How to determine the cable cross-section. Calculation of cable cross-section by power: practical advice from professionals
How to determine the wire cross-section? Both novice masters and ordinary guys come to this question sooner or later. Determining the wire cross-section helps to replace the wiring: the load was calculated and it became clear that the given conditions and current require certain wire cross-section parameters.
Let's say 2.5 mm2. You buy a cable or wire and install the wiring. After some time, the line is suddenly damaged. The channel is opened and it turns out that the insulation of the wires has melted. If the cross-section does not correspond to the load, such a situation is simply inevitable.
The reason for this could be:
- incorrect calculation and selection of wire cross-section;
- absence of technical information on the wire packaging or its unreliability.
Incorrect definition of the section can lead to negative consequences. It is super important to be able to correctly calculate section parameters. This way you can avoid unpleasant situations and save a decent amount.
Today the editors tell you how to find out the wire cross-section. Bonus - we have compiled several calculation options so that everyone can choose the best one for themselves.
How to determine wire cross-section: 3 main methods
Calipers
Pencil + ruler
Table
Option number 1: find out the wire cross-section using a caliper
Figure 1 - The wire has a mark and section. It happens that the cable is defective, that is, truncated. For example, you need a VVG 3x2.5, but in fact 3x2.1, although it says 3x2.5To determine the cross-section or triangular cross-section of a wire and cable core, a caliper is used. Below is an example of calculations using the device.
To get started, recall the formula for determining the area of a circle:
where n=3.14; r is the radius of the circle.
We will use a simplified version of the formula, adapted to our case:
d – diameter of the circle (core).
For clarity, divide the number n by 4. The result is the formula:
We determine the diameter of the wire: this is how we find out the cross-sectional area. To do this, you need to clean the wire core. Then measure its diameter using a caliper.
The result was 1.78 mm. This number must be put into the expression. Eventually:
Let's round to hundredths and get a wire cross-section of 2.79 mm2.
Option No. 2: determine the wire cross-section using a pencil and ruler
Not everyone has special equipment at home. And buying it for one-time use is stupid. We tell you how to determine the cross-section without professional instruments.
We take the wire and strip it along its length. 30-50 cm is enough. Then we wrap the wire around a pen or pencil. You need to wind them tightly to each other. Then we count the number of turns obtained and measure the length. Let's say we get 19 turns with a total length of 32 mm.
To find out the diameter, you need to divide the length by the number of turns. In our case, 32:19 = 1.684. According to the principle given in the first option, we substitute the diameter into the formula. We got 2.23 mm squared.
How accurate is this method? The magnitude of the error will depend on the number of turns: the more there are, the smaller the error.
IMPORTANT! This measurement option is not suitable for industrial scale, however, for home use it is what you need.
Option No. 3: determine the wire cross-section using the table
You can also use PUE compliance tables. They simplify the work, as they indicate the long-term permissible currents for copper and aluminum wires. This takes into account the types of insulation and shell.
Below is a simplified table for ease of use. With its help, you can determine the cross-section of three-wire, four-wire, and five-wire wires under single-phase and three-phase loads (220 V/380 V). To do this, it is enough to know the load current and its power.
- In the air (trays, boxes, voids, channels)
| Current, A | 220 V | 380 V |
|---|---|---|
| 19 | 4,1 | 12,5 |
| 25 | 5,5 | 16,4 |
| 35 | 7,7 | 23 |
| 42 | 9,2 | 27,6 |
| 55 | 12,1 | 36,2 |
| 75 | 16,5 | 49,3 |
| 95 | 20,9 | 62,5 |
| 120 | 26,4 | 78,9 |
| 145 | 31,9 | 95,4 |
| 180 | 39,6 | 118,4 |
| 220 | 48,4 | 144,8 |
| 260 | 57,2 | 171,1 |
| 305 | 67,1 | 200,7 |
| 350 | 77 | 230,3 |
| Current, A | 220 V | 380 V |
|---|---|---|
| - | - | - |
| 19 | 4,1 | 12,5 |
| 27 | 5,9 | 17,7 |
| 32 | 7 | 21 |
| 42 | 9,2 | 27,6 |
| 60 | 13,2 | 39,5 |
| 75 | 16,5 | 49,3 |
| 90 | 19,8 | 59,2 |
| 110 | 24,2 | 72,4 |
| 140 | 30,8 | 92,1 |
| 170 | 37,4 | 111,9 |
| 200 | 44 | 131,6 |
| 235 | 51,7 | 154,6 |
| 270 | 59,4 | 177,7 |
- In the ground
| Current, A | 220 V | 380 V |
|---|---|---|
| 27 | 5,9 | 17,7 |
| 38 | 8,3 | 25 |
| 49 | 10,7 | 32,5 |
| 60 | 13,2 | 39,5 |
| 90 | 19,8 | 59,2 |
| 115 | 25,3 | 75,7 |
| 150 | 33 | 98,7 |
| 180 | 39,6 | 118,5 |
| 225 | 49,5 | 148 |
| 275 | 60,5 | 181 |
| 330 | 72,6 | 217,2 |
| 385 | 84,7 | 253,4 |
| 435 | 95,7 | 286,3 |
| 500 | 110 | 329 |
| Current, A | 220 V |
|---|---|
Input wire cross-section: how to find out?
