In theory and practice, the choice of the area of the transverse wire cross-section by current(thickness) is given Special attention... In this article, analyzing the reference data, we will get acquainted with the concept of "cross-sectional area".
Calculation of the cross-section of the wires.
Science does not use the concept of "thickness" of a wire. In literary sources, the terminology is used - diameter and sectional area. In practice, the thickness of the wire is characterized by cross-sectional area.
Quite easy to calculate in practice wire cross section... The cross-sectional area is calculated using the formula, having previously measured its diameter (can be measured using a caliper):
S = π (D / 2) 2,
- S - wire cross-sectional area, mm
- D is the diameter of the conductive core of the wire. You can measure it with a caliper.
A more convenient form of the formula for the cross-sectional area of a wire:
S = 0.8D.
Small correction - is a rounded factor. Exact calculation formula:
In electrical wiring and wiring, in 90% of cases, copper wire is used. Copper wire has several advantages over aluminum wire. It is more convenient to install, with the same current strength it has a smaller thickness, more durable. But the larger the diameter ( cross-sectional area), the higher the price copper wire... Therefore, despite all the advantages, if the current exceeds 50 Amperes, aluminum wire is most often used. In a specific case, a wire with an aluminum core of 10 mm or more is used.
Measure in square millimeters cross-sectional area of wires... Most often in practice (in household electrics), there are such cross-sectional areas: 0.75; 1.5; 2.5; 4 mm.
There is another system for measuring cross-sectional area (wire thickness) - the AWG system, which is used mainly in the United States. Below is the section table wires according to the AWG system, as well as conversion from AWG to mm.
It is recommended to read the article about the choice of wire cross-section for direct current. The article contains theoretical data and reasoning about the voltage drop, about the resistance of wires for different sections. Theoretical data will guide you which wire cross-section for current is most optimal for different allowable voltage drops. Also on real example object, in the article on the voltage drop on three-phase cable lines of long length, formulas are given, as well as recommendations on how to reduce losses. Wire losses are directly proportional to the current and wire length. And they are inversely proportional to the resistance.
There are three main principles, when selection of wire cross-section.
1. For the passage of electric current, the cross-sectional area of the wire (wire thickness) must be sufficient. The concept of sufficient means that when the maximum possible, in this case, electric current passes, the heating of the wire will be permissible (no more than 600C).
2. Sufficient wire cross-section so that the voltage drop does not exceed the permissible value. This mainly applies to long cable lines (tens, hundreds of meters) and high currents.
3. The cross-section of the wire, as well as its protective insulation, must ensure mechanical strength and reliability.
For powering, for example, chandeliers, they mainly use light bulbs with a total power consumption of 100 W (current a little more than 0.5 A).
When choosing the thickness of the wire, it is necessary to focus on the maximum operating temperature. If the temperature is exceeded, the wire and the insulation on it will melt and, accordingly, this will lead to the destruction of the wire itself. The maximum operating current for a wire with a certain cross-section is limited only by its maximum working temperature... And the time that the wire can work in such conditions.
The following is a table of wire cross-sections, with the help of which, depending on the current strength, you can choose the cross-sectional area of copper wires. The initial data is the cross-sectional area of the conductor.
Maximum current for different thickness copper wires. Table 1.
|
Conductor cross-section, mm 2 |
Current, A, for wires laid |
||
|
open |
in one pipe |
||
|
one two vein |
one three vein |
||
Highlighted the ratings of wires that are used in electrical engineering. "One two-wire" - a wire that has two wires. One Phase, the second - Zero - this is considered a single-phase power supply to the load. "One three-wire" - used with three-phase power supply to the load.
The table helps to determine at what currents, as well as in what conditions it is operated wire of this section.
For example, if “Max 16A” is written on the outlet, then a wire with a cross section of 1.5 mm can be laid to one outlet. It is necessary to protect the outlet with a switch for a current of no more than 16A, better even 13A, or 10 A. This topic is covered in the article “About replacement and choice circuit breaker».
From the data in the table it can be seen that a single-core wire means that no more wires pass near (at a distance of less than 5 wire diameters). When two wires are side by side, as a rule, in one common insulation - a two-wire wire. There is a more severe thermal regime, therefore, the maximum current is lower. The more is collected in a wire or a bundle of wires, the less the maximum current should be separately for each conductor, due to the possibility of overheating.
