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Calculation of the wire cross section in a three-phase network. How to choose the right cable section

Cable power table required to correctly calculate the cable cross section, if the power of the equipment is large, and the cable cross section is small, then it will heat up, which will lead to the destruction of the insulation and loss of its properties.

To calculate the conductor resistance, you can use the conductor resistance calculator.

For the transmission and distribution of electric current, cables are the main means, they provide normal work everything related to electric shock and how good this work will be depends on right choice cable cross-sections by power. A convenient table will help you make the necessary selection:

But in order to use the table, it is necessary to calculate the total power consumption of appliances and equipment that are used in the house, apartment or other place where the cable will be laid.

An example of power calculation.

Let's say that a closed electrical wiring is being installed in the house with a BB cable. On a piece of paper, you need to rewrite the list of equipment used.

But how now find out the power? You can find it on the equipment itself, where there is usually a tag with the main characteristics recorded.

Power is measured in Watts (W, W) or Kilowatts (kW, KW). Now you need to write the data, and then add them.

The resulting number is, for example, 20,000 W, which would be 20 kW. This figure shows how much all electrical receivers together consume energy. Next, you should consider how many devices will be used simultaneously for a long period of time. Let's say it turned out 80%, in this case, the simultaneity coefficient will be equal to 0.8. We calculate the cable cross-section by power:

20 x 0.8 = 16 (kW)

To select the cross section, you will need a cable power table:

If the three-phase circuit is 380 Volts, then the table will look like this:

These calculations are not particularly difficult, but it is recommended to choose a wire or cable with the largest cross-section of the wires, because it may be that it will be necessary to connect some other device.

Additional cable power table.

Hello!

I have heard about some of the difficulties that arise when choosing equipment and connecting it (which outlet is needed for the oven, hob or washing machine). So that you can quickly and easily solve this, as good advice I suggest you take a look at the tables below.

⃰ Residual current device

Electrical connection at 220V/380V

When choosing a wire, first of all, you should pay attention to the rated voltage, which should not be less than in the network. Secondly, you should pay attention to the material of the cores. Copper wire has more flexibility than aluminum wire and can be soldered. Aluminum wires must not be laid on combustible materials.

You should also pay attention to the cross section of the wires, which should correspond to the load in amperes. You can determine the current strength in amperes by dividing the power (in watts) of all connected devices by the voltage in the network. For example, the power of all devices is 4.5 kW, the voltage is 220 V, which is 24.5 amperes. Find the required cable section from the table. It will be copper wire with a cross section of 2 mm 2 or aluminum wire with a cross section of 3 mm 2. When choosing a wire of the section you need, consider whether it will be easy to connect it to electrical devices. The insulation of the wire must comply with the laying conditions.

Wire marking.

The 1st letter characterizes the material of the conductive core:
aluminum - A, copper - the letter is omitted.

The 2nd letter stands for:
P - wire.

The 3rd letter indicates the insulation material:
B - sheath made of polyvinyl chloride plastic compound,
P - polyethylene sheath,
R - rubber shell,
H - nairite shell.
The brands of wires and cords may also contain letters characterizing other structural elements:
Oh - braid,
T - for laying in pipes,
P - flat,
F-t metal folded sheath,
G - increased flexibility,
And - increased protective properties,
P - a braid of cotton yarn impregnated with an anti-rotting compound, etc.
For example: PV - copper wire with PVC insulation.

Installation wires PV-1, PV-3, PV-4 are designed to supply power to electrical appliances and equipment, as well as for stationary installation of lighting networks. PV-1 is produced with a single-wire conductive copper conductor, PV-3, PV-4 - with twisted conductors made of copper wire. The cross section of the wires is 0.5-10 mm 2. The wires are coated with PVC insulation. They are used in AC circuits with a rated voltage of not more than 450 V with a frequency of 400 Hz and in DC circuits with voltage up to 1000 V. Working temperature limited to -50…+70 °С range.

The PVS installation wire is designed to connect electrical appliances and equipment. The number of cores can be equal to 2, 3, 4 or 5. Conductive core made of soft copper wire has a cross section of 0.75-2.5 mm 2 . It is produced with twisted conductors in PVC insulation and the same sheath.

It is used in power networks with a rated voltage not exceeding 380 V. The wire is designed for a maximum voltage of 4000 V, with a frequency of 50 Hz, applied for 1 minute. Working temperature — in the range of -40…+70 °С.

