When choosing a cable for a home or apartment to connect an electrical device, it is necessary to calculate the cross-section of the conductor that will be used for connection.
To determine the amount of load and electricity consumed, it is necessary to carry out appropriate calculations to select the correct cable.
In the article we will talk about how to determine the cross-section of a wire by current and power and what is required for this.
Cable cross-section
First, let's determine that the cable cross-section is the cut area of the current-carrying core.
If there is only one wire in a round core, then the cross-sectional area is determined in the same way as the area of the circle.
If there are several wires in the core, then the cross-section is considered to be the sum of all cross-sections of the wires located in this section.
In Russia, the official document regulating cross-section standards according to GOST is the “Rules for the Construction of Electrical Installations” or PUE. And the cross-section value is indicated in tables called “Permissible current loads on the cable.”
The section is selected based on the criteria:
- current strength;
- power;
- material type;
- number of supplied phases.
Wires with a large cross-section are safe because they can withstand heavy loads, but their cost sometimes incurs unjustified costs. It will be correct to install the electrical network in the house based on the planned connection of devices and equipment, choosing the optimal cross-section for this.
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
Permissible heating temperatures of current-carrying cable cores
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
Cable correction factors depending on ambient temperature
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.
Calculation of wire cross-section
If it is not possible to use a table, you can determine the cross section using a formula and a caliper. The formula is as follows: S=0.785*D², in which D is the diameter of the core being measured.
If there are more than one wires, then you will have to count the number of wires in one core, then measure the diameter of one, calculate using the formula and multiply by the number of wires in the core. The resulting indicator will be the cross-section of the core.
Relation between current and cross section
The ratio of cross-section and current is the main criterion when selecting electrical wires. After all, the larger the area and current strength that passes through the cord, the greater the load it can withstand.
To determine the load, it is necessary to calculate how many electrical appliances and what power will be connected to each loop.
You can find out the power consumption of the device from the passport or from the information on the tag.
For example, a kettle consumes 1.5 kW, and a refrigerator only 0.8 kW.
Introduction
A cable is a wire covered with insulation that serves to transmit electricity from a source to a consumer. Today's market is ready to offer customers many types of similar wires: aluminum, copper, single-core, multi-core, single- and double-insulated, with a cross-section from 0.35 mm2 to 25 mm2 or more. But most often, to connect household consumers, cables with a thickness of 0.5 to 6 square meters are used - this is quite enough to power any equipment.
Classic cable for wiring in an apartment
Why is it necessary to select insulated conductors, and not buy the first one you come across? The thing is that the thickness of the conductor determines the current strength it can withstand. For example, the permissible current for copper wires 1 mm thick is up to 8 Amperes, aluminum – up to 6 Amps.
Why not just buy the thickest wire possible? Because the thicker it is, the more expensive it is. In addition, a thick cable needs to be hidden somewhere, a groove must be cut for it in the ceiling and walls, and holes must be made in the partitions. In a word, there is no point in overpaying, because you won’t be driving a KAMAZ to buy bread.
If you choose a wire with a smaller diameter, it simply cannot withstand the current passing through it and will begin to heat up. This leads to insulation melting, short circuit and fire. Therefore, you should never rush when choosing a high-quality cable for connecting any devices - first think about what exactly will work on the new line, and then choose the thickness and type of cable.
What is the difference between a cable and a wire?
Most often, the terms cable and wire are used interchangeably. And a simple person, using these words, means the same thing by them. However, if you delve deeper into this topic, you can see that these are different things.
For example, a wire is one conductive core enclosed in a sheath, and a cable is a collection of several insulated wires combined into one.
Therefore, when calculating, it is necessary to take the cross-section of the wire, and not the entire cable.
Cable selection
First, you need to decide what you need the wire for, and then determine the wire from what material is most suitable for the room, and which one is specifically needed: stranded or single-core.
Each of them has both advantages and disadvantages.
Single or stranded
When installing wiring, wires of the PVC, VVGng, and PPV brands are usually used.
In addition, if there will be no impact on the wire and it will be in a stationary state, it is better to choose a stranded wire.
Due to the large number of conductors, stranded wires will oxidize much more, which means this is detrimental for rooms with temperature changes or high humidity. But it is worth noting that stranded wire is more flexible.
Copper or aluminum
Currently, only copper wiring is used, especially in residential buildings. Despite the fact that aluminum was previously used for reasons of economy, copper is now used due to its quality.
Aluminum is brittle and also very difficult to weld or solder.