However, this table is not very convenient with practical side... Often, the initial parameter is the power of the electricity consumer, and not the electric current. Therefore, you need to choose a wire.
We determine the current, having a power value. To do this, divide the power P (W) by the voltage (V) - we get the current (A):
I = P / U.
To determine the power, having a current indicator, you need to multiply the current (A) by the voltage (V):
P = IU
These formulas are used in cases of active load (consumers in residential premises, light bulbs, irons). For reactive load, a coefficient from 0.7 to 0.9 is mainly used (for the operation of powerful transformers, electric motors, usually in industry).
The following table suggests the initial parameters - the consumed current and power, and the determined values - the wire cross-section and the tripping current of the protective circuit breaker.
Based on power consumption and current - choice wire cross-sectional area and a circuit breaker.
Knowing the power and current, in the table below you can choose wire size.
Table 2.
|
Max. power, |
Max. load current, |
Cross section |
Machine current, |
Critical cases in the table are highlighted in red, in these cases it is better to play it safe, not saving on the wire, choosing a thicker wire than indicated in the table. And the current of the machine, on the contrary, is smaller.
You can easily select from the table wire cross-section by current, or wire cross-section by power... Select a circuit breaker for the specified load.
In this table, all data are given for the following case.
- Single phase, voltage 220 V
- Temperature environment+ 300C
- Laying in the air or in a box (located in an enclosed space)
- Three-core wire, in general insulation (wire)
- The most common TN-S system is used with a separate earth conductor
- In very rare cases, the consumer reaches the maximum power. In such cases, the maximum current can act for a long time without negative consequences.
Recommended choose a larger section(the next from the series), in cases where the ambient temperature will be 200C higher, or there will be several wires in the bundle. This is especially important in cases where the value of the operating current is close to the maximum.
In doubtful and controversial points, such as:
large starting currents; possible future load increase; fire hazardous premises; large temperature differences (for example, the wire is in the sun), it is necessary to increase the thickness of the wires. Or, for reliable information, refer to the formulas and reference books. But basically, the tabular reference data is applicable for practice.
Also, the thickness of the wire can be found out by the empirical (obtained empirically) rule:
The rule for choosing the cross-sectional area of the wire for the maximum current.
Needed cross-sectional area for copper wire, based on the maximum current, you can choose by applying the rule:
The required wire area is equal to the maximum current divided by 10.
Calculations according to this rule without a margin, therefore, the result obtained must be rounded to big side to the nearest standard size. For example, you need wire cross-section mm, and the current is 32 Amperes. It is necessary to take the closest one, of course, to the larger side - 4 mm. It can be seen that this rule fits well into the tabular data.
It should be noted that this rule works well for currents up to 40 Amperes. If there are more currents (outside the dwelling, such currents are at the input), you need to choose a wire with an even larger margin, and divide it by 8 not by 10, but by 8 (up to 80 A).
The same rule is used to find the maximum current through a copper wire, if its area is known:
The maximum current is equal to the cross-sectional area, multiplied by 10.
About aluminum wire.
Unlike copper, aluminum is less conductive to electric current. For aluminum ( wire of the same cross-section, as copper), at currents up to 32 A, the maximum current will be less than for copper by 20%. At currents up to 80 A, aluminum passes the current worse by 30%.
A rule of thumb for aluminum:
The maximum current of the aluminum wire is cross-sectional area, multiply by 6.
With the knowledge gained in this article, you can choose a wire according to the ratios "price / thickness", "thickness / operating temperature", as well as "thickness / maximum current and power".
The main points about the cross-sectional area of the wires are highlighted, but if something is not clear, or there is something to add - write and ask in the comments. Subscribe to the SamElektrik blog to receive new articles.
The Germans treat the maximum current, depending on the cross-sectional area of the wire, somewhat differently. A recommendation for choosing a circuit breaker (protective) switch is located in the right column.
Table of the dependence of the electric current of the circuit breaker (fuse) on the cross-section. Table 3.
This table is taken from the "strategic" industrial equipment perhaps that is why the impression may be created that the Germans are reinsured.
