Which cable is better to use for wiring in an apartment: brands, sections, choice. How to select and correctly calculate the cable cross-section Wire cross-section 3 phases

Content:

Before connecting the load to the network, it is important to ensure that the supply cable cores are thick enough. If the permissible power is significantly exceeded, the insulation and even the core itself may be destroyed due to overheating.

Before calculating the cable cross-section by power, you should calculate the sum of the powers of the connected electrical appliances. In most modern apartments, the main consumers are:

• Refrigerator 300 W
• Washing machine 2650 W
• Computer 550 W
• Lighting 500 W
• Electric kettle 1150 W
• Microwave oven 700 W
• TV 160 W
• Water heater 1950 W
• Vacuum cleaner 600 W
• Iron 1750 W
• Total 10310 W = 10.3 kW

In total, most modern apartments consume approximately 10 kW. Depending on the time of day, this parameter can decrease significantly. However, when choosing a conductor cross-section, it is important to focus on a larger value.

You need to know the following: the calculation of cable cross-section for single-phase and three-phase networks is different. But in both cases, three parameters should be taken into account first:

• Current strength(I),
• Voltage(U),
• Power consumption (P).

There are also several other variables, their meaning varies from case to case.

Calculation of wire cross-section for a single-phase network

Calculation of wire cross-section by power is carried out using the following formula:

I = (P × K u) / (U × cos(φ))

Where,

• I- current strength;
• P- power consumption of all electrical appliances in total;
• K and- simultaneity coefficient, usually the standard value of 0.75 is taken for calculations;
• U- phase voltage, it is 220 (V), but can range from 210 to 240 (V);
• Cos(φ)- for household single-phase appliances this value is unchanged and equals 1.

If you need to quickly calculate the current, you can omit the value of cos (φ) and even K and. The resulting value will differ downward (by 15%) if a formula of this type is applied:

I=P/U

Having found the current using the calculation formula, you can safely proceed to selecting the power cable. More precisely, its cross-sectional area. There are special tables that present data that allows you to compare the current value, power consumption and cable cross-section.

The data varies greatly for conductors made from different metals. Today, for residential electrical wiring, only hard copper cable, aluminum is practically not used. Although in many old houses all lines are laid using aluminum.

The cross-section of the copper cable is selected according to the following parameters:

Calculation of wire cross-section in an apartment - Table

It often happens that the calculation results in a current that is between the two values ​​​​presented in the table. In this case, the nearest larger value must be used. If, as a result of calculations, the current value in a single-core wire is 25 (A), it is necessary to select a cross-section of 2.5 mm 2 or more.

Calculation of cable cross-section for a three-phase network

To calculate the cross-section of the power cable used in a three-phase network, you must use the following formula:

I = P / (√3 × U × cos(φ))

Where,

• I- current strength by which the cross-sectional area of ​​the cable will be selected;
• U- phase voltage, 220 (V);
• Cosφ- phase shift angle;
• P- an indicator of the total power of all electrical appliances.

Cosφ is very important in this formula. Since it directly affects the current strength. It is different for different equipment; most often this parameter can be found in the technical accompanying documentation, or it is indicated on the case.

The total power of consumers is found very simply: all powers are added up, the resulting value is used for calculations.

A distinctive feature of the choice of cable cross-sectional area for use in a three-phase network is that a thinner core can withstand a larger load. The required section is selected according to the standard table.

Selection of cable cross-section for a three-phase network - Table

Calculation of the wire cross-section for power in a three-phase network is carried out using a value such as √3 . This value is necessary to simplify the appearance of the formula.

U linear = √3 × U phase

Thus, if necessary, you can replace the product of the root and phase voltage with linear voltage. This value is equal to 380 (V) (U linear = 380 V).

When choosing a cable cross-section, both for a three-phase network and for a single-phase network, it is necessary to take into account permissible continuous current . This parameter indicates the current strength (measured in amperes) that the conductor can withstand for an unlimited amount of time. It is determined using special tables, they are available in the PUE. For aluminum and copper conductors, the data differs significantly.

Permissible current duration - Table

When the value specified in the table is exceeded, the conductor begins to heat up. The heating temperature is inversely proportional to the current strength.