IMPORTANT! The rating of the input circuit breaker must be agreed upon with the energy supply institution. Without this procedure, it is prohibited to change it, because this may affect the selectivity of the operation of protection devices located in the power circuit in the ASU/TP (including the allocated power for a specific housing).
You can find out the denomination parameters in two ways:
- in an organization specializing in energy saving;
- in the documentation, which specifies the technical conditions for connecting to networks.
For clarity, let’s take the conditional values of technical specifications. For example, the power is 5 kW, the rating of the input circuit breaker is 25 A and single-phase power supply (220 V).
It's simple. It is necessary to take into account the type of electrical wiring, the material used for the production of cores and the voltage. We select the cross-section so that the long-term permissible current of the cable is higher than the rating of the input circuit breaker.
For example, we decided to install a three-core copper cable of the VVGng brand, laying it in an open way. Therefore, the suitable cross-section will be no less than 4 mm2, which means that you need to buy VVGng (3x4).
Remember about the “conditional shutdown current” of the machine: a machine that provides a rated current of 25 A has a “conditional shutdown current” of 1.45 25 = 36.25 A.
In such conditions, the machine will turn off in a cold state after about an hour.
A cable with a cross section of 4 mm2 has a continuous permissible current of 35 A, the “conditional shutdown current” indicator is 36.25 A. The values do not differ much, you don’t have to change anything. However, it is better to choose a 6 mm2 input cable with a continuous current rating of 42 A.
How to determine what cross-section of the wire of the outlet lines?
Every electrical appliance has power. The parameters can be found in the product passport or on the sticker supplied by the manufacturer. It is measured in Watts. As an example, we will offer conditional parameters.
Let's say you need a power line for a washing machine. The power of the device is 2.4 kW. We use a copper three-core cable of the VVGng brand, lay it so that it is not visible. From this we conclude that the cross-section should be at least 1.5 mm2. This means that you will need a VVGng cable (3x1.5).
If you use the outlet only to connect the washing machine, these parameters will be sufficient. To protect the cable you will need a machine with a rated current of 10 A.
However, sockets are rarely installed for such reasons. For example, you will need to connect another device to the same network, and each electrical device has its own power. It is more advisable to use a copper cable with a cross-section of 2.5 mm2 for outlet lines. For protection, you need to install a circuit breaker with a rating of 16 A.
How to find out the wire cross-section of a three-phase motor?
Let's assume there is a three-phase asynchronous motor of type AIR71A4U2. Its power is 550 W, and the windings are connected in a star at a voltage of 380 V. To find out which cross-section will be optimal, you need to look at the rated current of the motor, which is indicated on the sticker of the device. Let's say it is 1.6 A.
IMPORTANT! Sometimes it happens that there is no sticker. Then you can look up the data in lookup tables. We will use copper cable and lay it over the air. To determine the cross section, refer to the table. We get 1.5 mm2. It can also be recognized by the power of the consumer.
Depending on the situation, you can choose the most convenient method. They say that experienced electricians are able to determine the cross-section visually by eye. But for now, we advise beginners to refrain and still use our recommendations. Good luck!
In order for the electrical wiring to function flawlessly, it is important to select the correct cross-section of wires and make a competent power calculation, because other characteristics depend on these indicators. Current moves through wires in the same way that water moves through pipes.
The safety of the entire premises depends on the quality of the electrical installation work. Here it is especially important to choose the right parameter such as cable cross-section. In order to calculate the cable cross-section by power, you need to know the technical characteristics of all electricity consumers that will be connected to it. You should also consider the length of the wiring and how it will be installed.
Selecting a cable for electrical wiring
Current moves through wires just like water flows through a pipe. Just as it is impossible to place a larger volume of liquid into a water pipe, it is also impossible to pass more than a certain amount of current through a cable. In addition, the cost of the cable directly depends on its cross-section. The larger the cross-section, the higher the price of the cable.
A water pipe with a cross-section larger than necessary is more expensive, and one that is too narrow will not allow the required amount of water to pass through. The same thing happens with current, with the only difference being that choosing a cable with a cross-section smaller than the specified value is much more dangerous. Such a wire overheats all the time, and the current power in it increases. Because of this, the light in the room will randomly turn off, and in the worst case, a short circuit will occur and a fire will start.
There is nothing wrong with the fact that the selected cable cross-section will be larger than necessary. On the contrary, wiring where the power and cross-section exceed the required value will last much longer, but the cost of all electrical installation work will immediately increase by at least 2-3 times, because the main cost of power supply lies precisely in the cost of the wires.
