Calculation of industrial electrical networks is carried out on the basis of several technical indicators. But when it comes to household lines, one parameter is usually taken as a basis - the power of household appliances and lighting. Therefore, calculating cable power is the only correct method to correctly assemble the electrical wiring of a house. Of course, you will also have to take into account the length of each cable, because modern private houses are sometimes entire palaces where kilometers of cable are laid. But the calculation is still based on power.
Let's start with the fact that the power characteristics of household appliances can be found on the devices themselves or in the accompanying documentation (passport, instructions, etc.). Please note that some devices indicate two values: average power value and maximum. For the calculation, it is the second one that is needed.
It should be noted that some household appliances operate in different modes. For example, a washing machine can consume only a few tens of watts in the rinse mode, or hundreds of watts in the washing mode, and several kilowatts in the water heating and boiling mode. That is, at a certain moment the machine consumes different power. No one can determine at what exact moment washing with boiling will be carried out, therefore, in order to make the correct selection of the cable, it is necessary to take as a basis the maximum power indicator.
By the way, you will also have to calculate the electrical wiring for the air conditioner in the same way. After all, this device will consume only 50-60 watts in simple ventilation mode, and 1.0-1.0 kW in air conditioning mode.
Selection of conductor cross-section by power and length
The length of the conductor determines the voltage supplied to the end point.
A situation may arise when the voltage at the point of consumption is insufficient to operate electrical appliances. In household electrical communications, these losses are neglected and a cable is used ten to fifteen centimeters longer than necessary. This surplus is spent on switching. When connected to a distribution board, the reserve is increased, taking into account the need to connect circuit breakers.
Cable laid in a closed waySource kadetbrand.ru
When laying long lines, one should take into account the inevitable voltage drop. Everyone has their own resistance, which is influenced by three main factors:
- Length measured in meters. As this indicator increases, losses increase.
- Cross section, measured in square millimeters. If this parameter is increased, the voltage drop decreases.
- The resistance of the conductor material, the value of which is taken from the reference data. They show the reference resistance of a wire with a cross-section of one millimeter and a length of one meter.
The product of resistance and current numerically represents the voltage drop. This value should not exceed five percent. If it exceeds this figure, then it is necessary to take a conductor with a larger cross-section.
More information on how to calculate cable cross-section in the video:
Section calculation using formulas
The selection algorithm is as follows:
The conductor area along the length and maximum power is calculated using the formula:
Source infopedia.su
Where:
P – power;
U – voltage;
cosф – coefficient.
For household electrical networks, the coefficient value is equal to one. For industrial communications, it is calculated as the ratio of active power to total power.
- The PUE table contains the current cross section.
- Wiring resistance is calculated:
Source textarchive.ru
Where:
ρ – resistance;
l – length;
S – cross-sectional area.
At the same time, do not forget that the current moves in both directions and in fact the resistance is equal to:
Source textarchive.ru
The voltage drop corresponds to the relationship:
Source moypatent.ru
In percentage terms, the voltage drop is as follows:
Source tex.stackovernet.com
If the result exceeds five percent, then the nearest cross section with a larger value is searched in the directory.
Such calculations are rarely performed by generic electricity consumers. There are specialized specialists and a lot of reference material for this. Moreover, there are many online calculators on the Internet, with the help of which all calculations can be made in a couple of clicks.
Visually calculate the cable cross-section using the formulas in the video:
Possible correction of the core cross-section for line resistance
Any conductor has its own resistance - we talked about this at the very beginning of the article, when we cited the resistivity values of materials, copper and aluminum.
Both of these metals have very decent conductivity, and in short sections the line’s own resistance does not have any significant effect on the overall parameters of the circuit. But if you plan to lay a long line, or, for example, a long-length carrying extension cord is being manufactured to work at a considerable distance from the house, then it is advisable to calculate your own resistance and compare the voltage drop it causes with the supply voltage. If the voltage drop is more than 5% of the nominal voltage in the circuit, the operating rules for electrical installations require the use of a cable with conductors of a larger cross-section.
For example, a carrying case for a welding inverter is made. If the resistance of the cable itself is excessive, the wires will overheat greatly under load, and the voltage may not be enough for the device to operate correctly.
The cable's own resistance can be calculated using the formula:
Rk = 2 × ρ × L/S
Rk—own resistance of the cable (line), Ohm;
2 - the cable length doubles, since the entire current path is taken into account, that is, “there and back”;
ρ is the resistivity of the cable core material;
L—cable length, m;
S is the cross-sectional area of the core, mm².
It is assumed that we already know what current we will have to deal with when connecting the load - this has already been discussed more than once in this article.
Knowing the current strength, it is easy to calculate the voltage drop using Ohm's law and then compare it with the nominal value.
Ur = Rk × I
ΔU (%) = (Ur / Unom) × 100
If the test result is more than 5%, then the cross-section of the cable cores should be increased by one step.
Another online calculator will help you quickly carry out such a check. It seems that he will not require additional explanations.
Long distance line voltage drop test calculator
As already mentioned, with a value of up to 5% you can not change anything. If it turns out more, the cross-section of the cable core is increased, also with subsequent checking.
* * * * * * *
So, the main issues regarding the required cable cross-section depending on the planned load on it were considered. The reader is free to choose any of the proposed calculation methods, whichever he likes best.
Let's end the article with a video on the same topic.
Section and installation method
Another factor that influences the choice of conductor cross-section is the method of laying the lines. There are two of them:
- open;
- closed.
In the first method, the wiring is placed in a special box or corrugated pipe and is located on the wall surface. The second option involves embedding the cable inside the decoration or main body of the walls.
Here the thermal conductivity of the environment plays a major role. In the ground, heat is removed from the cable better than in air. Therefore, with the closed method, wires with a smaller cross-section are taken than with the open method. The table below shows how the laying method affects the cross-section of the conductor.
Laying method and conductor cross-section Source m-strana.ru
Power cable for AIR electric motors
The correct choice of the cross-section of the power supply cable is the cornerstone of the successful operation of any industrial enterprise that uses electrical machines, of which the lion's share is made up of AIR electric motors.