The temperature in a certain area can increase not only due to an incorrectly selected cross-section, but also due to poor contact.For example, in the place where wires are twisted. Quite often this happens as a result of direct contact between aluminum cables and copper cables. The surface of metals oxidizes and becomes covered with an oxide film, which significantly impairs contact. This is where the cable gets hot.

The article discusses the main criteria for choosing a cable cross-section and gives examples of calculations.

In markets you can often see handwritten signs indicating which one the buyer needs to purchase depending on the expected load current. Do not believe these signs, as they are misleading. The cable cross-section is selected not only by the operating current, but also by several other parameters.

First of all, it is necessary to take into account that when using a cable at the limit of its capabilities, the cable cores heat up by several tens of degrees. The current values ​​shown in Figure 1 assume heating of the cable cores to 65 degrees at an ambient temperature of 25 degrees. If several cables are laid in one pipe or tray, then due to their mutual heating (each cable heats all other cables), the maximum permissible current is reduced by 10 - 30 percent.

Also, the maximum possible current decreases at elevated ambient temperatures. Therefore, in a group network (a network from panels to lamps, plug sockets and other electrical receivers), as a rule, cables are used with currents not exceeding 0.6 - 0.7 of the values ​​​​shown in Figure 1.

Rice. 1. Permissible long-term current of cables with copper conductors

Based on this, the widespread use of circuit breakers with a rated current of 25A to protect socket networks laid with cables with copper conductors with a cross-section of 2.5 mm2 is dangerous. Tables of reduction coefficients depending on temperature and the number of cables in one tray can be found in the Electrical Installation Rules (PUE).

Additional restrictions arise when the cable is longer. In this case, voltage losses in the cable can reach unacceptable values. As a rule, when calculating cables, the maximum loss in the line is no more than 5%. Losses are not difficult to calculate if you know the resistance value of the cable cores and the calculated load current. But usually, to calculate losses, they use tables of the dependence of losses on the load moment. The load moment is calculated as the product of the cable length in meters and the power in kilowatts.

Data for calculating losses at a single-phase voltage of 220 V are shown in Table 1. For example, for a cable with copper conductors with a cross-section of 2.5 mm2, with a cable length of 30 meters and a load power of 3 kW, the load moment is 30x3 = 90, and the losses will be 3%. If the calculated loss value exceeds 5%, then it is necessary to select a cable with a larger cross-section.

Table 1. Load moment, kW x m, for copper conductors in a two-wire line for a voltage of 220 V at a given conductor cross-section

Using Table 2, you can determine the losses in a three-phase line. Comparing tables 1 and 2, you can see that in a three-phase line with copper conductors with a cross section of 2.5 mm2, losses of 3% correspond to six times the load torque.

A triple increase in the load torque occurs due to the distribution of load power over three phases, and a double increase due to the fact that in a three-phase network with a symmetrical load (the same currents in the phase conductors) the current in the neutral conductor is zero. With an asymmetrical load, cable losses increase, which must be taken into account when choosing the cable cross-section.

Table 2. Load moment, kW x m, for copper conductors in a three-phase four-wire line with zero for a voltage of 380/220 V at a given conductor cross-section (to enlarge the table, click on the figure)

Cable losses have a significant impact when using low-voltage lamps, such as halogen lamps. This is understandable: if 3 Volts drop on the phase and neutral conductors, then at a voltage of 220 V we most likely will not notice this, and at a voltage of 12 V, the voltage on the lamp will drop by half to 6 V. That is why transformers for powering halogen lamps need to be maximally bring it closer to the lamps. For example, with a cable length of 4.5 meters with a cross-section of 2.5 mm2 and a load of 0.1 kW (two 50 W lamps), the load torque is 0.45, which corresponds to a loss of 5% (Table 3).

Table 3. Load moment, kW x m, for copper conductors in a two-wire line for a voltage of 12 V at a given conductor cross-section

The tables above do not take into account the increase in resistance of conductors due to heating due to current flowing through them. Therefore, if the cable is used at currents of 0.5 or more of the maximum permissible current of the cable of a given cross-section, then a correction must be introduced. In the simplest case, if you expect losses of no more than 5%, then calculate the cross section based on losses of 4%. Also, losses may increase if there are a large number of cable core connections.