For correct and safe installation cables for wiring, it is imperative to make a preliminary calculation of the estimated power consumption. Failure to comply with the requirements for the selection of the cross-section of the cable used for wiring can lead to melting of the insulation and a fire.
Calculation of the cable cross-section for a specific wiring system can be divided into several stages:
- breakdown of electricity consumers by groups;
- determination of the maximum current for each segment;
- selection of cable cross-section.
All consuming electrical appliances should be divided into several groups so that the total power consumption by one group does not exceed approximately 2.5-3 kW. This will allow you to pick up a copper cable with a cross section of no more than 2.5 square meters. mm. The power of some of the main household appliances is shown in Table 1.
Table 1. The value of the power of the main household appliances.
Consumers united in one group should be geographically located in approximately the same place, since they are connected to the same cable. If the entire connected object is powered from a single-phase network, then the number of groups and the distribution of consumers do not play a significant role.
Then the percentage of discrepancy can be calculated by the formula = 100% - (Pmin / Pmax * 100%), where Pmax is the maximum total power per phase, Pmin is the minimum total power per phase. The smaller the power discrepancy percentage, the better.
Calculation of the maximum current for each consumer group
After the power consumption has been found for each group, the maximum current can be calculated. It is better to take the demand coefficient (Kc) equal to 1 everywhere, since the use of all elements of one group at the same time is not excluded (for example, you can turn on all household appliances belonging to one consumer group at the same time). Then the formulas for a single-phase and three-phase network will look like:
Icalc = Pcalc / (Unom * cosφ)
for a single-phase network, in this case, the voltage in the network is 220 V,
Icalc = Pcalc / (√3 * Unom * cosφ)
for a three-phase network, network voltage 380 V.
When installing electrical wiring in recent decades, the method using. This is due to a whole set of properties that corrugated pipe, but at the same time, when working with it, you must adhere to certain rules.
You can often find both in theory and in practice the terms triangle and star connection, phase and linear voltage - an interesting one will help to understand their differences.
The cosine value for household appliances and incandescent lighting is assumed to be 1, for led lighting- 0.95, for fluorescent lighting - 0.92. The arithmetic mean cosine is found for the group. Its value depends on the cosine of the device that consumes the greatest power in this group. Thus, knowing the currents in all sections of the wiring, you can start choosing the cross-section of wires and cables.
Selection of cable cross-section by power
At known values the calculated maximum current, you can proceed to the selection of cables. This can be done in two ways, but the easiest way is to select the required cable cross-section according to the tabular data. Parameters for the selection of copper and aluminum cable are shown in the table below.
Table 2. Data for selection of cable cross-section with copper conductors and aluminum cable.
When planning electrical wiring, it is preferable to choose cables from the same material. The connection of copper and aluminum wires with ordinary twisting is prohibited by the rules fire safety, since when the temperature fluctuates, these metals expand in different ways, which leads to the formation of gaps between the contacts and the release of heat. If it becomes necessary to connect cables from different materials, it is best to use the terminals specially designed for this.
Video with formulas for calculating cable cross-section
Cable power table it is required to correctly calculate the cable cross-section, if the power of the equipment is large, and the cable cross-section is small, then it will heat up, which will lead to the destruction of the insulation and the loss of its properties.
To calculate the resistance of a conductor, you can use the Conductor Resistance Calculator.
For the transmission and distribution of electric current, the main means are cables, they provide normal work everything related to electric shock and how high-quality this work will be depends on the right choice cable cross-section by power... A handy table will help you make necessary selection:
|
Current section |
||||
|
Voltage 220V |
Voltage 380V |
|||
|
Current. A |
Power. kW |
Current. A |
Power, kWt |
|
|
Cross section Current |
Aluminum conductors of wires and cables |
|||
|
Voltage 220V |
Voltage 380V |
|||
|
Current. A |
Power. kW |
Current. A |
Power, kWt |
|
But in order to use the table, it is necessary to calculate the total power consumption of devices and equipment that are used in a house, apartment or other place where the cable will be routed.
An example of power calculation.
Let's say that a closed electrical wiring is being installed in the house with an explosive cable. It is necessary to rewrite the list of equipment used on a sheet of paper.
But how now find out the power? You can find it on the equipment itself, where there is usually a tag with recorded basic characteristics.
Power is measured in Watts (W, W) or Kilowatts (kW, KW). Now you need to write the data and then add it up.