The installation wire PUNP is designed for laying stationary lighting networks. The number of cores can be 2.3 or 4. The cores have a cross section of 1.0-6.0 mm 2 . The current-carrying core is made of soft copper wire and has a plastic insulation in a PVC sheath. It is used in electrical networks with a rated voltage of not more than 250 V with a frequency of 50 Hz. The wire is designed for a maximum voltage of 1500 V with a frequency of 50 Hz for 1 minute.

Power cables of the VVG and VVGng brands are intended for transmission electrical energy in stationary AC installations. The conductors are made of soft copper wire. The number of cores can be 1-4. Cross-section of conductive wires: 1.5-35.0 mm 2. The cables are produced with an insulating sheath made of polyvinyl chloride (PVC) plastic compound. VVGng cables have low flammability. Are applied with a rated voltage no more than 660 V and a frequency of 50 Hz.

The NYM brand power cable is designed for industrial and household fixed installation indoors and on outdoors. The cable wires have a single-wire copper core with a cross section of 1.5-4.0 mm 2, insulated with PVC compound. The flame retardant outer sheath is also made of light gray PVC compound.

Here, it seems, is the main thing that it is desirable to understand when choosing equipment and wires to them))

The question of choosing the cable section for 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. End result - short circuit. The thing is that the load creates a certain current density. And if the cable cross section is small, then the current density in it will be large. Therefore, before buying, it is necessary to calculate the cable cross-section according to the load.

Of course, you should not just randomly choose a wire with 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, it is best to start with the question, how to calculate the load on the cable? And only then, according to this indicator, select the electric wire itself.

Power calculation

The easiest way is to calculate the total power that a house or apartment will consume. This calculation will be used to select the wire section from the power line pole to the introductory machine to the cottage or from the access panel to the apartment to the first junction box. In the same way, wires are calculated for loops or rooms. It is clear that the input cable will have the largest section. And the further from the first junction box, the figure will decrease.

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

After that, all the powers must be added. This is the total power of the house or apartment. Exactly the same calculation must be done along the contours. But there is one point of contention here. Some experts recommend multiplying the total by a reduction factor of 0.8, adhering to the rule that not all devices will be connected to the circuit at the same time. Others, on the contrary, suggest multiplying by a multiplying factor of 1.2, thereby creating a certain reserve for the future, in view of the fact that there is a high probability of additional household appliances. In our opinion, the second option is the best one.

Cable selection

Now, knowing the total power indicator, you can select the wiring section. The PUE has tables that make it easy to make this choice. Here are a few examples for an electric line energized with 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.

There is exactly the same table for a 380 volt electrical network.

Calculation of the current load

Exactly this exact value calculation carried out on 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 - 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 by the indicator of current strength that the cable cross section is determined according to the same tables in the PUE. Again, here are a few examples.

• Current strength 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 by power, here it is also best to multiply the current strength indicator by a multiplying factor of 1.5.

Odds

Exist certain conditions, at which the current inside the wiring can increase or decrease. For example, in open electrical wiring when the wires are laid along the walls or ceiling, the current strength will be higher than in a closed circuit. It is directly related to temperature. environment. The larger it is, the more current this cable can pass.

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

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 permissible current load is reduced by 10-30 percent. The same goes for open wiring inside heated rooms. Therefore, we can conclude: when calculating the cable cross-section, depending on the current load at elevated operating temperatures, you can choose wires of a smaller area. This, of course, is a good savings. By the way, there are also tables of reducing coefficients in the PUE.

There is one more point that concerns the length of the used electric cable. The longer the wiring, the greater the voltage loss in the sections. Losses equal to 5% are used in any calculations. 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 wiring resistance and current load. Although best option- use the PUE table, in which the dependence of the load moment and losses is established. In this case, the load moment is the product of the power consumption in kilowatts and the length of the cable itself in meters.

Let's look at an example in which an installed cable 30 mm long in a 220-volt AC mains can withstand a load of 3 kW. In this case, the load moment will be equal to 3 * 30 \u003d 90. We look at the PUE table, which shows that this moment corresponds to a loss of 3%. That is, it is less than the face value of 5%. What is allowed. As mentioned above, if the calculated losses would exceed the five percent barrier, then a larger cable would have to be purchased and installed.

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 immediately visible.

Currently, aluminum wires are rarely used in wiring. But you need to know that their resistance is 1.7 times greater 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 by phases. In this case, in the zero circuit, the current should 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 according to loads, it is necessary to take into account various coefficients (reducing and increasing). On your own, if you are an electrician at the level of an amateur or a novice master, this is not easy to do. 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.