A correctly selected section will allow:
- avoid overheating of wires;
- prevent short circuit;
- save on repair costs.
Calculation using formulas
A sufficient cross-sectional area will allow maximum current to pass through the wires without overheating. Therefore, when designing electrical wiring, first of all, find the optimal wire cross-section depending on the power consumption. To calculate this value, the total current must be calculated. It is determined based on the power of all devices connected to the cable.
To choose the optimal wire cross-section, knowing the power, you should remember Ohm's law, as well as the rules of electrodynamics and other electromechanical formulas. Thus, the current strength (I) for a section of a network with a voltage of 220 Volts, namely this voltage is used for a home network, is calculated by the formula:
I=(P1+P2+…+Pn)/220, where:
(P1+P2+…+Pn) – the total power of each electrical appliance used.
For networks with a voltage of 380 Volts:
I=(P1+P2+…+Pn)/ √3/380.
Power ratings of some household electrical appliances
| electrical appliance | Power, W | electrical appliance | Power, W |
|---|---|---|---|
| Blender | up to 500 | Heated towel rail | 900-1700 |
| Fan | 750-1700 | Dishwasher | 2000 |
| Video recorder | up to 500 | Vacuum cleaner | 400-2000 |
| Storage water heater | 1200-1500 | Juicer | up to 1000 |
| Instantaneous water heater | 2000-5000 | Washing machine | 3000 |
| Hood (ventilation) | 500-1000 | Washer and dryer | 3500 |
| Grill | 1200-2000 | Hand dryer | 800 |
| Oven | 1000-2000 | TV | 100-400 |
| Computer | 400-750 | Toaster | 600-1500 |
| Air conditioner | 1000-3000 | Humidifier | 200 |
| Coffee maker | 800-1500 | Iron | 500-2000 |
| Food processor | up to 100 | Hair dryer | 450-2000 |
| Microwave | 850 | deep fryer | 1500 |
| Microwave oven combination | 2650 | Fridge | 200-600 |
| Mixer | up to 500 | Electric shaver | up to 100 |
| Meat grinder | 500-1000 | Electric lamps | 20-250 |
| Heater | 1000-2400 | Electric stove | 8000-10000 |
| Double boiler | 500-1000 | Electric kettle | 1000-2000 |
But these are vague formulas and simplified calculations. Detailed calculations take into account the permissible loads, which for a copper cable will be 10 A/mm², and for an aluminum cable – 8 A/mm². The load determines how much current can pass through a unit area unhindered.
Correction of power indicators
Also, when calculating, an amendment is added in the form of the demand coefficient (Kс). This coefficient shows which devices are used on the network constantly, and which for a certain time. A special calculator and tables showing power calculations simplify all these calculations.
Demand coefficients of auxiliary receivers (Kс)
But what to do if the characteristics indicate 2 types of power: active and reactive? Moreover, the first of them measures in the usual kV, and the second - kVA. Our networks carry alternating current, the magnitude of which varies over time. Therefore, for all consumers there is active power, which is calculated as the average value of all instantaneous current and power variables. Devices with active power include incandescent lamps and electric heaters. For such energy consumers, the phases of current and voltage coincide. If the electrical circuit involves units that accumulate energy, for example, transformers or electric motors, then they may exhibit deviations in amplitude. Due to this phenomenon, reactive power arises.
For networks where there is reactive and active power, one more correction must be taken into account - the power factor (cosφ) or the reactive component.
Thus, the formula is obtained:
S= Kс*(P1+P2+…+Pn)/(220*cosφ*Рд), where:
- S – cross-sectional area,
- Рд – permissible load value.
In addition, possible current power losses that occur during passage through the wires are considered. When using a cable with several cores, you need to multiply the loss by the number of these cores.
Important! For all these calculations you will need not just a calculator, but also deep knowledge of physics. It will not be possible to make an accurate calculation right away without theoretical knowledge.
Finding area by diameter
Sometimes even a scrupulous calculation does not help; a short circuit occurs in the circuit. This is due to the fact that the stated technical characteristics often do not correspond to the actual value. Therefore, in order to find out how to make a power calculation, it is important to be sure that the store will offer a suitable electrical wire cross-section. To do this, we use a simple formula:
S=0.785d 2 , where:
- d is the diameter of the core;
- S – cross-sectional area.
You can determine the exact diameter of the wire and calculate the cross-section using a caliper or a micrometer, which is more accurate.
If the cable consists of several thin wires, then first look at the diameter of one of them, and then multiply the obtained data by their number:
Stotal=n*0.785di 2, where:
- di is the area of an individual wire;
- n – number of wires;
- Stotal – total cross-sectional area.