“Weak” electrical wiring can lead to overload and emergency engine shutdowns. In addition, this is unsafe and is fraught with accidents at work - overheating of the wire, melting of the insulation, short circuit and fire!
On the other hand, an excessively thick cable section is an unreasonable waste of the budget, but in economics calculation and planning are important.
Order an electric motor by phone
Pivot table
There are tables that allow you to determine the required cross-section using several parameters at once - current, power, conductor material, and so on. They are more convenient to use and one of them is located below. It indicates the cross-section of the wire in terms of current and power, and also takes into account the installation method.
Wire cross-section by current and power - table for copper and aluminum conductorsSource tvz2.ru
Perhaps the article came out a little boring and full of technical terms. However, the information contained in it should not be neglected. Since the reliability and safety of the functioning of the home electrical network depends on how correctly the wiring was chosen.
Selecting a circuit breaker based on short-circuit current
The selection of a circuit breaker for protection against short circuit (short circuit) is carried out based on the calculated value of the short circuit current at the end of the line. The calculation is relatively complicated, the value depends on the power of the transformer substation, the cross-section of the conductor and the length of the conductor, etc.
From the experience of carrying out calculations and designing electrical networks, the most influential parameter is the length of the line, in our case the length of the cable from the panel to the outlet or chandelier.
Because in apartments and private houses this length is minimal, then such calculations are usually neglected and circuit breakers with characteristic “C” are selected; you can, of course, use “B”, but only for lighting inside an apartment or house, because such low-power lamps do not cause a high inrush current, and already in the network for kitchen appliances with electric motors, the use of machines with characteristic B is not recommended, because the machine may be triggered when the refrigerator or blender is turned on due to a jump in the starting current.
We measure the cross-section of wires depending on the diameter
The cross-section of a cable or other types of conductors is determined in several ways. The main thing is to take care of preliminary measurements. To do this, it is recommended to remove the top layer of insulation.
About measuring instruments, description of the process
Calipers and micrometers are the main tools that help in measurements. Most often, preference is given to devices of the mechanical group. But it is also permissible to choose electronic analogues. Their main difference is digital special screens.
Electronic caliper
Vernier calipers are among the tools found in every household. Therefore, it is often chosen when measuring the diameter of wires and cables. This also applies to cases where the network continues to operate - for example, inside an outlet or panel device.
The following formula helps determine the cross-section based on the diameter:
S = (3.14/4)*D2.
D is a letter indicating the diameter of the wire.
If there is more than one core in the structure, then measurements are carried out for each of the component elements separately. After this, the results obtained are added to each other.
Then everything can be calculated using the following formula:
Stotal= S1+ S2+…
Stotal is an indication of the total cross-sectional area.
S1, S2 and so on are the cross sections defined for each of the cores.
It is recommended to measure the parameter at least three times to ensure accurate results. The conductor is turned in different directions every time. The result is an average value that is as close as possible to reality.
A regular ruler can be used if a caliper or micrometer is not at hand. The following manipulations are expected to be performed:
- Complete cleaning of the insulation layer near the core.
- Winding the turns around the pencil, as tightly as possible to each other. The minimum number of such components is 15-17 pieces.
- The winding is measured by its overall length.
- The final value is divided by the number of turns.
The accuracy of the measurement is questionable if the turns do not fit evenly on the pencil, leaving gaps of certain sizes. To ensure higher accuracy, it is recommended to measure the product from different sides. It is difficult to wind thick strands onto regular pencils. It is still better to use calipers.
The cross-sectional area of the wire is calculated using the formula described earlier. This is done after completing the main measurements. You can rely on special tables.
The micrometer is recommended to be used in cases where the composition contains ultra-thin conductors. Otherwise, there is a high probability of mechanical damage.
Table of correspondence of wire diameters and their cross-sectional area
What brand of cable is best to use to connect the motor?
When selecting cable brands for powering asynchronous electric motors, take into account the conditions in which they will operate. To provide power to mobile electrical installations that often move between construction sites or other work, preference should be given to flexible brands of cable products. There are a number of mechanisms driven by asynchronous electric motors, which make constant movements during operation. For example, beam cranes, electric hoists and other lifting equipment. The electric drive of such mechanisms is connected to the electrical network only with flexible copper cables with rubber insulation. The current cores of flexible cables are assembled with bundles of thin copper wire, so they can withstand repeated bending.
The most suitable and popular brands for such engines are KG, KPGS, KPG1U
For installation in the ground, it is advisable to choose an armored cable (for example, VBShv or AVBShv) to exclude the possibility of accidental damage in hard ground. It is allowed to lay PVC-sheathed cables in trenches with a sand cushion. These types of cables can also be laid in trays.
Three main ways to determine wire diameter
There are several methods, but each of them is based on determining the diameters of the core with subsequent calculations of the final results.
Method one. Using instruments. Today there are a number of instruments that help measure the diameter of a wire or wire strand. This is a micrometer and caliper, which come in both mechanical and electronic (see below).
This option is primarily suitable for professional electricians who are constantly installing electrical wiring. The most accurate results can be obtained using a caliper. This technique has the advantage that it is possible to measure the wire diameter even on a section of a working line, for example, in a socket.
After you have measured the diameter of the wire, you need to make calculations using the following formula:
It must be remembered that the number “Pi” is 3.14, so if we divide the number “Pi” by 4, we can simplify the formula and reduce the calculation to multiplying 0.785 by the diameter squared.
Method two. We use a ruler. If you decide not to spend money on a device, which is logical in this situation, then you can use a simple proven method for measuring the cross-section of a wire or wire?. You will need a simple pencil, ruler and wire. Strip the core of insulation, wind it tightly onto a pencil, and then use a ruler to measure the total length of the winding (as shown in the figure).
Then divide the length of the wound wire by the number of cores. The resulting value will be the diameter of the wire cross-section.