The resulting number is, for example, 20,000 W, this will be 20 kW. This figure shows how much energy is consumed by all electrical consumers together. Next, you should consider how many devices will be used simultaneously for a long period of time. Let's say it turned out 80%, in this case, the coefficient of simultaneity will be 0.8. We calculate the cable cross-section by power:
20 x 0.8 = 16 (kW)
To select a cross-section, you will need a cable power table:
|
Current section |
Copper conductors of wires and cables |
|||
|
Voltage 220V |
Voltage 380V |
|||
|
Current. A |
Power. kW |
Current. A |
Power, kWt |
|
|
10 |
15.4 |
|||
If the three-phase circuit is 380 Volts, then the table will look like this:
|
Current section |
Copper conductors of wires and cables |
|||
|
Voltage 220V |
Voltage 380V |
|||
|
Current. A |
Power. kW |
Current. A |
Power, kWt |
|
|
16.5 |
||||
|
10 |
15.4 |
|||
These calculations are not particularly difficult, but it is recommended to choose a wire or cable with the largest cross-section of conductors, because it may be that it will be necessary to connect some other device.
Additional cable power table.
The quality of the electro installation works affects the safety of the whole building. The determining factor in carrying out such work is the indicator of the cable cross-section. To carry out the calculation, you need to find out the characteristics of all connected electricity consumers. It is necessary to calculate the cable cross-section for power. The table is needed to view the required indicators.
High quality and suitable cable ensures safe and durable operation of any network
The optimal cable cross-sectional area allows the maximum amount of current to flow without heating up. When carrying out a wiring project, it is important to find correct value for a wire diameter that would be suitable for certain conditions of power consumption. To perform calculations, you need to determine the indicator of the total current. In this case, you need to find out the power of all equipment that is connected to the cable.
Before work, the cross section of the wire and the load are calculated. The table will help you find these values. For a standard 220 volt network, the approximate current value is calculated as follows, I (current) = (P1 + P2 +…. + Pn) / 220, Pn - power. For example, the optimal current for an aluminum wire is 8 A / mm, and for a copper wire - 10 A / mm.
The table shows how to carry out calculations, knowing specifications
Load calculation
Even after determining the desired value, you can make certain corrections for the load. After all, not often all devices work simultaneously in the network. To make the data more accurate, it is necessary to multiply the cross-section value by Kc (correction factor). If all equipment is turned on at the same time, then this coefficient does not apply.
To perform the calculations, use the table for calculating the cable cross-section for power correctly. It should be borne in mind that there are two types of this parameter: reactive and active.
An alternating current flows in electrical networks, the indicator of which can vary. Active power is needed to calculate the average. Electric heaters and incandescent lamps have active power. If there are electric motors and transformers in the network, then some deviations may occur. At the same time, reactive power is formed. In calculations, the reactive load indicator is reflected in the form of a coefficient (cosph).
Useful information! In everyday life, the average value of cosph is 0.8. And for a computer, this figure is 0.6-0.7.
Length calculation
Length calculations are necessary in the construction of production lines when the cable is subjected to heavy loads. For calculations, use the table of cable cross-sections for power and current. When the current moves along the lines, power losses appear, which depend on the resistance that appears in the circuit.
By technical parameters, most great importance voltage drop should not be more than five percent.
Using the wire size table by power
In practice, a table is used to carry out calculations. The calculation of the cable cross-section by power is carried out taking into account the shown dependence of the current and power parameters on the cross-section. There are special standards for the construction of electrical installations, where you can see information on the measurements you want... Common values are shown in the table.
To select a cable for a certain load, it is necessary to carry out some calculations:
- calculate the indicator of the current strength;
- round to the highest using the table;
- pick up the closest standard parameter.
Related article:
Video step by step installation will allow you to do all the work yourself without contacting specialists. What needs to be prepared for work and how to avoid mistakes, we will tell you in the article.
Formula for calculating power by current and voltage
If there are already any cables available, then a vernier caliper should be used to find out the desired value. In this case, the cross section is measured and the area is calculated. Since the cable has a rounded shape, the calculation is made for the area of the circle and looks like this: S (area) = π (3.14) R (radius) 2. You can correctly determine, using the table, the cross-section of the copper wire in terms of power.
