How to choose a wire size. Cable cross-section. How to choose it correctly. How to calculate the cross section of a stranded wire

We bring to your attention an interesting and informative video on how to correctly calculate the cable cross-section in terms of power and length:

The choice of cross-sectional wires is the main element of a power supply project of any scale, from a room to large networks. This will determine the current that can be taken into the load and power. Choosing the right wires also ensures electrical and fire safety., and provides an economical budget for your project.

This article will tell you how to calculate the wire cross-section by power consumption yourself. You need to know this not only when in the house, but also when carrying out work in cars, for example. If the cross-section of the wire turns out to be insufficient, then it will begin to heat up very much, which will lead to a significant loss of the level of safety. Considering all the recommendations that will be set out below, you can independently calculate the parameters of the wires for installing power supply in the house. But if you are not confident in your abilities, it is better to contact specialists in this field. Moreover, it should be noted that the calculation of the wire cross-section for the power consumption (12V and 220V) is carried out in the same way.

Calculating the length of the wiring

For any type of electronic system, the most important condition for stable and trouble-free operation is a competent calculation of the cross-sections of all wires for current and power. The first step is to calculate the maximum length of the entire wiring. There are several ways to do this:

1. Measurement of the distance from panels to sockets, switches according to the installation diagram. Moreover, this can be done with a ruler on a pre-prepared wiring plan - it is enough to multiply the obtained lengths by the scale.
2. And the second, more accurate way is to arm yourself with a ruler and walk through all the rooms, taking measurements. Moreover, it must be borne in mind that the wires must somehow be connected, so there must always be a margin - at least one or two centimeters from each edge of the wiring.

Now you can proceed to the next step.

Calculation of the load on the wiring

To calculate the total load, you need to add up all the minimum power of consumers around the house. Let's say you are calculating for the kitchen, it has lamps, a microwave oven, an electric kettle and a stove, a dishwasher, and so on. All powers must be summed up (look at the back covers for the power consumption, but you will have to calculate the current yourself using this parameter). Then multiply by a correction factor of 0.75. It is also called the coefficient of simultaneity. Its essence is clear from the name itself. This figure, which will be obtained as a result of the calculations, you will need in the future to calculate the parameters of the wires. Please note that the entire power supply system must be safe, reliable and durable. These are the main requirements that must be taken into account when calculating the wire cross-section for the power consumption of 12V and 220V.

Consumption current of electrical installations

Now how to calculate the current consumption of an electrical device. You can do this in your head, or you can use a calculator. See the instructions for the device, what is the value of the power consumption of it. Of course, an alternating current with a voltage of 220 volts flows in the household electrical network. Therefore, using a simple formula (power consumption divided by supply voltage), you can calculate the current. For example, an electric kettle has a power of 1000 W. So, if we divide 1000 by 220, we get a value approximately equal to 4.55 amperes. It is made very simply in terms of power consumption. How to do this is described in the article. In the operating mode, the kettle consumes 4.55 amperes from the network (for protection, it is necessary to install a circuit breaker of a larger rating). Note, however, that this is not always the exact value. For example, if there is a motor in the design of an electrical appliance, the obtained value can be increased by about 25% - the current consumption of the motor in the starting mode is much higher than during idling.

But you can use a set of rules and standards. There is such a document as the Electrical Installation Rules, it is he who regulates all the rules for conducting wiring not only in private estates, but also in factories, factories, etc. According to these rules, the wiring standard is the ability to withstand a load of 25 amperes for a long time. Therefore, in apartments, all electrical wiring must be carried out only using a copper wire, its cross section is at least 5 sq. mm. Each core must have a cross section of over 2.5 sq. mm. The conductor diameter should be 1.8 mm.

In order for all electrical wiring to work as safely as possible, a circuit breaker is being installed at the input. It will protect the apartment from short circuits. Also, recently, most of the owners of living space are installing protective shutdown devices, which instantly act on a change in resistance in the circuit. In other words, if you accidentally touch them while energized, they will instantly de-energize and you will not get hit. it is necessary to calculate by current, and it is necessary to choose with a margin, so that there is always an opportunity to install any electrical appliance in the house. Competent calculation of the wire cross-section by power consumption (you will learn how to make the correct choice of wires from this material) is a guarantee that the power supply will function correctly and efficiently.