But the following must be taken into account:
- the more cores you wind on a pencil, the more accurate the result will be; the number of turns should be at least 15;
- press the coils tightly against each other so that there is no free space between them, this will significantly reduce the error;
- take measurements several times (change the measuring side, the direction of the ruler, etc.). Several results obtained will again help you avoid a large error.
Please note the disadvantages of this measurement method:
- You can only measure the cross-section of thin wires, since it will be difficult for you to wind a thick wire around a pencil.
- To begin with, you will need to purchase a small piece of the product before making the main purchase.
The formula discussed above is suitable for all measurements.
Method three. We use the table. In order not to carry out calculations using the formula, you can use a special table that indicates the diameter of the wire? (in millimeters) and conductor cross-section (in square millimeters). Ready-made tables will give you more accurate results and will significantly save your time, which you will not have to spend on calculations.
| Conductor diameter, mm | Conductor cross-section, mm² |
| 0.8 | 0.5 |
| 1 | 0.75 |
| 1.1 | 1 |
| 1.2 | 1.2 |
| 1.4 | 1.5 |
| 1.6 | 2 |
| 1.8 | 2.5 |
| 2 | 3 |
| 2.3 | 4 |
| 2.5 | 5 |
| 2.8 | 6 |
| 3.2 | 8 |
| 3.6 | 10 |
| 4.5 | 16 |
How to find out what cross-section is needed for different voltages (3, 10, 6, 5, 2, 100, 1, and 8 kW)
First of all, you need to understand how a wire differs from a cable. There are several conductor strands in the cable braid, and the wire is a single strand in a casing. In turn, the conductor can be single-wire or multi-wire.
Single core models
Several factors influence the determination of wire cross-section:
Conductive materials include copper and aluminum. The advantages of aluminum are lower cost, but it is brittle, the melting point is 657 ° C and an oxide film is formed on the surface, which resists the transmission of current. After several bends, the wire breaks, so the service life of an electrical network with an aluminum core is lower than with a copper one.
Dependence of current, power and core cross-section
When choosing a cable, you must be guided by several criteria:
- the strength of the electric current that the cable will pass;
- power consumed by energy sources;
- current load exerted on the cable.
Power
The most important parameter during electrical installation work (in particular, cable laying) is throughput. The maximum power of electricity transmitted through it depends on the cross-section of the conductor.
Therefore, it is extremely important to know the total power of the energy consumption sources that will be connected to the wire
Typically, manufacturers of household appliances, appliances and other electrical products indicate on the label and in the documentation accompanying them the maximum and average power consumption. For example, a washing machine can consume electricity ranging from tens of W/h during rinsing mode to 2.7 kW/h when heating water.
The average power of all electrical appliances and lighting devices in an apartment rarely exceeds 7500 W for a single-phase network. Accordingly, the cable cross-sections in the electrical wiring must be selected to this value.
On a note. It is recommended to round the cross-section towards higher power due to a possible increase in power consumption in the future. Typically, the next largest cross-sectional area from the calculated value is taken.
So, for a total power of 7.5 kW, it is necessary to use a copper cable with a core cross-section of 4 mm2, which is capable of transmitting about 8.3 kW. The cross-section of the conductor with an aluminum core in this case must be at least 6 mm2, passing a current power of 7.9 kW.
Marking labels for electrical appliances and household appliances indicating their rated power
Electric current
Often the power of electrical equipment and equipment may not be known to the owner due to the absence of this characteristic in the documentation or completely lost documents and labels. There is only one way out in such a situation - to calculate using the formula yourself.
P = U*I, where:
- P – power, measured in watts (W);
- I – electric current strength, measured in amperes (A);
- U is the applied electrical voltage, measured in volts (V).
When the strength of the electric current is unknown, it can be measured with control and measuring instruments: an ammeter, a multimeter, and a clamp meter.
Measuring current with clamp meters
After determining the power consumption and electric current, you can use the table below to find out the required cable cross-section.
Load
Calculation of the cross-section of cable products based on current load must be carried out to further protect them from overheating. When too much electric current passes through conductors for their cross-section, destruction and melting of the insulating layer can occur.
The maximum permissible long-term current load is the quantitative value of the electric current that can pass the cable for a long time without overheating. To determine this indicator, it is initially necessary to sum up the powers of all energy consumers. After this, calculate the load using the formulas:
- I = P∑*Ki/U (single-phase network),
- I = P∑*Kи/(√3*U) (three-phase network), where:
- P∑ – total power of energy consumers;
- Ki – coefficient equal to 0.75;
- U – electrical voltage in the network.
| Section of cable and wire products | Electrical voltage 220 V | Electrical voltage 380 V | ||
| Current strength, A | power, kWt | Current strength, A | power, kWt | |
| 2,5 | 27 | 5,9 | 25 | 16,5 |
| 4 | 38 | 8,3 | 30 | 19,8 |
| 6 | 50 | 11 | 40 | 26,4 |
| 10 | 70 | 15,4 | 50 | 33 |
| 16 | 90 | 19,8 | 75 | 49,5 |
| 25 | 115 | 25,3 | 90 | 59,4 |
| 35 | 140 | 30,8 | 115 | 75,9 |
| 50 | 175 | 38,5 | 145 | 95,7 |
| 70 | 215 | 47,3 | 180 | 118,8 |
| 95 | 260 | 57,2 | 220 | 145,2 |
| 120 | 300 | 66 | 260 | 171,6 |
Determining the cross-section of a cable product is a particularly important process in which miscalculations are unacceptable. You need to take into account all factors, parameters and rules, trusting only your calculations. The measurements taken must coincide with the tables described above - if they do not contain specific values, they can be found in the tables of many electrical engineering reference books.