Cables with aluminum conductors have a resistance 1.7 times greater than cables with copper conductors, and accordingly their losses are 1.7 times greater.

The second limiting factor for long cable lengths is exceeding the permissible resistance value of the phase-zero circuit. To protect cables from overloads and short circuits, as a rule, circuit breakers with a combined release are used. Such switches have thermal and electromagnetic releases.

The electromagnetic release provides instantaneous (tenths and even hundredths of a second) shutdown of the emergency section of the network in the event of a short circuit. For example, a circuit breaker designated C25 has a 25 A thermal release and a 250 A electromagnetic release. Automatic circuit breakers of group “C” have a multiplicity of the breaking current of the electromagnetic release to the thermal one from 5 to 10. But the maximum value is taken.

The total resistance of the phase-zero circuit includes: the resistance of the step-down transformer of the transformer substation, the resistance of the cable from the substation to the input switchgear (SDU) of the building, the resistance of the cable laid from the ASU to the switchgear (RU) and the resistance of the cable of the group line itself, the cross-section of which is required define.

If the line has a large number of connections of cable cores, for example, a group line of a large number of lamps connected by a cable, then the resistance of the contact connections must also be taken into account. Very accurate calculations take into account the arc resistance at the fault point.

The total resistance of the phase-zero circuit for four-core cables is given in Table 4. The table takes into account the resistance of both the phase and neutral conductors. Resistance values ​​are given at a cable core temperature of 65 degrees. The table is also valid for two-wire lines.

Table 4. Circuit impedance phase - zero for 4-core cables, Ohm/km at core temperature 65 o C

In urban transformer substations, as a rule, transformers with a capacity of 630 kV or more are installed. A and more, having an output resistance Rtp less than 0.1 Ohm. In rural areas, transformers of 160 - 250 kV can be used. And having an output resistance of about 0.15 Ohm, and even transformers for 40 - 100 kV. A, having an output impedance of 0.65 - 0.25 Ohm.

Supply network cables from city transformer substations to house ASUs are usually used with aluminum conductors with a phase conductor cross-section of at least 70 - 120 mm2. If the length of these lines is less than 200 meters, the resistance of the phase-neutral circuit of the supply cable (Rpc) can be taken equal to 0.3 Ohm. For a more accurate calculation, you need to know the length and cross-section of the cable, or measure this resistance. One of the devices for such measurements (Vector device) is shown in Fig. 2.

Rice. 2. Device for measuring the resistance of the phase-zero circuit "Vector"

The line resistance must be such that in the event of a short circuit, the current in the circuit is guaranteed to exceed the operating current of the electromagnetic release. Accordingly, for the C25 circuit breaker, the short circuit current in the line must exceed the value of 1.15x10x25=287 A, here 1.15 is the safety factor. Therefore, the resistance of the phase-zero circuit for the C25 circuit breaker should be no more than 220V/287A=0.76 Ohm. Accordingly, for the C16 circuit breaker the circuit resistance should not exceed 220V/1.15x160A=1.19 Ohms and for the C10 circuit breaker - no more than 220V/1.15x100=1.91 Ohms.

Thus, for an urban apartment building, taking Rtp = 0.1 Ohm; Rpk=0.3 Ohm when using a cable with copper conductors with a cross-section of 2.5 mm2 in the socket network, protected by a C16 circuit breaker, the cable resistance Rgr (phase and neutral conductors) should not exceed Rgr=1.19 Ohm - Rtp - Rpk = 1.19 - 0.1 - 0.3 = 0.79 Ohm. From Table 4 we find its length - 0.79/17.46 = 0.045 km, or 45 meters. For most apartments this length is sufficient.

When using a C25 circuit breaker to protect a cable with a cross-section of 2.5 mm2, the circuit resistance must be less than 0.76 - 0.4 = 0.36 Ohm, which corresponds to a maximum cable length of 0.36/17.46 = 0.02 km, or 20 meters.