In order to correctly lay the electrical wiring, ensure the uninterrupted operation of the entire electrical system and eliminate the risk of fire, it is necessary to calculate the loads on the cable before purchasing the cable to determine the required cross section.

There are several types of loads, and for maximum quality installation electrical systems, it is necessary to calculate the loads on the cable for all indicators. The cable section is determined by the load, power, current and voltage.

Calculation of the power section

In order to produce, it is necessary to add up all the indicators of electrical equipment operating in the apartment. Calculation of electrical loads on the cable is carried out only after this operation.

Calculation of the cable cross-section by voltage

The calculation of electrical loads on the wire necessarily includes. There are several types of electrical network - single-phase 220 volts, as well as three-phase - 380 volts. In apartments and residential premises, as a rule, a single-phase network is used, therefore, in the calculation process, this moment must be taken into account - the voltage must be indicated in the tables for calculating the cross section.

Calculation of the cable section according to the load

Table 1. Installed power (kW) for open cables

Table 2. Installed power (kW) for cables laid in a gate or pipe

Each electrical appliance installed in the house has a certain power - this indicator is indicated on the nameplates of the appliances or in the technical passport of the equipment. To implement, you need to calculate the total power. When calculating the cable cross-section according to the load, it is necessary to rewrite all electrical equipment, and you also need to think about what equipment can be added in the future. Since the installation is made for a long time, you need to take care of this issue so that a sharp increase in load does not lead to an emergency.

For example, you get the sum of the total voltage of 15,000 watts. Since the voltage in the vast majority of residential premises is 220 V, we will calculate the power supply system taking into account a single-phase load.

Next, you need to consider how many equipment can work simultaneously. As a result, you will get a significant figure: 15,000 (W) x 0.7 (simultaneity factor 70%) = 10,500 W (or 10.5 kW) - the cable must be rated for this load.

You also need to determine what material the cable cores will be made of, since different metals have different conductive properties. In residential areas, copper cable is mainly used, since its conductive properties far exceed those of aluminum.

It should be borne in mind that the cable must necessarily have three cores, since grounding is required for the power supply system in the premises. In addition, it is necessary to determine what type of installation you will use - open or hidden (under plaster or in pipes), since the calculation of the cable section also depends on this. After you have decided on the load, the material of the core and the type of installation, you can see the desired cable section in the table.

Calculation of the cable cross-section by current

First you need to calculate the electrical loads on the cable and find out the power. Let's say that the power turned out to be 4.75 kW, we decided to use a copper cable (wire) and lay it in a cable channel. is produced according to the formula I \u003d W / U, where W is the power, and U is the voltage, which is 220 V. In accordance with this formula, 4750/220 \u003d 21.6 A. Next, we look at table 3, we get 2, 5 mm.

Table 3. Permissible current loads for a cable with copper conductors laid hidden

In order for the wiring to function flawlessly, it is important to choose the right cross-section of wires and make a competent calculation of power, because other characteristics depend on these indicators. Current flows through wires just like water flows through pipes.

From the quality of the electrical work depends on the safety of the whole room. Here it is especially important to choose the right parameter such as cable cross-section. In order to calculate the cable cross-section by power, you need to know specifications all consumers of electricity that will be connected to it. You should also consider the length of the wiring and how it will be installed.

Current flows through wires just like water flows through a pipe. How in water pipe it is impossible to place a liquid of a larger volume, and it is impossible to pass more than a certain amount of current through the cable. In addition, the cost of the cable directly depends on its cross section. How larger section, the higher the price of the cable.

The water pipe is larger than necessary, costs more, and too narrow will not let through the right amount water. The same thing happens with the current, with the only difference that the choice of a cable with a cross section less than a given value is much more dangerous. Such a wire overheats all the time, the current power in it increases. Because of this, the light in the room will be arbitrarily cut down, and in the worst case, a short circuit will occur, a fire will start.

There is nothing wrong with the fact that the selected cable section will be more than necessary. On the contrary, wiring, where the power and cross section exceed desired value, will last much longer, but the cost of all electrical work will immediately increase by at least 2-3 times, because the main costs for power supply are precisely in the cost of wires.
A properly selected section will allow:

• avoid overheating of wires;
• prevent short circuit;
• save on repair costs.