Calculation of permissible current for heating cores
If a conductor of a suitable cross-section is selected, this will eliminate voltage drop and overheating of the line. Thus, the cross-section determines how optimal and economical the operating mode of the electrical network will be. It would seem that you can simply take and install a cable with a huge cross-section. But the cost of copper conductors is proportional to their cross-section, and the difference when installing electrical wiring in just one room can amount to several thousand rubles
Therefore, it is important to be able to correctly calculate the cable cross-section: on the one hand, you guarantee the safety of the network operation, on the other hand, you will not spend extra money on purchasing an overly “thick” conductor
To select the wire cross-section, two important criteria must be taken into account - permissible heating and voltage loss. Having received two values for the cross-sectional area of the conductor using different formulas, choose the larger value, rounding it to the standard. Overhead power lines are especially sensitive to voltage loss.
At the same time, for underground lines and cables placed in corrugated pipes, it is important to take into account the permissible heating. Thus, the cross-section should be determined depending on the type of wiring
Id - permissible load on the cable (heating current). This value corresponds to the current flowing through the conductor over a long period of time. In the process of this, a set, long-term permissible temperature (Td) appears. The calculated current strength (Ip) must correspond to the permissible current (Id), and to determine it you need to use the formula:
Iр=(1000*Pн*kз)/√(3*Un*hд*cos j),
Where:
- Pn — rated power, kW;
- Kz - load factor (0.85-0.9);
- Un — rated voltage of the equipment;
- hd - equipment efficiency;
- cos j - equipment power factor (0.85-0.92).
Even if we take into account the same current values, the thermal output will be different depending on the ambient temperature. The lower the temperature, the more efficient the heat transfer
Temperatures differ depending on the region and time of year, so in the PUE you can find tables for specific values. If the temperature differs significantly from the calculated one, correction factors will have to be used. The base temperature indoors or outdoors is 25 degrees Celsius. If the cable is laid underground, the temperature changes by 15 degrees Celsius. However, it is underground that it remains constant.
Wire diameter measurement
According to the standard, the diameter of the wire must correspond to the declared parameters, which are described in the marking. But the actual size may differ from the declared one by 10-15 percent. This is especially true for cables that are manufactured by small companies, but large manufacturers can also have problems. Before purchasing an electrical wire for transmitting large currents, it is recommended to measure the diameter of the conductor. For this purpose, various methods can be used, differing in error. Before performing measurements, it is necessary to remove insulation from the cable cores.
Measurements can be made directly in the store if the seller allows you to remove the insulation from a small section of the wire. Otherwise, you will have to purchase a small piece of cable and take measurements on it.
Micrometer
Maximum accuracy can be achieved using micrometers that have a mechanical and electronic circuit. On the shaft of the instrument there is a scale with a division value of 0.5 mm, and on the circle of the drum there are 50 marks with a division value of 0.01 mm. The characteristics are the same for all micrometer models.
When working with a mechanical device, the following sequence of actions should be followed:
- By rotating the drum, the gap between the screw and the heel is set close to the measured size.
- Use a ratchet to bring the screw closer to the surface of the part being measured. The eyeliner is performed by rotating the hand without effort until the ratchet operates.
- Calculate the transverse diameter of the part according to the readings on the scales located on the stem and drum. The diameter of the product is equal to the sum of the values on the rod and the drum.
Mechanical micrometer measurement
Working with an electronic micrometer does not require rotation of the units; it displays the diameter value on the liquid crystal screen. It is recommended to check the settings before using the device as electronic devices measure in millimeters and inches.
Vernier caliper
The device has reduced accuracy compared to a micrometer, which is quite sufficient for measuring a conductor. Vernier calipers are equipped with a flat scale (vernier), a circular dial or a digital display on a liquid crystal display.
To measure the transverse diameter, you must:
- Clamp the conductor to be measured between the jaws of the caliper.
- Calculate the value on the scale or view it on the display.
An example of calculating size on a vernier
Ruler
Measuring with a ruler gives a rough result. To perform measurements, it is recommended to use tool rulers, which have greater accuracy. Using wooden and plastic school products will give a very approximate diameter value.
To measure with a ruler you need:
- Remove insulation from a piece of wire up to 100 mm long.
- Wind the resulting piece tightly around a cylindrical object. The turns must be complete, that is, the beginning and end of the wire in the winding are directed in one direction.
- Measure the length of the resulting winding and divide by the number of turns.
Measuring the diameter with a ruler by the number of turns
In the example above there are 11 turns of wire which are about 7.5mm in length. By dividing the length by the number of turns, you can determine the approximate value of the diameter, which in this case is 0.68 mm.
On the websites of stores selling electrical wires, there are online calculators that allow you to calculate the cross-section based on the number of turns and the length of the resulting spiral.
How to choose the right one
When choosing the appropriate wire cross-section for installing the line, you must adhere to the following rules:
For your information! A cable with a cross-section of more than 4 mm² is considered to be of increased power. For each electrical appliance, a separate circuit breaker is installed in the panel and an autonomous line is laid.
Along the entire length of the line, joints and branches are eliminated to maintain its reliability. For indoor wiring, hard wire is used. Connections and branches are made in clamp boxes.
Ground loop
Section according to GOST or TU
A large assortment of electrical products helps to quickly solve problems associated with electrical installation work. The quality of these products plays a very important role and all products must comply with GOST requirements.
Often, manufacturers, wanting to save money, find loopholes to deviate from GOST requirements and themselves develop technical production conditions (TU) taking into account the permitted errors.
As a result, the market is oversaturated with low-quality and cheap goods that need to be double-checked before purchasing.
If the cables available in retail outlets of a suitable price do not correspond to the declared characteristics, the only thing that can be done is to purchase a wire with a reserve cross-section. Power reserve will never negatively affect the quality of electrical wiring
It would also be useful to pay attention to products from manufacturers who value their name - although they are more expensive, they are a guarantee of quality, and wiring replacement is not done so often that you can save on it
We count: 20 x 0.8 = 16 (kW)
To make a choice of cable cross-section based on power, look at our tables:
For a three-phase 380 Volt circuit it will look like this:
As you can see, it's not difficult. I would also like to add that I advise you to choose a cable or wire with the largest cross-section of cores, in case you want to connect something else .