When using a C10 circuit breaker to protect a group lighting line made with a cable with copper conductors with a cross-section of 1.5 mm2, we obtain the maximum permissible cable resistance of 1.91 - 0.4 = 1.51 Ohm, which corresponds to a maximum cable length of 1.51/29, 1 = 0.052 km, or 52 meters. If such a line is protected by a C16 circuit breaker, then the maximum line length will be 0.79/29.1 = 0.027 km, or 27 meters.

First of all, an electrician must be able to correctly calculate the cross-section of the cable to be laid, since if the cross-section is chosen incorrectly, the electrical network will not last long. In everyday life, this knowledge will be useful to everyone who makes repairs, changes wiring, purchases new electrical equipment, and at the same time thinks about the reliability of the electrical network and their own safety.

A precisely selected wiring cross-section will ensure the following:

1. Will provide long-term, uninterrupted operation of your equipment.
2. Will exclude the possibility of fires.
3. Will deliver from the need to replace the wiring.
4. Will allow avoid additional costs for purchasing a product with a large cross-section.

How to choose cable cross-section based on power?

For correct calculation you need:

1. Calculate the number of household appliances in the room (it is advisable to take into account the appliances that you plan to purchase in the future), their total power.
2. All devices divide into 2 groups: those that will operate continuously, and those that will be used rarely, then sum up their powers and determine the approximate operating time of the wiring at full load.
3. Add to the resulting value of 5% - “margin of safety”.
4. Final value must be divided by the network operating coefficient, the result will be the required wire power indicator, after which, using a special current flow table, we determine the cross-section of the cores for the resulting value.
5. Select product made of aluminum, copper, or aluminum-copper, the cross-section of which is suitable for your power value, taking into account the network voltage (220V for a household power supply, 380V for an industrial power supply).

You need to know that the materials for conductive products are aluminum, copper and aluminum-copper, and each of them has advantages and disadvantages.

Features of Aluminum Cable:

1. Lighter and cheaper than copper ones.
2. Possess 1.73 times less conductive than copper.
3. Susceptible to oxidation, after which they lose conductivity.
4. After long-term use stop keeping their shape.
5. At home soldering is not possible.

Features of copper cable:

1. Possess high elasticity and mechanical strength.
2. Are different small amount of electrical resistance.
3. Great amenable soldering and tinning.
4. They're standing much more than aluminum ones.

An aluminum-copper cable is an aluminum conductor clad on the outside with copper (the amount of copper is 10-30%) using a thermomechanical method.

Features of aluminum-copper cable:

1. Conductivity is better than that of an aluminum product, but worse than that of a copper product.
2. With time, the characteristics of this product do not deteriorate, unlike aluminum wires.
3. Much lower cost, compared to copper.
4. Aluminum copper, unlike copper and aluminum, is not of interest to thieves, since non-ferrous metal collectors do not accept aluminum-copper due to the difficulty of separating the two metals.

How to find out the power?

Power is measured in watts, kilowatts (W, kW, w, kWt). On every modern electrical equipment (domestic and industrial), the power is indicated on the tag along with other characteristics of the product. If this parameter is missing for some reason, we recommend using Table 1.

Table 1 – average power values ​​of household electrical appliances:

Calculation examples

Permissible current for cables and wires:

Example 1. Calculation for a single-phase 220V network.

Most often, apartment buildings are powered from a single-phase network with a voltage of 220V. Let’s say the total power of household electrical appliances, taking into account an additional 5% - the “safety margin”, is 7.6 kW (average electrical load in an apartment) - now you can start choosing the cable material.

To do this, we find the value of the nearest suitable cable cross-section in the corresponding table of the publication “Rules for Electrical Installations” (Table 2), in our case it will be:

• 4 mm sq. for copper (designed for continuous loads of 8.3 kW);
• 6 mm. sq. for aluminum (designed for continuous loads of 7.9 kW);
• 6 mm. sq. for aluminum copper (see section on advice from professionals);

Example 2. Calculation for a three-phase network with a voltage of 380V.

In this case, the connection is made to one of 3 phases and a common “zero” - this rule applies exclusively to single-phase devices, of which the vast majority are found in a modern home.

Do not forget about three-phase home appliances - pumps, welding machines, motors, etc., when connecting the load must be evenly distributed between 3 phases (7.6 kW / 3 phases = 2.6 kW per phase).