Tables for calculations
It is not entirely correct to resort to complex calculations for calculations every time. The industry produces wires of a certain cross-section. If, after accurate calculations and calculations, the cable cross-section for power is 3.2 square millimeters, then it will not be possible to find such a wire, because there are wires with a cross-section of 2.5 mm 2, 3 or 4 mm 2.
Attention! In order to find out the cable cross-sections, you need a table where all the data is regulated and compiled in accordance with the PUE - rules for the design of electrical installations.
In order to determine the cable cross-section at a known load, you need to:
- calculate the current strength;
- round up to a higher value according to the data in the table;
- then find the closest standard section size.
Permissible continuous current for wires and cords with rubber and polyvinyl chloride insulation with copper conductors
| Current cross-section provo- conductor, mm 2 | Current, A, for wires laid | |||||
|---|---|---|---|---|---|---|
| Open That | in one pipe | |||||
| two one- vein | three one- vein | four one- vein | one two- vein | one three- vein |
||
| 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 | - | - | - | - | - |
It's easy to make this calculation. First you need to determine the total power of all electrical appliances used in the network. For this you will need a table, and the missing data for each electrical appliance can be taken from the product passport. The resulting amount must be multiplied by 0.8 - the demand coefficient, if the electrical appliances will not be used all at once, or left unchanged during constant operation. Now the resulting value must be divided by the voltage in the network and add a constant value of 5. This will be the required current indicator. Let's say the current is 20A.
Note! In residential areas, three-wire electrical wiring and enclosed wiring are used. This must be remembered when calculations are made using the table.
Next you will need a table from the PUE. We take the column where the current values for a three-core core are given, and select the closest ones: 17 and 22. It is better to take the cross-section with a margin, so in the example under consideration the required value will be equal to 22. As you can see, this value corresponds to a three-core cable with a cross-section of 2 mm 2 .
You can additionally consider how this calculation is made for an aluminum cable according to the PUE, although for safety reasons such wires cannot be used in residential buildings. Old houses still have aluminum wiring, but it is recommended to replace it during major renovations. In addition, aluminum electrical wire crumbles at bends and has less conductivity at the joints. Exposed parts of aluminum quickly oxidize in air, which leads to significant losses of electricity at the joints.
Calculator
Today, specialists use not only a table, but also a special calculator to determine the cross-section. This calculation greatly simplifies the calculations. The calculator is easy to find on the Internet. To calculate the cross-sectional size, you need to know the following parameters:
- AC or DC current is used;
- wire material;
- power of all devices used;
- mains voltage;
- power supply system (single or three-phase);
- type of wiring.
These indicators are loaded into the calculator and the required cross-sectional value of the wires is obtained.
Calculation by length
Calculating the cross-section along the length is important when constructing industrial-scale networks, when areas are subject to constant heavy load, and the cable must be pulled over significant distances. After all, during the passage of current through the wires, power losses occur due to electrical resistance in the circuit. Power loss (dU) is calculated as follows:
dU = I*p*L/S, where:
- I – current strength;
- p – resistivity (copper – 0.0175, aluminum – 0.0281);
- L – cable length;
- S is the cross-sectional area we have already calculated.
According to the technical specifications, the maximum voltage drop along the length of the wire should not exceed 5 percent. Otherwise, you should select a wire with a larger cross-section.
Peculiarities
There are certain standards according to which the cable cross-section is calculated. If you are not sure which electrical wire is needed, then you can use these rules: electrical appliances in the apartment are divided into lighting group and others; for powerful electrical appliances, for example, washing machines or electric ovens, a connection from separate wires is used; the standard wire cross-section for a lighting group in an apartment is 1.5 mm 2, and for other wires – 2.5 mm 2. Such standards are used because the rated power of the incoming current cannot be greater.
Three-phase current is required when high-power industrial devices are used. Therefore, to determine the cable cross-section at enterprises, it is necessary to accurately calculate all additional coefficients, and also be sure to take into account power losses and voltage fluctuations. For electrical work in an apartment or private house, such complex calculations are not carried out.
For installation of acoustic equipment, wires with minimal resistance are used. This is necessary in order to eliminate distortion as much as possible and improve the quality of the transmitted signal. Therefore, for speaker systems, cables measuring 2x2.5 or 2x1.5 with a length of at least 3 meters are better suited, and the subwoofer is connected with the shortest cable 2.5-4 mm 2.
Examples
Let's consider a general diagram for choosing the cable cross-section in an apartment:
- First you need to determine the places where the sockets and lighting fixtures will be located;
- Next, you need to determine which devices will be used at each output;
- Now you can draw up a general connection diagram and calculate the cable length, adding at least 2 cm for wire connections;
- Based on the data obtained, we calculate the size of the cable cross-section using the formulas given above.
I=2400W/220V=10.91A, round up and get 11A.