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Useful advice: if you suddenly find yourself in an unfamiliar area at night. Don't use your cell phone to light your way
Source
General information about cable and wire
When working with conductors, it is necessary to understand their designation. There are wires and cables that differ from each other in their internal structure and technical characteristics. However, many people often confuse these concepts.
A wire is a conductor that has in its design one wire or a group of wires woven together and a thin common insulating layer. A cable is a core or a group of cores that has both its own insulation and a common insulating layer (sheath).
Each type of conductor will have its own methods for determining cross sections, which are almost similar.
Conductor materials
The amount of energy that a conductor transmits depends on a number of factors, the main one of which is the material of the current-carrying conductors. The following non-ferrous metals can be used as the core material of wires and cables:
- Aluminum. Cheap and lightweight conductors, which is their advantage. They are characterized by such negative qualities as low electrical conductivity, a tendency to mechanical damage, high transient electrical resistance of oxidized surfaces;
- Copper. The most popular conductors, which have a high cost compared to other options. However, they are characterized by low electrical and transition resistance at the contacts, fairly high elasticity and strength, and ease of soldering and welding;
- Aluminum copper. Cable products with aluminum cores coated with copper. They are characterized by slightly lower electrical conductivity than their copper counterparts. They are also characterized by lightness, average resistance and relative cheapness.
Different types of cables based on core material
Important! Some methods for determining the cross-section of cables and wires will depend specifically on the material of their conductor component, which directly affects the throughput power and current strength (method for determining the cross-section of conductors by power and current)
Quick Selection: Useful Standards and Ratios
To save time, calculations are usually tabulated, especially since the range of cable products is quite limited. The following table shows the calculation of the cross-section of copper and aluminum wires according to power consumption and current strength, depending on the purpose - for open and closed wiring. The diameter is obtained as a function of load power, metal and type of wiring. The mains voltage is considered to be 230 V.
The table makes it possible to quickly select a cross-section or diameter if the load power is known. The found value is rounded up to the nearest value from the nomenclature series.
The following table summarizes the data on permissible currents by cross-section and power of materials of cables and wires for calculation and quick selection of the most suitable ones:
Calculation of the cross-section of electrical wiring according to the power of connected electrical appliances
To select the cross-section of cable wire cores when laying electrical wiring in an apartment or house, you need to analyze the fleet of existing electrical household appliances from the point of view of their simultaneous use. The table provides a list of popular household electrical appliances indicating the current consumption depending on the power.
You can find out the power consumption of your models yourself from the labels on the products themselves or data sheets; often the parameters are indicated on the packaging. If the current consumed by an electrical appliance is not known, it can be measured using an ammeter.
Typically, the power consumption of electrical appliances is indicated on the housing in watts (W or VA) or kilowatts (kW or kVA). 1 kW=1000 W.
Table of power/current consumption of household electrical appliances
| electrical appliance | Power consumption, W | Current strength, A |
| Washing machine | 2000 – 2500 | 9,0 – 11,4 |
| Jacuzzi | 2000 – 2500 | 9,0 – 11,4 |
| Electric floor heating | 800 – 1400 | 3,6 – 6,4 |
| Stationary electric stove | 4500 – 8500 | 20,5 – 38,6 |
| microwave | 900 – 1300 | 4,1 – 5,9 |
| Dishwasher | 2000 – 2500 | 9,0 – 11,4 |
| Freezers, refrigerators | 140 – 300 | 0,6 – 1,4 |
| Electric meat grinder | 1100 – 1200 | 5,0 – 5,5 |
| Electric kettle | 1850 – 2000 | 8,4 – 9,0 |
| Electric coffee maker | 630 – 1200 | 3,0 – 5,5 |
| Juicer | 240 – 360 | 1,1 – 1,6 |
| Toaster | 640 – 1100 | 2,9 – 5,0 |
| Mixer | 250 – 400 | 1,1 – 1,8 |
| Hairdryer | 400 – 1600 | 1,8 – 7,3 |
| Iron | 900 –1700 | 4,1 – 7,7 |
| Vacuum cleaner | 680 – 1400 | 3,1 – 6,4 |
| Fan | 250 – 400 | 1,0 – 1,8 |
| TV | 125 – 180 | 0,6 – 0,8 |
| Radio equipment | 70 – 100 | 0,3 – 0,5 |
| Lighting devices | 20 – 100 | 0,1 – 0,4 |
Current is also consumed by the refrigerator, lighting fixtures, radiotelephone, chargers, and TV in standby mode. But in total this power is no more than 100 W and can be ignored in calculations.
If you turn on all the electrical appliances in the house at the same time, you will need to select a wire cross-section capable of passing a current of 160 A. You will need a finger-thick wire! But such a case is unlikely. It’s hard to imagine that someone is capable of grinding meat, ironing, vacuuming and drying hair at the same time.
Calculation example. You got up in the morning, turned on the electric kettle, microwave, toaster and coffee maker. The current consumption will accordingly be:
7 A + 8 A + 3 A + 4 A = 22 A
Taking into account the switched on lighting, refrigerator and, in addition, for example, a TV, the current consumption can reach 25 A.
Selecting a wire cross-section for connecting electrical appliances to a three-phase 380 V network
When operating electrical appliances, for example, an electric motor, connected to a three-phase network, the consumed current no longer flows through two wires, but through three and, therefore, the amount of current flowing in each individual wire is somewhat less. This allows you to use a smaller cross-section wire to connect electrical appliances to a three-phase network.
To connect electrical appliances to a three-phase network with a voltage of 380 V, for example an electric motor, the wire cross-section for each phase is taken 1.75 times less than for connecting to a single-phase network of 220 V
Attention, when choosing a wire cross-section for connecting an electric motor based on power, you should take into account that the nameplate of the electric motor indicates the maximum mechanical power that the motor can create on the shaft, and not the electrical power consumed
For example, you need to connect an electric motor that consumes power from a 2.0 kW network. The total current consumption of an electric motor of such power in three phases is 5.2 A. According to the table, it turns out that a wire with a cross-section of 1.0 mm2 is needed, taking into account the above 1.0 / 1.75 = 0.5 mm2. Therefore, to connect a 2.0 kW electric motor to a three-phase 380 V network, you will need a three-core copper cable with a cross-section of each core of 0.5 mm2.