Materials for making wires

As a rule, the installation of electrical wiring in a private house or apartment is done using three-core wires. Moreover, each core has separate insulation, they all have different colors - brown, blue, yellow-green (standard). A core is exactly that part of the wire through which the current flows. It can be either single-wire or multi-wire. Some brands of wire use a cotton braid over the wires. Materials for the manufacture of wire cores:

1. Steel.
2. Copper.
3. Aluminum.

Sometimes you can find combined, for example, stranded copper wire with several steel conductors. But these were used to implement field telephone communications - a signal was transmitted over copper, and steel was used for the most part to carry out attachment to poles. Therefore, this article will not talk about such wires. For apartments and private houses, copper wire is ideal. It is durable, reliable, performance is much higher than that of cheap aluminum. Of course, the price of a copper wire bites, but it's worth mentioning that its service life (guaranteed) is 50 years.

Wire brands

For wiring, it is best to use two brands of wires - VVGng and VVG. The first one has the ending "-ng", which means that the insulation does not burn. It is used to carry out electrical wiring inside structures and buildings, as well as in the ground, in the open air. Works stably in the temperature range -50 ... +50. The guaranteed service life is at least 30 years. The cable can be with two, three or four conductors, the cross-section of each is in the range of 1.5 ... 35 sq. mm. Please also pay attention to the fact that it is necessary to calculate the cross-section of the wire according to the power consumption and length (in the case of an overhead long line).

Carefully make sure that there is no letter "A" before the name of the wire (for example, AVVG). This suggests that the inner cores are made of aluminum. There are also foreign analogues - NYM cable, which has a round shape, meets the standards adopted in Germany (VDE0250). Copper conductors, insulation is not subject to combustion. The round shape of the wire is much more convenient if it is necessary to carry out installation through the wall. But for conducting wiring indoors, it turns out to be more convenient flat domestic.

Aluminum wires

They are lightweight, and most importantly, low cost. Therefore, they are useful for those cases when you need to lay long lines through the air. If all work is done correctly and correctly, you will get the perfect air line, since aluminum has one huge advantage - it is not subject to oxidation (unlike copper). But often, aluminum wiring was also used in houses (usually in old ones). The wire used to be easier to get, and it cost a penny. It should be noted that the calculation of the cross-section of the wire according to the power consumption (the features of this process are known to every electrician) is the main stage in creating a project for the power supply of the house. But you need to pay attention to one feature - the cross-section of the aluminum wire must be larger than the copper one in order to withstand the same load.

Table for calculating the cross-section by power

It should also be mentioned that the maximum permissible current load is much less than for copper ones. The table below will help you calculate the cross-section of the cores

Wire cross-section depending on the type of wiring

There are two types of electrical wiring in houses - open and closed. As you can imagine, this nuance must also be taken into account when making calculations. Concealed wiring is installed inside ceilings, as well as in grooves and channels, in pipes, etc. Enclosed wiring has higher requirements, since it has a lower cooling capacity. And any wire with prolonged exposure to a large load heats up very much. Therefore, in the case when calculating the cross-section of the wire according to the power consumption, be sure to take into account the effect on heating. It is also necessary to take into account the following parameters:

1. Long-term current load.
2. Loss of tension.

As the length of the wire increases, the voltage decreases. Therefore, in order to reduce voltage losses, it is necessary to increase the cross-section of the conductors of the wire. If we are talking about a small house or even a room, then the value of losses is extremely low, they can be neglected. But if a long line is being calculated, this cannot be avoided. After all, the calculation of the cross-section of the wire according to the power consumption (the effect of the length is very large) depends on such a parameter as the length of the line.

Wire power calculation

So, you need to know the following characteristics:

1. The material of which the cable conductors are made.
2. Maximum power consumption.
3. Supply voltage.

Please note that any current will increase the temperature and generate some heat. Moreover, the amount of heat is proportional to the total power that is dissipated on a piece of wiring. If you choose the wrong cross-section, then excessive heating will occur, and the result can be disastrous - ignition of the electrical wiring and fire. Therefore, it is worthwhile to carry out an accurate calculation of the wire cross-section according to the power consumption. The risk factors are too large and there are many.

Optimal parameters

Optimal cross-sections:

1. For wiring sockets - 2.5 sq. mm.
2. Lighting group - 1.5 sq. mm.
3. High power electrical appliances (hot plates) - 4-6 sq. mm.