Formula calculation

A sufficient cross-sectional area will make it possible to pass the maximum current through the wires without overheating. Therefore, when designing electrical wiring, first of all, they find the optimal wire cross-section depending on the power consumption. To calculate this value, the total current must be calculated. It is determined based on the power of all devices connected to the cable.

To choose the optimal wire cross-section, knowing the power, one should remember Ohm's law, as well as the rules of electrodynamics and other electromechanical formulas. So, the current strength (I) for a section of the network with a voltage of 220 Volts, namely, this voltage is used for home network, calculated by the formula:

I=(P1+P2+…+Pn)/220, where:

(P1 + P2 + ... + Pn) - the total power of each used electrical appliance.

For networks with a voltage of 380 Volts:

I=(P1+P2+…+Pn)/ √3/380.

Power ratings of some household electrical appliances

But these are vague formulas and a simplified calculation. The detailed calculations take into account the value allowable loads, which for a copper cable will be 10A / mm², and for aluminum - 8 A / mm². The load determines how much current can pass through a unit area without hindrance.

Power correction

Also, when calculating, an amendment is added in the form of a demand coefficient (Kc). This coefficient shows which devices are used in the network constantly, and which for a certain time. A special calculator and tables showing the calculation of power simplify all these calculations.

Demand coefficients of auxiliary receivers (Ks)

But what to do if the characteristics indicate 2 types of power: active and reactive? Moreover, the first of them measures in the usual kV, and the second - in kVA. In our networks, an alternating current flows, the value of which varies with time. Therefore, for all consumers there is active power, which is calculated as the average of all instantaneous alternating currents and powers. Active power devices include incandescent lamps, electric heaters. For such energy consumers, the phases of current and voltage coincide. If, however, units that accumulate energy, for example, transformers or electric motors, are involved in the electrical circuit, then they may have deviations in amplitude. Due to this phenomenon, reactive power arises.

For networks where there is reactive and active power, one more correction must be taken into account - the power factor (cosφ) or the reactive component.

Thus, the formula is obtained:

S= Kc*(P1+P2+…+Pn)/(220*cosφ*Rd), where:

• S is the cross-sectional area,
• Rd - allowable load.

In addition, possible current losses that occur during passage through the wires are considered. When using a cable with several cores, you need to multiply the loss by the number of these cores.

Important! For all these calculations, you will need not just a calculator, but also deep knowledge in the field of physics. It is impossible to make an accurate calculation immediately without theoretical knowledge.

Finding area by diameter

Sometimes even a rigorous calculation does not help, a short circuit occurs in the circuit. This is due to the fact that the declared technical characteristics often do not correspond to the real value. Therefore, in order to find out how to make a power calculation, it is important to be sure that the store will offer a suitable electrical wire according to the cross section. To do this, we use a simple formula:

S=0.785d 2 where:

• d is the core diameter;
• S is the cross-sectional area.

You can determine the exact one, you can calculate the cross section using a caliper or micrometer, which is more accurate.

If the cable consists of several thin wires, then first they look at the diameter of one of them, and then the obtained data is multiplied by their number:

Stot=n*0.785di 2 , where:

• di is the area of ​​a single wire;
• n is the number of wires;
• Stot is the total cross-sectional area.

Tables for calculations

Every time resorting to complex calculations for the calculation is not entirely correct. The industry produces wires of a certain section. If, after accurate calculations and calculations, a cable cross section of 3.2 square millimeters in size is obtained, then it will not be possible to find such a wire, because there are wires with a cross section of 2.5 mm 2, 3 or 4 mm 2.

Attention! In order to find out the cable cross-sections, a table is needed where all the data is regulated, and also drawn up in accordance with the PUE - the rules for electrical installations.

In order to determine the cable cross-section at a known load, it is necessary:

• calculate the current strength;
• round up to a larger value according to the data in the table;
• then find the nearest standard section value.

Permissible continuous current for wires and cords with rubber and PVC insulation with copper conductors

It is easy to make such a calculation. First you need to determine the total power of all electrical appliances used in the network. For this, a table will be needed, and the missing data for each electrical appliance can be taken from the product passport. The resulting amount must be multiplied by 0.8 - the demand coefficient, if electrical appliances are not used all at once, or left unchanged when permanent job. Now the resulting value must be divided by the voltage in the network and add a constant value of 5. This will be the required current indicator. Let's say the current is 20A.

Note! In residential premises, a three-wire electrical wire and closed wiring are used. This must be remembered when the calculation is done according to the table.