It is much easier to choose the wire cross-section for connecting a three-phase motor based on the current consumption, which is always indicated on the nameplate. For example, the current consumption of a 0.25 kW motor for each phase at a supply voltage of 220 V (the motor windings are connected in a delta configuration) is 1.2 A, and at a voltage of 380 V (the motor windings are connected in a star configuration) it is only 0.7 A.
Taking the current strength indicated on the nameplate, according to the table for selecting the wire cross-section for apartment electrical wiring, we select a wire with a cross-section of 0.35 mm2 when connecting the electric motor windings in a “triangle” pattern or 0.15 mm2 when connecting in a “star” pattern.
Odds
There are certain conditions under which the current inside the wiring can increase or decrease. For example, in open electrical wiring, when the wires are laid along walls or ceilings, the current strength will be higher than in a closed circuit. This is directly related to the ambient temperature. The larger it is, the more current this cable can carry.
That is, it turns out that if several wires are laid at once in one tray, corrugation or pipe, 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 applies to 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 is, of course, a good saving. By the way, there are also tables of reducing coefficients in the PUE.
There is one more point that concerns the length of the electrical cable used. The longer the wiring, the greater the voltage loss in the sections. Any calculations use a loss of 5%. That is, this is the maximum. If the losses are greater than this value, then the cross-section of the cable will have to be increased. By the way, it’s not difficult to independently calculate current losses if you know the wiring resistance and current load. Although the best option is to use the PUE table, which establishes the relationship between load torque and losses. In this case, the load torque 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 an alternating current network with a voltage of 220 volts can withstand a load of 3 kW. In this case, the load moment will be equal to 3*30=90. We look at the PUE table, which shows that losses of 3% correspond to this moment. That is, it is less than the nominal value of 5%. What is acceptable. As mentioned above, if the calculated losses exceeded the five percent barrier, then it would be necessary to purchase and install a cable of a larger cross-section.
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 that means their losses are just as many times greater.
As for three-phase networks, the load torque here is six times greater. This depends on the fact that the load itself is distributed over three phases, and this is a corresponding exponential increase in torque. Plus a double increase due to the symmetrical distribution of power consumption across phases. In this case, the current in the zero circuit must be zero. If the phase distribution is asymmetrical, 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.
How to calculate cable cross-section by power?
First step. The total power of all electrical appliances that can be connected to the network is calculated:
Psum = (P1 + P2 + .. + Pn) × Kс
- P1, P2 .. – power of electrical appliances, W;
- Kc – demand coefficient (probability of simultaneous operation of all devices), default is 1.
Second step. Then the rated current in the circuit is determined:
I = Psum / (U × cos ϕ)
- Psum – total power of electrical appliances;
- U – network voltage;
- cos ϕ – power factor (characterizes power losses), default is 0.92.
Third step. At the last stage, tables are used in accordance with the PUE (Electrical Installation Rules).
Table of copper cable cross-section by current according to PUE-7
| Conductor cross-section, mm2 | Current, A, for wires laid | |||||
| open | in one pipe | |||||
| two single-core | three single-core | four single-core | one two-wire | one three-wire | ||
| 0.5 | 11 | — | — | — | — | — |
| 0.75 | 15 | — | — | — | — | — |
| 1 | 17 | 16 | 15 | 14 | 15 | 14 |
| 1.2 | 20 | 18 | 16 | 15 | 16 | 14.5 |
| 1.5 | 23 | 19 | 17 | 16 | 18 | 15 |
| 2 | 26 | 24 | 22 | 20 | 23 | 19 |
| 2.5 | 30 | 27 | 25 | 25 | 25 | 21 |
| 3 | 34 | 32 | 28 | 26 | 28 | 24 |
| 4 | 41 | 38 | 35 | 30 | 32 | 27 |
| 5 | 46 | 42 | 39 | 34 | 37 | 31 |
| 6 | 50 | 46 | 42 | 40 | 40 | 34 |
| 8 | 62 | 54 | 51 | 46 | 48 | 43 |
| 10 | 80 | 70 | 60 | 50 | 55 | 50 |
| 16 | 100 | 85 | 80 | 75 | 80 | 70 |
| 25 | 140 | 115 | 100 | 90 | 100 | 85 |
| 35 | 170 | 135 | 125 | 115 | 125 | 100 |
| 50 | 215 | 185 | 170 | 150 | 160 | 135 |
| 70 | 270 | 225 | 210 | 185 | 195 | 175 |
| 95 | 330 | 275 | 255 | 225 | 245 | 215 |
| 120 | 385 | 315 | 290 | 260 | 295 | 250 |
| 150 | 440 | 360 | 330 | — | — | — |
| 185 | 510 | — | — | — | — | — |
| 240 | 605 | — | — | — | — | — |
| 300 | 695 | — | — | — | — | — |
| 400 | 830 | — | — | — | — | — |
Table of cross-section of aluminum cable by current according to PUE-7
| Conductor cross-section, mm2 | Current, A, for wires laid | |||||
| open | in one pipe | |||||
| two single-core | three single-core | four single-core | one-two-core | one three-wire | ||
| 2 | 21 | 19 | 18 | 15 | 17 | 14 |
| 2.5 | 24 | 20 | 19 | 19 | 19 | 16 |
| 3 | 27 | 24 | 22 | 21 | 22 | 18 |
| 4 | 32 | 28 | 28 | 23 | 25 | 21 |
| 5 | 36 | 32 | 30 | 27 | 28 | 24 |
| 6 | 39 | 36 | 32 | 30 | 31 | 26 |
| 8 | 46 | 43 | 40 | 37 | 38 | 32 |
| 10 | 60 | 50 | 47 | 39 | 42 | 38 |
| 16 | 75 | 60 | 60 | 55 | 60 | 55 |
| 25 | 105 | 85 | 80 | 70 | 75 | 65 |
| 35 | 130 | 100 | 95 | 85 | 95 | 75 |
| 50 | 165 | 140 | 130 | 120 | 125 | 105 |
| 70 | 210 | 175 | 165 | 140 | 150 | 135 |
| 95 | 255 | 215 | 200 | 175 | 190 | 165 |
| 120 | 295 | 245 | 220 | 200 | 230 | 190 |
| 150 | 340 | 275 | 255 | — | — | — |
| 185 | 390 | — | — | — | — | — |
| 240 | 465 | — | — | — | — | — |
| 300 | 535 | — | — | — | — | — |
| 400 | 645 | — | — | — | — | — |
In the Electrical Installation Rules, 7th edition, there are no tables of cable cross-section by power, there is only data on current strength. Therefore, when calculating sections using load tables on the Internet, you risk getting incorrect results.