Therefore, when connecting a load to a 3-phase network, the value of the total power is multiplied by a special coefficient, due to which the cross-sectional value decreases. For example, when connecting a load of 7.6 kW, for a 1-phase network you will need a copper wire - 4 mm sq., for a 3-phase network - 1.5 mm sq.

Note that it is much easier to carry out calculations for home conditions than for industrial facilities, since in the latter case the following are added to the indicators that must be taken into account when calculating:

• seasonal loads;
• simultaneity factor;
• demand factor;

Online calculators

To facilitate calculations and accurately select the required cross-section size, we have selected working online calculators that will quickly and accurately perform calculations for you to determine the required cross-section:

Consequences of incorrect selection of section

Selection of cross-section by power– an extremely responsible process. For example, if the cross-section of the home electrical network cable is designed for a power of up to 6 kW, at a load of 7.5 kW (only connecting just one household appliance, such as a microwave oven or electric kettle, to the home electrical network), the cable will overheat.

When overheating reaches a critical value, it will first begin to melt and then the cable insulation will ignite:

1. It is the incorrectly selected wire cross-section is the most common cause of domestic fires.
2. Also, if the insulation fails, a short circuit may occur, as a result of which all household appliances may fail.
3. Anyway, you will have to spend a lot of money to restore and replace, at least the wiring of the house.
4. At an industrial enterprise Incorrectly selected cables can lead to much more tragic consequences.

That is why this issue must be taken very seriously.

1. Aluminum wiring it is best to replace with aluminum-copper - the same diameter (this rule also applies to Table 2). If you replace a copper cable with an aluminum-copper one, the cross-section of the new cable should correspond to the copper one as 5 to 6.
2. With three-phase power supply It is best to divide the devices into groups so that the load on each phase is approximately the same.
3. At the time of buying, you need to pay attention to the markings, since sellers can cheat - passing off aluminum-copper cables as copper, thereby causing significant damage to your wallet. To prevent this from happening, you must:
   
   • Pay attention to the markings (domestic aluminum-copper products are marked with the letter combination AM).
   • If there is no marking, or the cable was manufactured abroad (not taking into account the CIS countries), it is enough to scrape off the top layer - the copper core is homogeneous, unlike aluminum copper.
4. Last time Laying cables using corrugated pipes (corrugations) is becoming increasingly common. Below are the advantages of corrugation, as well as operating features:
   
   • The reduced flammability of the corrugations minimizes the likelihood of a fire when the wiring is shorted.
   • The corrugation protects the wiring from mechanical stress and damage.
   • The longer it is, the more difficult it is to thread the wire into the corrugation; therefore, its end is first attached to a thin wire, which is much easier to thread through the corrugation.
5. For household electrical wiring, it is recommended to use stranded wires as they are more flexible.

It is important to select the wire cross-section for those who independently conduct electricity into the house.

The correctness of the cross-section determines the uninterrupted supply of current, the absence of risks of failures, the stability of the devices, as well as the safety of the residents of the house, which is important for human life.

If you take the wrong wire, that is, choose the wrong cross-section, the following consequences may occur:

• the cable will overheat;
• high temperature will cause the insulation to melt;
• there is a risk of occurrence;
• possible fire;
• Devices powered by the network may burn out during operation.

How to choose wires?

Long-term permissible current load is the main thing you should pay attention to when choosing a wire in a store.

Each type of wire must be supplied for sale with a passport, where all this data is indicated.

What is the continuous load capacity? This is the maximum total power of devices that consume current.

If the limit is exceeded, operation of the wiring is unacceptable.

So, it is logical to assume that to calculate the required cross-section we take into account the total power of appliances, devices and other items that consume energy (even the most common phone chargers need to be taken into account).

If we carry out, it is better to install the wiring with a reserve of maximum capacity, since the repairs are being done, probably, not for one year, but over time, the devices are made more powerful, and you will probably want to buy something additional.

Speaking about the effectiveness of copper or aluminum in the production of cables for electrical wiring, it is worth confidently saying that aluminum has a greater set of advantages compared to copper. In its favor we can note:

• resistant to mechanical damage;
• does not break when bent;
• durable;
• flexible;
• no oxidation;
• If you compare copper and aluminum in operation, then two different wires with the same cross-section will be able to transmit different amounts of energy. Of course, copper wins the battle for the right to a more reliable component.