In doing so, please note that copper wires can withstand the following loads:

1. Wire 1.5 sq. mm - up to 4.1 kW (current load - 19 amperes).
2. 2.5 sq. mm - up to 5.9 kW (for current - up to 27 amperes).
3. 4-6 sq. mm - more than 8-10 kW.

Therefore, as the load increases, you will always have a fairly large reserve.

Conclusion

Now you know how to calculate the wire cross-section based on the power consumption (you now know the definition of important characteristics and other small factors). Based on all of the above data, you can independently, without resorting to the help of professionals, draw up a correct power supply plan for your house or apartment.

Electric cable cross-section.

Electric cable cross-section- this is one of the fundamental components of correct electrical wiring in an apartment. This means the comfortable operation of electrical appliances and equipment, as well as the safety of consumers, that is, all of us. The purpose of this article is to clarify, for an apartment electrical network, based on the power of the electrical appliances used. And also tell which wire is needed for a particular section of home wiring.

Before starting a conversation on the main topic of the article, let me remind you of some terms.
● Lived- this, in the general sense, is a separate conductor (copper or aluminum), which can be either a solid conductor or consist of several twisted together in a bundle or, clothed in a common braid, separate wires.
● Wire Is a product that consists of one single-wire or multi-wire core, dressed in a light protective sheath.
● Installation wire- This is a wire that is used for electrical wiring intended for lighting or power networks. It can be one -, two - or three-core.
Is a wire with a conductor cross-section of up to 1.5 mm2. Cords are used to power light mobile (portable) electrical appliances and equipment. It consists of a multi-wire core, due to which it has increased ductility.
● Electric cable Is a product consisting of several insulated wires, on top of which there is from one to several protective sheaths.

To select a cable (wire) of the required cross-section for intra-apartment wiring, you need to use the table above, and to determine the current load on the cable, you can use the formula used earlier:
I races. = P/U No.

where:

I races. - calculated long-term permissible current load;
P- power of the connected equipment;
U No. - mains voltage;

Let's say you need to choose a cable for connecting an electric boiler with a power of 3 kW. Substituting the initial values ​​into the formula, we get:

Iras. = 3000 W / 220 V = 13.63 A,

rounding this value up, we get 14 A.

For a more accurate calculation of the current load, there are various coefficients, depending on the environmental conditions and the way the cable is laid. There is also a coefficient of intermittent treatment. But they all, to a greater extent, relate to a three-phase 380 V network, so they are not required for our calculations. But to increase the safety margin of the conductor, we apply the average value of 5 A. And we get:

14 A + 5 A = 19 A

In the column of table 1. 3. 4. "Three-core wires" we are looking for the value of 19 A. If it is not there, you need to choose the largest one closest to it. This is the value of 21 A. A cable with a conductor cross-section of 2.5 mm² can withstand such a continuous current load. We conclude that to connect an electric boiler (or other electrical equipment) with (consuming) a power of 3 kW, a three-core copper cable with a conductor cross-section of 2.5 mm² is required.

In the event that you need to connect an outlet (or block of outlets) from which several electrical appliances will be powered, you can use the above formula, in which the "P" value will be equal to the sum of the power of devices or equipment simultaneously connected to the outlet (block of outlets).
Since all electrical appliances with a power exceeding 2 kW are recommended to be connected to the power supply through a separate supply (a separate branch from the in-house electrical panel), it can be concluded that a copper (preferably) cable with a conductor cross-section of 2.5 mm² is required for the socket group of apartment wiring. Due to the fact that the lighting devices do not have high power, the wire for the wiring supplying them with electricity must have a conductor cross-section of at least 1.5 mm².

This is about copper wiring. But what about the wiring with aluminum conductors. There is a simple way to calculate the cross-section of an aluminum conductor of a wire.

Due to the fact that the electrical conductivity of aluminum is 65.9% of the electrical conductivity of copper, then when devices with the same power consumption are connected to them (wires or cables), the cross-section of the aluminum conductor must be larger than the copper one. For instance. Referring to the calculations made above in the text, it was determined that the cross-section of the copper core in the wire for connecting a 3 kW boiler should be 2.5 mm². When using a cable with an aluminum conductor, according to table. 1.3.4, the cross-section of the conductor must be chosen by a factor higher, ie - 4 mm².
Referring to the PUE Ch. 1. p. 3. tab. 1. 3. 5 can confirm this assumption.

Tab. 1.5.