Formula for calculation
The following formula is suitable here - Icalc = P / Unom where:
Example (wire cross-section for 3 kW power):
The standard correction factor for household equipment is 5 A. It is added to the result: 14 +5 = 19 A.
The resulting figure is inserted into the table, from which it can be seen that a copper wire for a given current strength should be selected with a cross-section of 2 mm², aluminum - 4 mm².
Then what wire cross-section is needed for 6 kW? For a line with such power, the cross-section of the aluminum wire is 6 mm², and the copper wire is 4 mm².
Cable selection according to PUE and GOST tables
When purchasing a wire, it is recommended to look at the GOST standard or the technical specifications according to which the product is manufactured. GOST requirements are higher than similar parameters of technical specifications, so you should prefer products made according to the standard.
Tables from the rules for electrical installations (PUE) represent the dependence of the strength of the current transmitted through the conductor on the cross-section of the conductor and the method of installation in the main pipe. The permissible current decreases as the individual cores increase in size or as multi-core cable insulation is used. The phenomenon is associated with a separate clause in the PUE, which stipulates the parameters of the maximum permissible heating of the wires. The main pipe is understood as a box, including a plastic one or when laying wiring in a bundle on a cable tray.
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The parameters in the tables are indicated taking into account the operating temperature of the core of 65 ° C and only phase wires (zero buses are not taken into account). If a standard three-core cable for supplying single-phase current is laid in a room pipe, then its parameters are taken into account in the data column for one two-core wire. Below is information for cables made from different materials. Please note that the tables are used to select wires. In the case of determining the type of cables, other data is used, which is also available in the PUE.
The second way to select a cable is the tables of the GOST 16442-80 standard, which exist in two versions - for copper and. In this information, the choice is made depending on the type of laying and the number of cores in the cables.
What affects the heating of wires
If the wiring heats up during the operation of household appliances, then you should immediately take all necessary measures to eliminate this problem. There are many factors influencing the heating of wires, but the main ones include the following:
- Insufficient cable cross-sectional area. To put it in accessible language, we can say this: the thicker the wires of the cable, the more current it can transmit without overheating. The value of this value is indicated in the marking of cable products. You can also measure the cross-section yourself using a caliper (you should make sure that the wire is not live) or by the type of wire.
- The material from which the wire is made. Copper conductors transmit voltage better to the consumer and have lower resistance compared to aluminum conductors. Naturally, they heat up less.
- Type of core The cable can be single-core (the core consists of one thick rod) or multi-core (the core consists of a large number of small wires). A multi-core cable is more flexible, but is significantly inferior to a single-core cable in terms of the permissible strength of the transmitted current.
- Cable laying method. Tightly laid wires located in the pipe heat up noticeably more than open wiring.
- Material and quality of insulation. Inexpensive wires, as a rule, have low quality insulation, which negatively affects their resistance to high temperatures.
Why do you need to specify cable cross-sections?
On most wires and cables, the manufacturer is required to apply markings indicating their type, the number of conductors and their cross-section. If the wire is marked as 3x2.5, this means that the diameter of the wire is 2.5 mm². Actual values may differ from those indicated by approximately 30%, because some types of wiring (in particular PUNP) are produced according to outdated standards, which allow an error of the indicated number of percent and generally it appears on the smaller side. As a result, if you use a cable with a smaller cross-section than the calculated one, then the effect for the wire will be approximately the same if a thin polyethylene hose were connected to a fire hydrant. This can lead to dangerous consequences: overheating of the electrical wiring, melting of the insulation, and changes in the properties of the metal. Therefore, before making a purchase, it is imperative to check that the cross-sectional area of the conductor does not differ from that declared by the manufacturer.
Ways to find out the real diameter of the wire
The easiest and most accurate method to measure the diameter of a wire strand is to use special tools such as a caliper or micrometer (electronic or mechanical). In order for the measurement to be accurate, the wire being measured must be cleared of insulation so that the tool does not cling to it. You also need to inspect the tip of the wire so that it is free of kinks - sometimes they appear if the wire is cut through with blunt pliers. Once the diameter is measured, you can begin to calculate the cross-sectional area of the wire core.
A micrometer will give a more reliable reading than a caliper.
If you don’t have an accurate measuring tool at hand, there is another way to find out the cross-section - you will need a screwdriver (pencil or any tube) and a measuring ruler. You will also have to buy at least one meter of wire (50 cm is enough, if they sell that much) and remove the insulation from it. Next, the wire is wound tightly, without gaps, onto the tip of a screwdriver and the length of the wound section is measured with a ruler. The resulting winding width is divided by the number of turns and the result will be the required diameter of the wire, by which you can already search for the cross-section.
How to take measurements is shown in detail in this video:
What formulas should you use?
What such a cross-section of a wire is known from the basics of geometry or drawing is the intersection of a three-dimensional figure with an imaginary plane. At the points of their contact, a flat figure is formed, the area of which is calculated by suitable formulas. The wire core is most often cylindrical in shape and has a circle in cross-section; accordingly, the cross-section of the conductor can be calculated using the formula:
S = ϖ R²
R – radius of the circle, equal to half the diameter;
ϖ = 3.14
There are wires with flat conductors, but there are few of them and it is much easier to find the cross-sectional area of them - just multiply the sides.
