). It is useful to analyze the calculated and actual values ​​for all objects of the same technology level of the same level of the power supply system, similar to table. 1.2 and 1.3. This will allow you to create a personal information bank and ensure the accuracy of calculations. The method of specific electricity consumption is applicable for 2UR sections (installations) (second, third ... Level of the Power System), missile defense departments and 4UR workshops, where the technological products are homogeneous and quantitatively change little (an increase in output, as a rule, reduces the specific electricity consumption Auy).

Maximum power method

In real conditions, the long-term operation of the consumer does not mean the constancy of the load at the point of its connection for more than high level power supply systems. As a statistical quantity Lud, determined for some previously identified object by power consumption A and volume L /, there is some averaging over a known, more often monthly or annual, interval. Therefore, the application of formula (2.30) gives not the maximum, but the average load. To select the transformers of the missile defense system, you can take Pcr = Pmax. In the general case, especially for 4UR (workshop), it is necessary to take into account Kmah as T to take the actual annual (daily) number of hours of production with the maximum use of active power.

Specific Load Density Method

The specific load density method is close to the previous one. Specific power (load density) y is set and the area of ​​the building of the structure or site, department, workshop is determined (for example, for machine building and metalworking shops y = 0.12 ... 0.25 kW / m2; for oxygen converter shops y = = 0.16 ... 0.32 kW / m2). A load exceeding 0.4 kW / m2 is possible for some areas, in particular, for those where there are single power consumers with a unit capacity of 1.0 ... 30.0 MW.

Technological graph method

The technological schedule method is based on the operation schedule of a unit, line or group of machines. For example, the operating schedule of an arc steel-making furnace is specified: the melting time (27 ... 50 min), the oxidation time (20 ... 80 min), the number of melts, technological coordination with the operation of other steel-making units are indicated. The graph allows you to determine the total energy consumption for the melt, the average for the cycle (taking into account the time until the start of the next melting), and the maximum load for calculating the supply network.

Ordered chart method

The method of ordered diagrams, which was applied in a directive way in the 1960s and 1970s. for all levels of the power supply system and at all stages of design, in the 1980s-1990s. transformed into the calculation of loads according to the coefficient of the calculated active power. In the presence of data on the number of electrical receivers, their power, operating modes, it is recommended to use it to calculate the elements of the power supply system 2UR, SAM (wire, cable, busbar, low-voltage equipment) supplying a power load with voltage up to 1 kV (simplified for the effective number of receivers of the entire shop, i.e. for a network with a voltage of 6 - 10 kV 4UR). The difference between the method of ordered diagrams and the calculation by the calculated active power factor consists in replacing the maximum coefficient, always understood unambiguously as the ratio Pmax / Pav (2.16), by the calculated active power factor Ap. The calculation order for a node element is as follows:

A list (number) of power electrical receivers is compiled with an indication of their nominal PHOMi (installed) power;

The work shift with the highest power consumption is determined and the typical day is agreed (with the technologists and the power system);

The features of the technological process that affect power consumption are described, power consumers with high load unevenness are distinguished (they are considered differently - according to the maximum effective load);

Excluded from the calculation (list) of electrical receivers: a) low power; b) reserve according to the terms of calculation of electrical loads; c) included occasionally;

Determined are groups of t power consumers having the same type (mode) of operation;

From these groups, subgroups are distinguished that have the same value of the individual utilization factor a: and /;

Electric consumers of the same operating mode are allocated and their average power is determined;

The average reactive load is calculated;

The group utilization factor Кн of active power is found;

The effective number of power consumers in a group of n power receivers is calculated:

where the effective (reduced) number of power consumers is the number of power receivers of the same power that are homogeneous in terms of operating mode, which gives the same value of the calculated maximum P as a group of power receivers that are different in power and mode of operation.

When the number of electrical receivers in the group is four or more, it is allowed to take pe equal to n ( real number electrical consumers), provided that the ratio of the rated power of the largest electrical consumer Pmutm to the rated power of the smaller electrical consumer House mm is less than three. When determining the value of n, it is allowed to exclude small electrical receivers, the total power of which does not exceed 5% of the rated power of the entire group;

According to the reference data and the heating time constant T0, the value of the calculated coefficient Kp is taken;

The calculated maximum load is determined:

Electrical loads individual nodes of the power supply system in networks with voltages above 1 kV (located on 4UR, 5UR) were recommended to be determined in the same way with the inclusion of losses in.