Important information! Most manufacturers reduce the cross-section to save material. Therefore, when making a purchase, use a caliper and measure the wire yourself, and then calculate the area. This will avoid overload problems. If the wire consists of several twisted elements, then you need to measure the cross section of one element and multiply by their number.
What examples are there?
A certain scheme will allow you to do right choice cable cross-sections for your apartment. First of all, plan where the lights and sockets will be placed. You should also figure out which technique will connect to each group. This will allow you to draw up a plan for connecting all elements, as well as calculate the length of the wiring. Do not forget to add 2 cm at the joints of the wires.
Determination of the cross-section of the wire, taking into account different types load
Applying the obtained values, the value of the current strength is calculated by the formulas and the cross-section is determined from the table. For example, you need to know the cross-section of the wire for household appliance, the power of which is 2400 watts. We consider: I = 2400/220 = 10.91 A. After rounding, 11 A.
Different coefficients are used to determine the exact figure for the cross-sectional area. These values are especially relevant for a 380 V network.To increase the safety margin, it is worth adding another 5 A.
It should be borne in mind that three-core wires are used for apartments. Using the tables, you can select the closest current value and the corresponding wire cross-section. You can see what wire cross-section is needed for 3 kW, as well as for other values.
At the wires different types its own subtleties of calculations are provided. Three-phase current is used where equipment of significant power is needed. For example, this is used for production purposes.
To identify the required parameters in production, it is important to accurately calculate all the coefficients, as well as to take into account the power losses during voltage fluctuations. When performing electrical work at home, you do not need to carry out complex calculations.
You should be aware of the differences between aluminum and copper wire. The copper version is more high price, but at the same time surpasses the analogue in terms of technical characteristics. Aluminum products can crumble at the folds, and also oxidize and have a lower thermal conductivity. For safety reasons, only copper products are used in residential buildings.
Basic materials for cables
Since the alternating current moves through three channels, a three-core cable is used for installation work. When installing acoustic devices, cables are used that have minimum value resistance. This will help improve signal quality and eliminate possible interference. To connect such structures, wires are used, the size of which is 2 * 15 or 2 * 25.
Pick up optimal indicator cross-sections for use in everyday life will help some average values. For sockets, it is worth purchasing a 2.5 mm2 cable, and for lighting design - 1.5 mm2. Equipment with a higher capacity requires a cross-section of 4-6 mm2.
A special table will help if you have any doubts in the calculations. To determine accurate indicators, it is necessary to take into account all factors that affect the current in the circuit. These are the length of the individual sections, the method of installation, the type of insulation and permissible value overheating. All data helps to increase productivity on a production scale and more efficient use of electrical energy.
Calculation of the cable and wire cross-section for power and current, for connecting a private house (video)
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In order for the wiring to function flawlessly, it is important to choose the right cross-section of the wires and make a competent calculation of the power, because other characteristics depend on these indicators. Current flows through wires in the same way that water flows through pipes.
The safety of the entire room depends on the quality of the electrical installation work carried out. It is especially important here to choose the right parameter such as the cable cross-section. In order to calculate the power cable cross-section, 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.
The current moves through wires just like water flows through a pipe. How in water pipe you cannot put a liquid of a larger volume, and it is impossible to pass more than a certain amount of current through the cable. In addition, the cost of a cable directly depends on its cross-section. The larger the cross-section, the higher the price of the cable will be.
A water pipe with a cross section that is larger than needed is more expensive, and too narrow will not allow the right amount water. The same thing happens with the current, with the only difference that the choice of a cable with a cross-section less than a given value is much more dangerous. Such a wire overheats all the time, the current power in it increases. Because of this, the light in the room will be arbitrarily cut off, and in worst case a short circuit will occur, a fire will start.
There is nothing wrong with the fact that the selected cable cross-section will be more 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 work will immediately increase at least 2-3 times, because the main costs of power supply are precisely in the cost of wires.
A correctly selected section will allow:
- avoid overheating of wires;
- prevent short circuit;
- save on repair costs.
Calculation using formulas
Sufficient cross-sectional area will allow the maximum current to pass through the wires without overheating. Therefore, when designing electrical wiring, first of all, they find the optimal wire cross-section, depending on the power consumption. To calculate this value, the total current must be calculated. It is determined based on the power of all devices connected to the cable.