To get a more accurate result you need to keep in mind:
- The more turns (there should be at least 15) screwed onto the screwdriver, the more accurate the result will be;
- There should be no distance between the turns; due to the gap, the error will be higher;
- You need to take several measurements, changing its beginning each time. The more there are, the higher the accuracy of the calculations.
The disadvantage of this method is that thin conductors can be used for measurements; it will be difficult to wind a thick cable.
Determine the wire cross-section using the table
Using formulas does not give a guaranteed result, and as luck would have it, they are forgotten at the most necessary moment. Therefore, it is better to determine the cross section according to the table where the calculation results are summarized. If you manage to measure the diameter of the core, then the cross-sectional area of the wire can be viewed in the corresponding column of the table:
If you need to find the total diameter of a stranded cable core, you will have to separately calculate the diameter of each wire and add the resulting values. Then everything is done in the same way as with a single-wire conductor - the result is found using a formula or table.
When measuring the cross-section of a wire, its core is carefully cleaned of insulation, since it is possible that its thickness will be greater than the standard. If for any reason there is doubt about the accuracy of the calculations, then it is better to choose cables or wires with a power reserve.
To approximately find out the cross-section of the wire that will be purchased, you need to add up the power of the electrical equipment that will be connected to it. Power consumption must be indicated in the device passport. Based on the known power, the total current that will flow through the conductor is calculated, and based on it, the cross-section is selected.
Device recommendations
The wiring device, among other things, requires design skills, which not everyone who wants to do it has. It is not enough to just have good electrical installation skills. Some people confuse design with drawing up documentation according to some rules. These are completely different things. A good project can be written out on pieces of paper from a notebook.
First of all, draw a plan of your premises and mark future sockets and lamps. Find out the power of all your consumers: irons, lamps, heating devices, etc. Then enter the power of the loads most frequently consumed in different rooms. This will allow you to choose the best cable options.
You will be surprised how many opportunities there are and what reserves there are for saving money. Once you have selected your wires, count the length of each line you draw. Put everything together, and then you will get exactly what you need, and as much as you need.
Each line must be protected by its own circuit breaker (circuit breaker), designed for a current corresponding to the permissible power of the line (the sum of the consumer powers). Label the machines located in the panel, for example: “kitchen”, “living room”, etc.
It is advisable to have a separate line for all lighting, then you can easily repair the socket in the evening without using matches. It is the sockets that are most often overloaded. Provide outlets with enough power - you don't know in advance what you'll have to plug into them.
In damp rooms, use only double-insulated cables! Use modern sockets (“Euro”) and cables with grounding conductors and connect the grounding correctly. Bend single-core wires, especially copper ones, smoothly, leaving a radius of several centimeters. This will prevent them from breaking. In cable trays and channels, the wires should lie straight, but freely; in no case should they be pulled like a string.
There should be a few extra centimeters in sockets and switches. When laying, you need to make sure that there are no sharp corners anywhere that could cut the insulation. It is necessary to tighten the terminals tightly when connecting, and for stranded wires this procedure should be repeated; they have a tendency for the cores to shrink, as a result of which the connection may become loose.
Copper wires and aluminum wires are not “friendly” with each other for electrochemical reasons; they cannot be connected directly. To do this, you can use special terminal blocks or galvanized washers. Connections must always be dry. Phase conductors must be white (or brown) and neutrals must always be blue.
We bring to your attention an interesting and educational video on how to correctly calculate the cable cross-section by power and length:
The choice of wire cross-section is the main element of a power supply project of any scale, from a room to large networks. The current that can be taken into the load and power will depend on this. Choosing the right wires also ensures electrical and fire safety and keeps your project budget-friendly.
How to calculate the cross-section of a stranded wire
Stranded wire, or as it is also called stranded or flexible, is a single-core wire twisted together. To calculate the cross-section of a stranded wire, you must first calculate the cross-section of one wire, and then multiply the resulting result by their number.
Let's look at an example. There is a multi-core flexible wire, in which there are 15 cores with a diameter of 0.5 mm. The cross-section of one core is 0.5 mm × 0.5 mm × 0.785 = 0.19625 mm2, after rounding we get 0.2 mm2. Since we have 15 wires in the wire, to determine the cable cross-section we need to multiply these numbers. 0.2 mm2×15=3 mm2. It remains to determine from the table that such a stranded wire will withstand a current of 20 A.
You can estimate the load capacity of a stranded wire without measuring the diameter of an individual conductor by measuring the total diameter of all twisted wires. But since the wires are round, there are air gaps between them. To eliminate the gap area, you need to multiply the result of the wire cross-section obtained from the formula by a factor of 0.91. When measuring the diameter, you need to make sure that the stranded wire does not flatten.
Let's look at an example. As a result of measurements, the stranded wire has a diameter of 2.0 mm. Let's calculate its cross-section: 2.0 mm × 2.0 mm × 0.785 × 0.91 = 2.9 mm2. Using the table (see below), we determine that this stranded wire will withstand a current of up to 20 A.
Current loads in DC networks
To determine current loads in networks with direct current, single-core wires are taken into account. What loads is the wire powering a 0.05 kW light bulb in a car headlight designed for? The on-board network is powered by the battery
DC voltage is 12 V. The current flowing through the wire from the battery to the headlight will be equal to:
I = P/U = 50/12 = 4.15 A.
From here the resistance is determined:
R = U/I = 12/4.15 = 2.9 Ohm.
Knowing the resistivity of copper and taking L = 2 m as the maximum wire length, substitute everything known into the formula.
This means that the copper conductor must have a cross-section:
S = (ρ*L)/R = (1.68*10-8*2)/2.9 = 1.9 mm2.
The PUE has many tables from which you can determine the current load of single-phase and three-phase AC circuits. It is not necessary to do mathematical calculations. It is enough to operate with known parameters and correctly determine the cross-section of the wire or cable.