When choosing a cable for electrical wiring, it is necessary to use not only the principles of economy, but also take into account the mechanical strength of the wire, as well as be guided by the Rules for the Arrangement of Electrical Installations. Which state that for wiring inside residential premises, it is necessary to use a cable with a conductor cross-section of at least 1.5 mm 2 (PUE Ch. 7; Section 7.1; Table 7.1.1). Thus, if, according to your calculations, a cable with a cross section of less than 1.5 mm 2 is sufficient for wiring, then, guided by the Rules and Safety Regulations, choose the recommended wiring.

All the necessary norms and rules, as well as tables can be viewed, and, if necessary, downloaded, in a file "Rules for the Arrangement of Electrical Installations" .

There is one more, simplest, method for selecting the cross-section of the wire for electrical wiring. It is probably used by all electricians. Its essence is that the cross-section is calculated on the basis of the current strength of 6 - 10 A per 1 mm 2 of the cross-sectional area for wires with copper conductors and 4 - 6 A per 1 mm 2 for an aluminum conductor. Thus, we can say that the operation of electrical wiring with a copper core at a current of 6 A per 1 mm 2 of the section is the most comfortable and safe. Whereas with a current density of 10 A per 1 mm 2 - it can only be used in a short-term mode. The same can be said for aluminum conductors.

Let's try using this method to select a wire for connecting equipment with a power of 3 kW, as in the example discussed above. After making calculations, a value of 14 A was obtained (3000 W / 220 V = 14 A). To select a cable with a copper conductor, take the smallest (for a greater margin of safety) value (from the "plug" 6 - 10 A per 1 mm 2) - 6 A. From this it can be seen that for a current strength of 14 A, a wire with a conductor cross section is needed

14 A / 6 A = 2.3 mm 2 ≈ 2.5 mm 2.

Which confirms our previous calculations.

As additional information, I can add: if you do not have a conductor of the required cross-section, then it can be replaced with several wires with a smaller cross-section, connected in parallel. So, for example, you need a cable with a cross section of 4 mm². You have at your disposal wires of the required length, but with a cross-section of 1 mm², 1.5 mm² and 2.5 mm². It is enough to take wires whose total cross-section is not less than the required one (one wire 1.5 mm² and one wire 2.5 mm² or two wires 1.5 mm² and one wire 1 mm²) and connect them in parallel (lay alongside each other and , "Twist" the ends). An example of this is stranded wire for extension cords. As you may have noticed, each of its conductors consists of many thin wires. And connected in parallel, in one "bundle", they give a conductor (core) of the required section. This achieves its elasticity while maintaining the required throughput. But this is only suitable for wiring to which low-power electrical devices are connected or if it is subjected to short-term peak loads. For other types of wiring, a wire (cable) is recommended, in which the cores consist of a solid (one, solid or stranded) conductor.

Having learned how to determine the cross-section of a wire that has a core of one (solid) wire, the question remains: "How to calculate the cross-section of a wire, the core of which consists of many wires?"

Stranded conductor cross-section.

Following the logic, you need to find out the cross-section of one separate wire and multiply by the number of them in the core. This is completely correct, but the hairs can be too fine and therefore it is not always possible to measure them. You can, of course, measure the diameter of the entire "bundle" of wires and, using the formula indicated in the photo "Calculation of the cross-section of a wire core relative to its diameter", determine the cross-section of the entire core. This is, in principle, sufficient for very rough calculations. But here you need to take into account the fact that the wires that make up the vein are round in cross section and, therefore, there is space between them in the twist. To make a more accurate calculation, you need to multiply the value obtained after calculating by the formula from the photo by 0.91. It is this factor that excludes the area of ​​gaps between the hairs in the multi-strand vein. For example, there is a wire with a stranded core with a diameter of 2.5 mm. Substitute the values ​​into the formula and get:

S = 3.14 × D ² / 4 = 3.14 × 2.5 ² / 4 = 4.90625 mm² ≈ 4.9 mm².
4.9 × 0.91 = 4.459 ≈ 4.5 mm².

Thus, the cross-section of a stranded conductor with a diameter of 2.5 mm is 4.5 mm². (this is just an example, so no need to bind it to real dimensions).

That, perhaps, is all that I wanted to say about how to calculate the cable cross-section... Armed with the information received, you can independently select an electrical wire or cable that will meet safety requirements.

Remember: improperly sized electrical wires can cause a fire!