To choose the optimal wire cross-section, knowing the power, one should recall Ohm's law, as well as the rules of electrodynamics and other electromechanical formulas. So, the current strength (I) for a section of a network with a voltage of 220 Volts, and it is this voltage that is used for a home network, is calculated by the formula:
I = (P1 + P2 +… + Pn) / 220, where:
(P1 + P2 +… + Pn) - total power of each electrical appliance used.
For networks with a voltage of 380 volts:
I = (P1 + P2 +… + Pn) / √3 / 380.
Power indicators 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 |
| Accumulative water heater | 1200-1500 | Juicer | up to 1000 |
| Instantaneous water heater | 2000-5000 | Washer | 3000 |
| Hood (ventilation) | 500-1000 | Washer-dryer | 3500 |
| Grill | 1200-2000 | Hand dryer | 800 |
| Oven | 1000-2000 | Television | 100-400 |
| A computer | 400-750 | Toaster | 600-1500 |
| Air conditioning | 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 |
| Combined microwave oven | 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 a simplified calculation. Detailed calculations take into account the value permissible loads which for copper cable will be 10A / mm², and for aluminum - 8 A / mm². The load determines how strong the current can pass through a unit of area without hindrance.
Power factor correction
Also, when calculating, an amendment is added in the form of a demand coefficient (Kc). This coefficient shows which devices are used in the network constantly, and which ones for a certain time. A special calculator and tables showing the power calculation simplify all these calculations.
Demand coefficients of auxiliaries receivers (Kс)
But what to do if the characteristics indicate 2 types of power: active and reactive? Moreover, the first of them measures in kV, familiar to everyone, and the second - in kVA. An alternating current flows in our networks, the magnitude of which changes over time. Therefore, for all consumers there is active power, which is calculated as the average value of all instantaneous alternating current and power. Devices with active power include incandescent lamps, electric heaters. For such energy consumers, the phases of the current and voltage coincide. If in electrical circuit units that store energy, for example, transformers or electric motors, are involved, then they may have deviations in amplitude. Due to this phenomenon, reactive power arises.
For networks where there is reactive and active power, one more correction must be taken into account - the power factor (cosφ) or the reactive component.
Thus, the formula is obtained:
S = Kс * (P1 + P2 +… + Pn) / (220 * cosφ * Рд), where:
- S - cross-sectional area,
- Рд - permissible load value.
In addition, possible current losses that occur during the passage through the wires are considered. When using a cable with several cores, the loss must be multiplied by the number of these cores.
Important! All these calculations will require not only a calculator, but also a deep knowledge of physics. It will not be possible to make an accurate calculation right away without theoretical knowledge.
Search for an 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 declared technical characteristics often do not correspond real value... Therefore, in order to find out how to calculate the power, it is important to be sure that the store offers a suitable electrical wire with a cross section. To do this, we will use a simple formula:
S = 0.785d 2, where:
- d is the diameter of the core;
- S is the cross-sectional area.
You can determine the exact one, you can make the calculation of the section using a caliper or a micrometer, which is more accurate.
If the cable consists of several thin wires, then first they look at the diameter of one of them, and then the data obtained is multiplied by their number:
Scot = n * 0.785di 2, where:
- di is the area of a single wire;
- n is the number of wires;
- Stot - total area section.
Calculation tables
It is not entirely correct to resort to complex calculations for the calculation every time. The industry produces wires of a certain section. If, after accurate calculations and calculations, you get a cable cross-section in terms of power measuring 3.2 square millimeter, 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 cross-section of the cable, a table is needed where all the data are regulated, and also compiled in accordance with the PUE - the rules for electrical installations.
In order to determine the cross-section of the cable at a known load, you must:
- calculate the current strength;
- round up to a higher value according to the data in the table;
- then find the closest standard cross-section value.
Permissible continuous current for wires and cords with rubber and PVC insulation with copper conductors
| Current section wire power vein, mm 2 | Current, A, for wires laid | |||||
|---|---|---|---|---|---|---|
| Open- then | 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 is easy to make such a calculation. First, you need to determine the total power of all electrical appliances used in the network. For this, a table will be needed, and the missing data for each electrical appliance can be taken from the product passport. The resulting amount must be multiplied by 0.8 - the demand coefficient, if electrical appliances will not be used all at once, or left unchanged when permanent work... Now the resulting value must be divided by the voltage in the network and add a constant value 5. This will be the required current indicator. Let's say the current is 20A.