Next, you need a table from the PUE. We take a column where the current values ​​\u200b\u200bare given for a three-core core, and select the closest ones: 17 and 22. It is better to take the section with a margin, therefore, in this example, the desired value will be 22. As you can see, this value corresponds to a three-core cable with a cross section of 2 mm 2 .

You can additionally consider how this calculation is made for aluminum cable according to the PUE, although for safety reasons such wires cannot be used in residential buildings. Still preserved in old houses aluminum wiring but during overhaul it is recommended to replace it. In addition, aluminum electrical wire crumbles at the bends and has less conductivity at the joints. Bare parts of aluminum quickly oxidize in air, which leads to significant losses of electricity at the junctions.

Calculator

Today, specialists use not only a table, but also a special calculator to determine the cross section. This calculation greatly simplifies the calculations. The calculator is easy to find on the Internet. To calculate the size by section, you need to know the following parameters:

• variable or D.C. used;
• wire material;
• power of all used devices;
• network voltage;
• power supply system (one or three-phase);
• wiring type.

These indicators are loaded into the calculator and the required wire cross-sectional value is obtained.

Length calculation

It is important to calculate the cross-section along the length in the construction of industrial-scale networks, when sections are subjected to constant heavy load, and the cable must be pulled over considerable distances. After all, during the passage of current through the wires, power losses occur due to electrical resistance in the circuit. The power loss (dU) is calculated as follows:

dU = I*p*L/S, where:

• I - current strength;
• p - resistivity (copper - 0.0175, aluminum - 0.0281);
• L is the cable length;
• S is the cross-sectional area already calculated by us.

According to the specifications, the maximum voltage drop along the length of the wire should not exceed 5 percent. Otherwise, you should choose a wire with a larger cross section.

Peculiarities

There are certain standards according to which the cable is calculated according to the cross section. If you are not sure which electrical wire is needed, then you can use these rules: electrical appliances in the apartment are divided into a lighting group and the rest; for powerful electrical appliances, for example, washing machines or electric ovens use a connection from separate wires; the standard wire section for the lighting group in the apartment is 1.5 mm 2, and for the rest of the wires - 2.5 mm 2. Such standards are used because the rated input power cannot be greater.

Three-phase current is required when high power production value appliances are used. Therefore, to determine the cable cross-section at enterprises, it is necessary to accurately calculate all additional factors, and it is also necessary to take into account power losses and voltage fluctuations. For electrical work in an apartment or a private house, such complex calculations are not carried out.

For the installation of acoustic equipment, wires with minimal resistance are used. This is necessary in order to remove distortions as much as possible and improve the quality. transmitted signal. Therefore, for acoustic systems 2x2.5 or 2x1.5 cables are better suited with a length of at least 3 meters, and the subwoofer is connected with the shortest cable 2.5-4 mm 2.

Examples

Consider general scheme to select the cable section in the apartment:

• First you need to determine the places where sockets and lighting fixtures will be located;
• Next, you need to determine which devices will be used at each output;
• Now you can draw up a general connection diagram and calculate the cable length, adding at least 2 cm to the wire connections;
• Based on the data obtained, we consider the size of the cable section according to the formulas given above.

I \u003d 2400W / 220V \u003d 10.91A, round up and get 11A.

As we already know, for exact definition cross-sectional area, different coefficients are used, but almost all of these data refer to a network with a voltage of 380V. To increase the safety margin, add another 5A to our current value:

For apartments, three-core cables are used. The table will show a current value close to our 16A, it will be 19A. We get that to install one washing machine a wire with a cross section of at least 2 mm 2 is required.

General theory

To determine the optimal cable cross-section for domestic needs, in the general case, the following rules are used:

• sockets require wires with a cross section of 2.5 mm²;
• for lighting - 1.5 mm²;
• for devices with increased power - 4-6 mm².

If there are doubts about the calculation of the cross section, then the PUE table is used. To determine the exact data on the cable cross-section, all factors affecting the passage of current through the circuit are taken into account. These include:

• type of wire insulation;
• the length of each section;
• laying method;
• temperature regime;
• humidity;
• permissible value of overheating;
• power difference of current receivers in one group.

All of these indicators lead to an increase in industrial scale energy efficiency, and avoid overheating.

Section selection. Video

In this video, the master shares his experience in choosing the cable section and the nominal value of the machine. He points to possible mistakes and gives good advice newcomers.

If after reading the article there are still some doubts, then the table or calculator described above will help you find the exact cross section of the wire in terms of power.