As we already know, different coefficients are used to accurately determine the cross-sectional area, but almost all of this data refers to a network with a voltage of 380V. To increase the safety margin, we add another 5A to our current value:
For apartments, three-core cables are used. The table will show a current value close to our 16A, it will be 19A. We find that to install one washing machine you need a wire with a cross-section of at least 2 mm 2.
General theory
To determine the optimal cable cross-section for domestic needs, the following rules are generally used:
- sockets require wires with a cross section of 2.5 mm²;
- for lighting – 1.5 mm²;
- for devices with increased power – 4-6 mm².
If there are doubts about the calculation of the cross-section, then the PUE table is used. To determine the exact data on the cable cross-section, all factors affecting the passage of current through the circuit are taken into account. These include:
- type of wire insulation;
- length of each section;
- laying method;
- temperature regime;
- humidity;
- permissible value of overheating;
- difference in power of current receivers in one group.
All these indicators make it possible to increase the efficiency of energy use on an industrial scale, as well as to avoid overheating.
Selection of section. Video
In this video, the master shares his experience in choosing the cable cross-section and machine rating. He points out possible mistakes and gives practical advice to beginners.
If after reading the article you still have any doubts, then the table or calculator described above will help you find the exact power cross-section of the wire.
The choice of cross-sectional area of wires (in other words, thickness) is given much attention in practice and in theory.
In this article we will try to understand the concept of “sectional area” and analyze reference data.
Calculation of wire cross-section
Strictly speaking, the concept of “thickness” for a wire is used colloquially, and the more scientific terms are diameter and cross-sectional area. In practice, the thickness of the wire is always characterized by its cross-sectional area.
S = π (D/2) 2, Where
- S– wire cross-sectional area, mm 2
- π – 3,14
- D– diameter of the conductor of the wire, mm. It can be measured, for example, with a caliper.
The formula for the cross-sectional area of a wire can be written in a more convenient form: S = 0.8 D².
Amendment. Frankly, 0.8 is a rounded factor. More precise formula: π (1/2) 2 = π/4 = 0.785. Thanks to attentive readers;)
Let's consider copper wire only, since in 90% of electrical wiring and installation it is used. The advantages of copper wires over aluminum wires are ease of installation, durability, and reduced thickness (at the same current).
But with an increase in diameter (sectional area), the high price of copper wire eats up all its advantages, so aluminum is mainly used where the current exceeds 50 Amperes. In this case, a cable with an aluminum core of 10 mm 2 or thicker is used.
The cross-sectional area of the wires is measured in square millimeters. The most common cross-sectional areas in practice (in household electrics): 0.75, 1.5, 2.5, 4 mm2
There is another unit for measuring the cross-sectional area (thickness) of a wire, used mainly in the USA - AWG system. On Samelektrika there is also a conversion from AWG to mm 2.
Regarding the selection of wires, I usually use catalogs from online stores, here is an example of copper. They have the largest selection I've ever seen. It’s also good that everything is described in detail - composition, applications, etc.
I also recommend reading my article, there are a lot of theoretical calculations and discussions about voltage drop, wire resistance for different cross-sections, and which cross-section to choose is optimal for different permissible voltage drops.
In the table solid wire– means that there are no more wires passing nearby (at a distance of less than 5 wire diameters). Twin wire– two wires side by side, usually in the same common insulation. This is a more severe thermal regime, so the maximum current is less. And the more wires in a cable or bundle, the less the maximum current for each conductor must be due to possible mutual heating.
I find this table not very convenient for practice. After all, most often the initial parameter is the power of the electricity consumer, and not the current, and based on this you need to choose a wire.
How to find the current knowing the power? You need to divide the power P (W) by the voltage (V), and we get the current (A):
How to find power knowing current? You need to multiply current (A) by voltage (V), we get power (W):
These formulas are for the case of active load (consumers in residential premises, such as light bulbs and irons). For reactive loads, a factor of 0.7 to 0.9 is usually used (in industry where large transformers and electric motors operate).
I offer you a second table in which initial parameters - current consumption and power, and the required values are the wire cross-section and the shutdown current of the protective circuit breaker.
Selecting the thickness of the wire and circuit breaker based on power consumption and current
Below is a table for selecting the wire cross-section based on known power or current. And in the right column is the choice of the circuit breaker that is installed in this wire.
table 2
| Max. power, kW |
Max. load current, A |
Section wires, mm 2 |
Machine current, A |
| 1 | 4.5 | 1 | 4-6 |
| 2 | 9.1 | 1.5 | 10 |
| 3 | 13.6 | 2.5 | 16 |
| 4 | 18.2 | 2.5 | 20 |
| 5 | 22.7 | 4 | 25 |
| 6 | 27.3 | 4 | 32 |
| 7 | 31.8 | 4 | 32 |
| 8 | 36.4 | 6 | 40 |
| 9 | 40.9 | 6 | 50 |
| 10 | 45.5 | 10 | 50 |
| 11 | 50.0 | 10 | 50 |
| 12 | 54.5 | 16 | 63 |
| 13 | 59.1 | 16 | 63 |
| 14 | 63.6 | 16 | 80 |
| 15 | 68.2 | 25 | 80 |
| 16 | 72.7 | 25 | 80 |
| 17 | 77.3 | 25 | 80 |
Critical cases are highlighted in red, in which it is better to play it safe and not skimp on the wire by choosing a wire thicker than indicated in the table. And the current of the machine is less.