Note! In residential premises, a three-core electrical wire is used and closed wiring... This must be remembered when the calculation is done according to the table.
Next, you need a table from the PUE. We take the column where the current values for a three-core conductor are given, and we select the closest ones: 17 and 22. It is better to take the cross-section with a margin, therefore, in the example under consideration, the desired value will be 22. As you can see, this value corresponds to a three-core cable with a cross section of 2 mm 2 ...
You can additionally consider how this calculation is made for 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 during overhaul it is recommended to replace it. In addition, the aluminum electrical wire crumbles at the folds and has less conductivity at the joints. Bare parts of aluminum are rapidly oxidized in air, which leads to significant losses of electricity at the junction.
Calculator
Today, experts use not only a table to determine the cross section, but also a special calculator. 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:
- variable or D.C. used;
- wire material;
- the power of all used devices;
- mains voltage;
- power supply system (one or three-phase);
- type of wiring.
These indicators are loaded into the calculator and received required value cross-sectional wires.
Length calculation
It is important to calculate the cross-section along the length in the construction of industrial-scale networks, when the sections are subjected to constant heavy loads, and the cable must be pulled over considerable distances. Indeed, during the passage of current through the wires, power losses occur due to electrical resistance in the chain. The power loss (dU) is calculated as follows:
dU = I * p * L / S, where:
- I is the current strength;
- p - resistivity(copper - 0.0175, aluminum - 0.0281);
- L is the cable length;
- S is the cross-sectional area already calculated by us.
According to technical specifications, the maximum voltage drop along the length of the wire should not exceed 5 percent. Otherwise, you should choose a wire with a larger cross-section.
Peculiarities
There are certain standards according to which the cable 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 a lighting group and the rest; for powerful electrical appliances, for example washing machines or electric ovens use a separate wire connection; the standard cross-section of the wire for the lighting group in the apartment is 1.5 mm 2, and for the rest of the wires - 2.5 mm 2. Such standards are used because the rated input current cannot be greater.
Three-phase current is required when high power production value devices are used. Therefore, to determine the cable cross-section at enterprises, it is necessary to accurately calculate all additional factors, and also be sure to take into account power losses and voltage fluctuations. For electrical work in an apartment or a private house, such complex calculations are not carried out.
For the installation of acoustic equipment, wires with a minimum resistance are used. This is necessary in order to remove distortions as much as possible and improve the quality of the transmitted signal. Therefore, for speaker systems, cables with a size of 2x2.5 or 2x1.5 are better suited with a length of at least 3 meters, and the subwoofer is connected with the shortest cable 2.5-4 mm 2.
Examples of
Consider a general scheme for choosing a 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 involved at each output;
- Now you can draw up a general connection diagram and calculate the cable length, adding at least 2 cm to the wire connections;
- Based on the data obtained, we consider the size of the cable section according to the formulas given above.
I = 2400W / 220V = 10.91A, round up and get 11A.
As we already know, for precise definition cross-sectional area, different coefficients are used, but almost all of these data refer to a network with a voltage of 380V. To increase the safety margin, add another 5A to our current value:
Three-core cables are used for apartments. The table will show the current value close to our 16A, it will be 19A. We get that to install one washing machine a wire with a cross section of at least 2 mm 2 is required.
General theory
For determining optimal section cables for household needs, in general, the following rules are used:
- for sockets, wires with a cross section of 2.5 mm² are required;
- 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 cross-section of the cable, all factors affecting the passage of current through the circuit are taken into account. These include:
- type of wire insulation;
- the length of each section;
- way of laying;
- temperature regime;
- humidity;
- permissible overheating value;
- power difference of current receivers in one group.
All these indicators help to improve the efficiency of electricity use on an industrial scale, as well as to avoid overheating.
Section selection. Video
In this video, the master shares his experience in choosing the cable cross-section and the nominal value of the machine. He points to possible mistakes and gives good advice beginners.
If, after reading the article, there are still some doubts, then the table or calculator described above will help you find the exact wire cross-section in terms of power.