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Content:

Before connecting the load to the mains, it is important to make sure that the cores of the supply cable are of sufficient thickness. If the permissible power is significantly exceeded, the insulation and even the core itself may be destroyed due to its overheating.

Before calculating the cable cross-section by power, you should calculate the sum of the power 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 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 be significantly reduced. However, when choosing a conductor cross-section, it is important to focus on a large value.

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

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

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

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

The calculation of the 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;
• To and- coefficient of simultaneity, 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 equal to 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 according to the calculation formula, you can safely proceed to the selection of the supply cable. More precisely, its cross-sectional area. There are special tables that provide data that allow you to compare the current value, power consumption and cable cross-section.

The data is very different for conductors made of different metals. Today, for apartment wiring, only rigid copper cable, aluminum is practically not used. Although in many old houses all lines are laid with the use of aluminum.

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

Calculation of the cross-section of the wire in the apartment - Table

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

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

To calculate the cross-section of the supply 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 cable cross-sectional area will be selected;
• U- phase voltage, 220 (V);
• Cos φ- phase shift angle;
• P- an indicator of the total power of all electrical appliances.

Cos φ in this formula is very important. 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 capacities are added, 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 large load. The required section is selected according to the typical table.

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

The calculation of the wire cross-section for power in a three-phase network is performed using such a value 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 by the linear voltage. This value is 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 according to special tables, they are available in the PUE. For aluminum and copper conductors, the data differ 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 amperage.

The temperature in a certain area may increase not only due to an incorrectly selected section, but also due to poor contact.For example, at the place where the wires are twisted. Quite often this happens as a result of direct contact between aluminum and copper cables. The surface of metals is oxidized, covered with an oxide film, which significantly worsens the contact. This is where the cable heats up.

Comfort and safety in the house depends on the correct choice of wiring cross-section. Overloading the conductor will overheat and the insulation may melt, resulting in a fire or short circuit. But it is unprofitable to take a cross-section larger than necessary, since the price of the cable increases.

In general, it is calculated depending on the number of consumers, for which they first determine the total power used by the apartment, and then multiply the result by 0.75. In the PUE, a table of loads along the cable section is used. From it, you can easily determine the diameter of the cores, which depends on the material and the passing current. Typically, copper conductors are used.

The cross-section of the cable core must exactly correspond to the calculated one - in the direction of increasing the standard size range. It is most dangerous when it is underestimated. Then the conductor constantly overheats, and the insulation quickly fails. And if you set the appropriate one, then its frequent operation will occur.

If the wire cross-section is overestimated, it will cost more. Although a certain margin is necessary, since in the future, as a rule, you have to connect new equipment. It is advisable to apply a safety factor of the order of 1.5.

Calculation of the total power

The total power consumed by the apartment falls on the main input, which enters the switchboard, and after it branches out on the line:

• lighting;
• outlet groups;
• separate powerful electrical appliances.

Therefore, the largest cross-section of the power cable is at the input. On the outlet lines, it decreases, depending on the load. First of all, the total power of all loads is determined. This is not difficult, since it is indicated on the bodies of all household appliances and in the passports for them.

All capacities add up. Calculations are made in the same way for each circuit. Experts suggest multiplying the amount by 0.75. This is due to the fact that all devices are not connected to the network at the same time. Others suggest choosing a larger section. This creates a reserve for the subsequent commissioning of additional electrical devices that may be purchased in the future. It should be noted that this option for calculating the cable is more reliable.

How to determine the wire size?

In all calculations, the cable cross-section appears. It is easier to determine its diameter by using the formulas:

• S =π D² / 4;
• D= √ (4 ×S/π).

Where π = 3.14.

S = N × D² / 1.27.

Stranded wires are used where flexibility is required. Cheaper solid conductors are used for fixed installations.

How to choose a cable for power?

In order to select the wiring, the table of loads for the cable cross section is used:

• If the open-type line is under a voltage of 220 V, and the total power is 4 kW, a copper conductor with a cross section of 1.5 mm² is taken. This size is usually used for lighting wiring.
• With a power of 6 kW, conductors of a larger cross-section are required - 2.5 mm². The wire is used for sockets to which household appliances are connected.
• Power of 10 kW requires 6 mm² wiring. Usually it is intended for the kitchen, where the electric stove is connected. Such a load is supplied via a separate line.

Which cables are better?