If the most powerful devices are planned to be connected to different sockets, then the wire cross-section can be 2.5 mm at the load that we showed in the example.

If, with the same indicator, high-power devices are connected to one outlet (or even to one room), then 4-6 mm is the ideal solution.

However, for a room where too strong appliances will not operate, a cross-section of 1.5 mm is sufficient for the entire room.

You also have to figure out... The diagram will help with this:

As a rule, the main thing in an apartment with two rooms that is powered by electricity is:

• boiler. The most powerful device, but not everyone has it. If you have a gas water heater installed, but in the future you plan to switch to a boiler, it is better to immediately take into account that it consumes about 2000 W;
• iron. Although we turn it on infrequently, this device consumes as much as 1700 W, which will significantly affect the amount of energy flow when it is turned on;
• Electric kettle. Consumes 1200 W. A kitchen attribute in almost every apartment;
• washing machine. Perhaps one of the leaders in energy intake. Consumes 2500 W;
• microwave oven - power varies, but averages 700 W;
• vacuum cleaner. Approximately 650 W;
• computer. 500 W;
• light. 500 W;
• fridge. 300 W;
• modern TV. 140 W.

Important: there are energy-saving devices, and there are ordinary ones. Radiators that look the same can vary significantly in the level of energy consumed, but on the box or in the passport for the device, this data must be indicated based on how much time the device consumes per hour.

Calculation of wire cross-section for a network phase

For single phase

• sum up the power of all devices that will be used in the apartment;
• we multiply the resulting amount by the simultaneity coefficient (this indicator is calculated based on the average data on the simultaneous activation of a number of devices, and is 0.75);
• divide the resulting number by the network voltage (in our case 220).

Calculation of wire cross-section for a three-phase network 380 W

We calculate in the following sequence:

In general, the formula will look like this:

Wire cross-section table

To find out the permissible load for a specific wire and calculate the wire cross-section, it is enough to compare the obtained data with ready-made tables.

True, a lot depends on what wire is used.

For copper wires:

For aluminum:

Important: if the cable consists of 4 or 5 cores, the result obtained is multiplied by a factor of 0.93.

Here you can learn everything from real professionals in their field.

So, the known power of each electrical appliance in the house, the known number of lighting fixtures and lighting points allow us to calculate the total power consumed. This is not an exact sum, since most values ​​for the powers of various devices are averages. Therefore, you should immediately add 5% of its value to this figure.

Average power readings for common electrical appliances

And many believe that this is enough to select almost standard copper cable options:

• cross section 0.5 mm2 for wires for lighting spotlights;
• cross section 1.5 mm2 for lighting wires for chandeliers;
• cross-section 2.5 mm2 for all sockets.

At the level of household use of electricity, such a scheme looks quite acceptable. Until the refrigerator and electric kettle decided to turn on in the kitchen at the same time, while you were watching TV there. The same unpleasant surprise overtakes you when you plug in a coffee maker, washing machine and microwave into one outlet.

Thermal calculation using correction factors

For several lines in one cable channel, the tabulated values ​​of the maximum current should be multiplied by the appropriate coefficient:

• 0.68 — for the number of conductors from 2 to 5 pcs.
• 0.63 — for conductors from 7 to 9 pcs.
• 0.6 — for conductors from 10 to 12 pcs.

The coefficient refers specifically to the wires (cores), and not to the number of passing lines. When calculating the number of laid wires, the neutral working wire or grounding wire is not taken into account. According to PUE and GOST 16442-80, they do not affect the heating of wires during the passage of normal currents.

Summarizing the above, it turns out that in order to correctly and accurately select the wire cross-section, you need to know:

1. The sum of all maximum powers of electrical appliances.
2. Network characteristics: number of phases and voltage.
3. Characteristics of cable material.
4. Tabular data and coefficients.

At the same time, power is not the main indicator for an individual cable line or the entire internal power supply system. When selecting a cross-section, be sure to calculate the maximum load current, and then check it with the rated current of the home circuit breaker.