Looking at the plate, you can easily choose current wire cross-section, or wire cross-section by power.
And also - select a circuit breaker for a given load.
This table shows the data for the following case.
- Single phase, voltage 220 V
- Ambient temperature +30 0 C
- Laying in the air or in a box (in a closed space)
- Three-core wire, in general insulation (cable)
- The most common TN-S system is used with a separate ground wire
- The consumer reaching maximum power is an extreme but possible case. In this case, the maximum current can operate for a long time without negative consequences.
If the ambient temperature is 20 0 C higher, or there are several cables in the bundle, then it is recommended to select a larger cross-section (the next one in the series). This is especially true in cases where the operating current value is close to the maximum.
In general, in case of any controversial and doubtful issues, for example
- possible future increase in load
- high inrush currents
- large temperature changes (electrical wire in the sun)
- fire hazardous premises
you need to either increase the thickness of the wires, or approach the choice in more detail - refer to formulas and reference books. But, as a rule, tabular reference data is quite suitable for practice.
The thickness of the wire can be determined not only from reference data. There is an empirical (experienced) rule:
Rule for choosing wire cross-sectional area for maximum current
You can select the required cross-sectional area of the copper wire based on the maximum current using this simple rule:
The required wire cross-sectional area is equal to the maximum current divided by 10.
This rule is given without reserve, back to back, so the result must be rounded up to the nearest standard size. For example, the current is 32 Amps. You need a wire with a cross section of 32/10 = 3.2 mm 2. We choose the closest one (naturally, in the larger direction) - 4 mm 2. As you can see, this rule fits well into the tabular data.
Important note. This rule works well for currents up to 40 Amps.. If the currents are greater (this is already outside the boundaries of an ordinary apartment or house, such currents are at the input) - you need to choose a wire with an even larger margin - divide not by 10, but by 8 (up to 80 A)
The same rule can be stated for finding the maximum current through a copper wire with a known area:
The maximum current is equal to the cross-sectional area multiplied by 10.
And in conclusion - again about the good old aluminum wire.
Aluminum conducts current less well than copper. This is enough to know, but here are some numbers. For aluminum (the same cross-section as the copper wire) at currents up to 32 A, the maximum current will be only 20% less than for copper. At currents up to 80 A, aluminum conducts current 30% worse.
For aluminum the rule of thumb would be:
The maximum current of an aluminum wire is equal to the cross-sectional area multiplied by 6.
I believe that the knowledge given in this article is quite enough to choose a wire based on the ratios “price/thickness”, “thickness/operating temperature” and “thickness/maximum current and power”.
That's basically all I wanted to tell you about wire cross-sectional area. If something is not clear or you have something to add, ask and write in the comments. If you are interested in what I will publish next on the SamElectric blog, subscribe to receive new articles.
Table for selecting a circuit breaker for different wire cross-sections
As you can see, the Germans are playing it safe and are providing for a larger reserve compared to us.
Although, perhaps this is because the table was taken from instructions from “strategic” industrial equipment.
Regarding the selection of wires, I usually use catalogs from online stores, here is an example of copper. They have the largest selection I've ever seen. It’s also good that everything is described in detail - composition, applications, etc.
A good Soviet book on the topic of the article:
/ Brochure from the Electrician's Library. Provides instructions and calculations necessary for selecting cross-sections of wires and cables up to 1000 V. Useful for those who are interested in primary sources., zip, 1.57 MB, downloaded: 158 times./
Determining what size wire you need is only half the battle. We still need to find the required cross section. The fact is that some manufacturers, in order to increase profits, produce cables with wires of a much smaller cross-section than stated in the accompanying documents. For example, the declared cores are 4 mm 2, but in reality - 3.6 mm 2 or even less. This is a decent difference. If it is not noticed in time, the wiring may heat up and this, in turn, can lead to a fire. Therefore, we will further talk about how to find out the cross-section of a wire by diameter, because the diameter can always be measured. Next, based on the measurement results, we will find out the actual parameters of the core.
When purchasing an electrical cable or wire, to check the cross-section of the core, you need to measure its diameter. There are several ways to do this. You can use measuring instruments such as calipers or micrometers. They measure the size of the exposed part of the conductor. The device is simply attached to the core, clamped between the jaws, and the result is displayed on the scale.
How to measure the diameter of the core - take a caliper or micrometer
For private use, the measurements are quite accurate, with a small error. Especially if the devices are electronic.