The calculation results are tabulated. This exhausts the calculation of loads according to the calculated active power factor.

The calculated maximum load of a group of electrical receivers Pmax can be found in a simplified way:

where Рnom - group rated power (the sum of rated powers, excluding reserve ones for calculating electrical loads); Rav.cm ~ average active power for the busiest shift.

The calculation by formula (2.32) is cumbersome, difficult to understand and apply, and most importantly, it often gives a twofold (or more) error. The method overcomes non-Gaussian randomness, uncertainty, and incompleteness of the initial information by making assumptions: electrical receivers of the same name have the same coefficients, reserve motors are excluded according to the conditions of electrical loads, the utilization factor is considered independent of the number of electrical receivers in the group, electrical receivers with an almost constant load schedule are distinguished, the smallest ones are excluded from the calculation. power receivers. The method is not differentiated for different levels of the power supply system and for different stages of project implementation (approval). The calculated maximum coefficient Kmax of active power is taken as tending to unity with an increase in the number of electrical receivers (in fact, this is not the case - statistics do not confirm this. For a department where there are 300 ... 1000 engines, and a workshop, where there are up to 6000 pcs., The coefficient can be 1 , 2 ... 1,4). The introduction of market relations, leading to automation, a variety of product output, moves electrical consumers from group to group.

Statistical definition YSr.cm for operating enterprises is complicated by the difficulty of choosing the busiest shift (transferring the start of work of different categories of workers within the shift, four-shift work, etc.). Uncertainty in measurements is manifested (imposition on the administrative-territorial structure). The restrictions on the part of the power system lead to modes when the maximum load Ptgx occurs in one shift, while the power consumption is greater in the other shift. When determining Рр, it is necessary to refuse Рср.см by excluding intermediate calculations.

A detailed consideration of the disadvantages of the method is caused by the need to show that the calculation of electrical loads based on the classical concepts of electrical circuit and load curves, theoretically cannot provide sufficient accuracy.

Statistical methods for calculating electrical loads are consistently defended by a number of specialists. The method takes into account that even for one group of mechanisms operating in a given production area, the coefficients and indicators vary within wide limits. For example, the inclusion coefficient for non-automatic machine tools of the same type varies from 0.03 to 0.95, A3 load - from 0.05 to 0.85.

The task of finding the maximum of the function Рр at a certain time interval is complicated by the fact that electrical receivers and consumers with different operating modes are powered from 2UR, SAM, 4UR. The statistical method is based on measuring the loads of the lines supplying characteristic groups of power consumers, without referring to the operating mode of individual power consumers and the numerical characteristics of individual graphs.

(xtypo_quote) The method uses two integral characteristics: the general mean load PQp and the general standard deviation, where the DP variance is taken for the same averaging interval. (/ xtypo_quote)

The maximum load is determined as follows:

The value of p is assumed to be different. In probability theory, the rule of three sigma is often used: Pmax = Pcp ± 3a, which, in a normal distribution, corresponds to a marginal probability of 0.9973. The probability of exceeding the load by 0.5% corresponds to p = 2.5; for p = 1.65, a 5% probability of error is provided.

The statistical method is a reliable method for studying the loads of an operating industrial enterprise, providing a relatively correct value of the maximum load Pi (miiX) declared by the industrial enterprise during the hours of passing the maximum in the power system. In this case, it is necessary to admit a Gaussian distribution of the work of electrical consumers (consumers).

The method of probabilistic modeling of load graphs implies a direct study of the probabilistic nature of sequential random changes in the total load of groups of electrical consumers in time and is based on the theory of random processes, with the help of which autocorrelation (formula (2.10)), cross-correlation functions and other parameters are obtained. Studies of the work schedules of electrical receivers of large unit capacity, the work schedules of workshops and enterprises determine the promising nature of the method for managing power consumption modes and aligning schedules.