Electricians are well aware of the German NUM cable for office and residential premises. In Russia, brands of cables are produced that are lower in characteristics, although they may have the same name. They can be distinguished by compound leakage in the space between the veins or by its absence.

The wire is produced in monolithic and multi-wire. Each core, as well as the entire twist from the outside, is insulated with PVC, and the filler between them is non-combustible:

• So, the NUM cable is used indoors, since the insulation on the street is destroyed by the sun's rays.
• And as an internal and widely used cable of the VVG brand. It is cheap and reliable enough. It is not recommended to use it for laying in the ground.
• VVG wire is made flat and round. No filler is applied between the veins.
• made with an outer shell that does not support combustion. The conductors are made round up to a cross-section of 16 mm², and above them - sector-shaped.
• The brands of PVA and SHVVP cables are made with multi-wire and are used mainly for connecting household appliances. It is often used as electrical wiring in the home. It is not recommended to use stranded conductors outdoors due to corrosion. In addition, the insulation will crack when bent at low temperatures.
• Armored and moisture-resistant cables AVBShv and VBShv are laid underground on the street. The armor is made of two steel strips, which increases the reliability of the cable and makes it resistant to mechanical stress.

Determination of current load

A more accurate result is given by calculating the cable cross-section for power and current, where geometric parameters are related to electrical ones.

For home wiring, not only the active load must be taken into account, but also the reactive one. The current strength is determined by the formula:

I = P / (U ∙ cosφ).

The reactive load is created by fluorescent lamps and motors of electrical appliances (refrigerator, vacuum cleaner, power tool, etc.).

Current example

Let's find out what to do if it is necessary to determine the cross-section of a copper cable for connecting household appliances with a total power of 25 kW and three-phase machines for 10 kW. This connection is made with a five-core cable laid in the ground. The house is powered from

Taking into account the reactive component, the power of household appliances and equipment will be:

• P household. = 25 / 0.7 = 35.7 kW;
• P rev. = 10 / 0.7 = 14.3 kW.

The currents at the input are determined:

• I everyday life. = 35.7 × 1000/220 = 162 A;
• I rev. = 14.3 × 1000/380 = 38 A.

If you distribute single-phase loads evenly over three phases, one will have a current:

I f = 162/3 = 54 A.

I f = 54 + 38 = 92 A.

All equipment will not work at the same time. Taking into account the margin, there is a current for each phase:

I f = 92 × 0.75 × 1.5 = 103.5 A.

In a five-core cable, only phase conductors are taken into account. For a cable laid in the ground, it is possible to determine for a current of 103.5 A the cross-section of the conductors is 16 mm² (table of loads by the cross-section of the cable).

A refined calculation of the current strength saves money, since a smaller cross-section is required. With a coarser calculation of the cable in terms of power, the conductor cross-section will be 25 mm², which will cost more.

Voltage drop across the cable

The conductors have a resistance that must be taken into account. This is especially important for a long cable length or with a small cross-section. PES standards have been established, according to which the voltage drop on the cable should not exceed 5%. The calculation is done as follows.

1. The resistance of the conductor is determined: R = 2 × (ρ × L) / S.
2. The voltage drop is found: U pad. = I × R. In relation to the linear percentage, it will be: U% = (U pad / U lin.) × 100.

The following notation is accepted in the formulas:

• ρ - resistivity, Ohm × mm² / m;
• S - cross-sectional area, mm².

Factor 2 indicates that current flows through two cores.

An example of calculating a cable for a voltage drop

• Wire resistance is: R = 2 (0.0175 × 20) / 2.5 = 0.28 Ohm.
• Conductor current: I = 7000/220 = 31.8 A.
• Carry voltage drop: U pad. = 31.8 x 0.28 = 8.9 V.
• Voltage drop percentage: U% = (8.9 / 220) × 100 = 4.1 %.

The carrier is suitable for the welding machine according to the requirements of the rules for the operation of electrical installations, since the percentage of voltage drop on it is within the normal range. However, its value on the supply wire remains large, which can adversely affect the welding process. Here it is necessary to check the lower permissible limit of the supply voltage for the welding machine.

Conclusion

To reliably protect the wiring from overheating with prolonged excess of the rated current, cable cross-sections are calculated based on long-term permissible currents. The calculation is simplified if the table of cross-section loads is used. A more accurate result is obtained if the calculation is based on the maximum current load. And for stable and long-term operation, a circuit breaker is installed in the wiring circuit.