For the second method, you only need a ruler and some kind of even rod. But in this case, you still have to do calculations, albeit very simple ones. More on this method later.
Ruler+rod
If there are no measuring instruments on the farm, you can get by with a regular ruler and any rod of the same diameter. This method has a high error, but if you try it will be quite accurate.
Take a piece of wire about 10-20 cm long and remove the insulation. We wind the bare copper or aluminum wire onto a rod of the same diameter (any screwdriver, pencil, pen, etc. will do). We lay the coils carefully, close to one another. The number of turns is 5-10-15. We count the number of full turns, take a ruler and measure the distance that the wound wire occupies on the rod. Then divide this distance by the number of turns. The result is the diameter of the conductor.
As you can see, there is an error here. Firstly, you can lay the wire loosely. Secondly, it is not enough to take accurate measurements. But if you do everything carefully, the discrepancies with the actual sizes will not be so large.
How to measure the diameter of stranded wire
If you need to find out the diameter of a stranded wire, measurements are carried out with one of the wires that make it up. The process is the same: remove the insulation, remove the braiding (if any), fluff the wires, highlighting one, take measurements in any way (micrometer or wound around a rod).
Multiply the found size by the number of wires in one conductor (spread and count). That's all, you have found the diameter of the stranded conductor. It remains to find out how to find out the cross-section of a wire by diameter, because when planning wiring, it is the cross-sectional area of the wires that is used.
How to calculate using the formula
Since the cross-section of the wire is a circle, we will use the formula for the area of a circle (in the photo). As you can see, you can calculate the wire cross-section using the measured diameter or calculate the radius (divide the diameter by 2). For clarity, let's give an example. Let the measured wire size be 3.8 mm. We substitute this figure into the formula and get: 3.14 / 4 * 3.8 2 = 11.3354 mm 2. You can round the result - it will be 11.3 mm 2. Impressive cable.
The second part of the formula uses the radius. This is half the diameter. That is, to find the radius, divide the diameter by 2, we get 3.8 / 2 = 1.9 mm 2. Next, we substitute it into the formula and get: 3.14 * 1.9 2 = 11.3354 mm 2.
The numbers match, as they should. So, with a wire diameter of 3.8 mm, its cross-sectional area is 11.34 mm 2. You know how to find out the cross-section of a wire using the formula. But it is not always possible to do calculations. In this case, tables can help.
Determination of wire cross-section by diameter using tables
For cable and wire products there is a certain set of cross-sections that are specified in the standards. Knowing what cross-section you need, use the table to find the diameter of the conductor. Next, you just need to find products with the required parameters.
| Conductor cross-section | Diameter |
|---|---|
| 0.5 mm2 | 0.8 mm |
| 0.75 mm2 | 0.98 mm |
| 1.0 mm2 | 1.13 mm |
| 1.5 mm2 | 1.38 mm |
| 2.0 mm2 | 1.6 mm |
| 2.5 mm2 | 1.78 mm |
| 4.0 mm2 | 2.26 mm |
| 6.0 mm2 | 2.76 mm |
| 10.0 mm 2 | 3.57 mm |
Now a little about how to work with this table. You go for products with certain parameters. For example, you know that you need a cable with a core cross-section of 4 mm 2. Having found the corresponding value in the table, we look for the required parameters in the cable products. In this case, you will need to find wires with a diameter of 2.26 mm. If we find similar parameters in a store or market, that’s already good. It happens that the parameters indicated on the tag are too high, i.e. the actual cross-section of the conductors is smaller.
There are two ways to find what you need. The first is to look for products that meet the stated parameters. Perhaps, after spending some time, you will be able to find it. But it will take a lot of time to search. There are too few responsible producers. By the way, there is a sign by which you can navigate. This is the price. She is significantly above average. This is because more copper or aluminum is wasted. If you use this feature, it will take less time.
The second option is to look at products with a declared high denomination. In our case, we reason like this: we need a 4-square wire. The next one is 6 mm 2. It is very likely that the parameters of this cable in real life will be close to the required 4 squares. Perhaps the cross-section of the conductors will be larger, but this is good - the wiring will definitely not heat up. The disadvantage of this option is that you will spend more money, since such cables cost more.
In general, you know not only how to find out the wire cross-section by diameter, but also how to choose the right one. Even if the stated characteristics do not coincide with the real ones.
The main and most common method of transmitting electricity to the consumer is an electrical wire and electrical cable. An electrical wire and electrical cable is an electrical product consisting of a metal conductor or several conductors. Each core is electrically insulated. All insulated conductors of a wire or electrical cable are placed in general insulation.
Currently, the industry produces a wide variety of electrical wires and electrical cables. Cables and wires are mainly copper and aluminum, i.e. the composition of the cable or wire cores is copper or aluminum.
Electrical cables and wires can be single-core or multi-core. The core of a cable or wire can be either single-wire (monolithic) or multi-wire. The cores are made mainly in round shape, however, often for electrical cables with a large cross-section, the shape of the stranded core can be made in the form of a triangle. Today we will tell you how to calculate the cross-section of a wire by diameter.
Marking of electrical cable (wire)
There is a standard range of wire and electrical cable cross-sections that are used. This is 1mm 2; 1.5mm 2; 2.5mm 2; 4mm 2; 6mm 2; 8mm 2; 10mm 2 etc. The type, cross-section and number of cores are indicated either on the tag that comes with the cable or wire, or on the product itself. For example, markings are often applied to the general insulation of cables and wires. Also, technical data of electrical conductors are indicated in the product passport.
Let's say there is a VVGng 3x2.5 cable available. This marking is deciphered quite simply: a copper cable with PVC insulation, in a PVC sheath, non-flammable, the number of cores is three, the cross-section of each core is 2.5 mm 2. If the letter “A” appears at the beginning of the marking, i.e. The cable type will be AVVG, which means that the cable has aluminum cores.
By marking the wire, you can also find out not only the type of wire itself, but also the number and cross-section of current-carrying wires. For example, PVS wire 3x1.5. The decoding is as follows: wire with PVC insulation and PVC sheath, connecting. The number of cores is also three, and the cross-section of each wire is 1.5 mm 2 .
Conductor cross-section
Each wire and cable core has its own cross-section. It can be either very small (1mm 2 or less) or very large (95mm 2 or more). The cross-section of the conductor affects the ability to withstand a certain amount of electric current for a long and short time. The larger the cross-section of the core, the greater the current it can withstand for an almost unlimited time.
An incorrectly selected cross-section during design may subsequently cause overheating of the conductor, damage (destruction) of its insulation during the process of high heating, which may result in a short circuit and, as a result, fire and fire.
Section mismatch
The cause of overheating of a cable or wire during operation may not always be an incorrect calculation of the cross-section. As often happens in practice, the reason is very simple. Not all manufacturers of cable and wire products are conscientious about the quality of their products. The fact is that very often the cross-section of manufactured cables and wires is actually underestimated, i.e. does not correspond to the declared value.
To avoid purchasing an electrical cable or wire with an undersized cross-section, you must first visually assess its actual cross-section. Almost any electrical specialist is able to “by eye” determine the cross-section of a conductor. But when this is not enough, a professional can independently calculate the cross-sectional area of the electrical conductor. The cross section is calculated using the usual mathematical formula:
S = π*D 2 /4– formula No. 1
S=π* R 2 – formula No. 2
where: π is a mathematical constant, which is always equal to approximately 3.14;
R – wire radius;
D – wire diameter.
The radius is equal to half the diameter:
R=D/2– formula No. 3
Calculation of the actual cross-section of the electrical conductor
Knowing the formula for calculating the cross-section of a conductor, you can calculate its actual value and find out how much the manufacturer’s declared cross-section value is underestimated or overestimated (which happens rarely).
Single-wire (monolithic core)
First, you need to remove the insulation layer from a wire core or an electrical cable core to expose the core itself. Then the diameter of the core is measured with a caliper. Because the vein is monolithic, then there will be only one measurement. After measuring the diameter of the core, you need to substitute the value of the diameter (radius) into one of the above formulas.
Example No. 1
Let’s say that a cable or wire has a declared core cross-section of 2.5 mm 2 . When measured, the core diameter turned out to be 1.7 mm. Substituting the value into formula No. 1, we get:
S = 3.14*1.7 2 /4 = 2.26865 ≈ 2.3mm 2
Calculation using formula No. 1 showed that the cross-section of the core is underestimated by 0.2 mm 2 from the standard value.
Now let’s calculate the actual value of the cross-section using formula No. 2, but first, let’s determine the radius using formula No. 3:
R = 1.7/2 = 0.85mm
We substitute the radius value into formula No. 2 and get:
S = 3.14*0.85 2 = 2.26865 ≈ 2.3mm
The calculation using the second formula turned out to be similar to the calculation using the first. Those. The cross-section of the cable core turned out to be underestimated by 0.2 mm 2.
Example No. 2
Let’s say the core diameter, when measured with a caliper, turns out to be 1.8 mm. Substituting this value into formula No. 1, we get:
S = 3.14*1.8 2 /4 = 2.5434 ≈ 2.5 mm 2
Those. the actual cross-section was 2.5 mm 2, which in principle corresponds to the standard value.
Stranded core
If you determine the cross-section of a stranded conductor, then you cannot measure the diameter using the monolithic conductor method, because the calculation will have a large error. To determine the cross-section of a stranded core, it is necessary to measure the diameter of each individual wire in the core.
If the total cross-section of the core is large enough, then measuring each wire is quite possible, because You can actually measure the diameter with a caliper. But if the stranded core has a small cross-section, then determining the diameter of each wire is very problematic due to the thinness of the conductor.