In terms of conducted power, copper wire also wins; the difference in the conducted current differs by more than 1 kW. The power advantage of copper wire is significant.
Related video: Selecting cable cross-section
Materials for making wires
As a rule, installation of electrical wiring in a private house or apartment is done using three-core wires. Moreover, each core has separate insulation, they all have different colors - brown, blue, yellow-green (standard). The core is the part of the wire through which current flows. It can be either single-wire or multi-wire. Some brands of wire use a cotton braid over the cores. Materials for making wire cores:
- Steel.
- Copper.
- Aluminum.
Sometimes you can find combined ones, for example, stranded copper wire with several steel conductors. But these were used for field telephone communications - the copper ones transmitted a signal, and the steel ones were used for the most part for attaching to supports. Therefore, this article will not discuss such wires. For apartments and private houses, copper wire is ideal. It is durable, reliable, and its performance is much higher than that of cheap aluminum. Of course, the price of copper wire is steep, but it is worth mentioning that its service life (guaranteed) is 50 years.
Why is the calculation made?
Calculation of the wire cross-section by power is required to ensure that the wires and cables meet the necessary requirements and standards, both for power and safety and reliability.
To prevent melting or short circuits from occurring, it is worth calculating the cross section in advance.
Useful video - How to choose wire cross-section
What you need to know
One of the main criteria for calculating the cross-section is the permissible current load of the wire. To determine the current value, you need to add up all the connected devices in the apartment.
The cable cross-section for power and the cable cross-section for current are calculated. 220 V and 380 V networks are common.
After calculating the cross-section, the current can be calculated using the following formula for a single-phase 220V network:
I = (P x Ki) / (U x cos φ)
where:
- P—total power of all electrical appliances, W;
- U—mains voltage, V;
- KI = 0.75 - coefficient of simultaneity;
- cos φ is an indicator that means connected electrical equipment.
To calculate the current in a three-phase 380V network:
I = P /(√3 x U x cos φ)
After finding the current strength, the cross-section can be found from the table of cable and wire cross-sections. If the resulting value does not coincide with the table value, the closest closest value is selected.
The length of the wire affects the voltage. The longer the line, the greater the voltage loss.
To avoid this, you need to increase the cross-section of the conductor. Since the wire has its own resistance (R), the calculation is made taking into account the following data using the formula:
R = p × L/S
Where:
- R – conductor resistance, Ohm;
- p – resistivity, Ohm mm²/m;
- L – wire length, m;
- S – cross-sectional area, mm².
Which wire is better to use
Feel free to choose copper wire, as it has the following advantages:
- stronger and softer;
- less susceptible to corrosion and oxidation;
- Conductivity is higher and can withstand higher current load.
The only negative is the cost. It is several times more expensive than the price of aluminum ones, but it is completely justified.
A few basic concepts
Why is it necessary to calculate the cross-section of wires? Is it possible to limit the selection “by eye”? No, you can’t, because it’s not at all difficult to go to two extremes:
- A conductor of insufficient cross-section begins to overheat greatly. This leads to melting of the wiring insulation, creating conditions for spontaneous combustion and short circuits. All this causes destructive fires, often accompanied by human tragedies.
- Conductors of excess diameter, of course, do not pose such dangers. But they are also significantly more expensive (especially if we are talking about copper cables) and are not so convenient to use. The result is completely unjustified material and labor costs.
So we should be guided by the principle of reasonable sufficiency. Moreover, anyone who has at least a little understanding of the basics of mathematics and physics can make the necessary calculations.
To begin with, let us recall some concepts that are probably already well known to many. But just so that in the further presentation there are no discrepancies.
So, one wire can be used as a conductor in wires and cables - from the point of view of electrical conductivity, this is the best option.
But to achieve flexibility in cable products, it is necessary to use more complex structures - many thin wires, usually twisted into a “pigtail”. The more such wires, the more flexible the conductor becomes.
However, this should not be confused with stranded wire. A separate conductor means a separate conductor. To make it clearer, look at the illustration.
The picture below shows examples of single-core wire. It’s just that on the left side there is a rigid single-wire version, and on the right side there is a more flexible multi-wire version.
Both left and right are single-core wires.
If a wire (cable) structurally combines two or more conductors isolated from each other, it becomes two-core, three-core, etc. But it can also remain single- or multi-wire.
Double stranded wire
Three-core power cables - with single-wire or multi-wire cores Rigid single-wire products are good for fixed sections of wiring, for example, embedded in walls. Stranded wires and cables are great for areas where mobility is needed - typical examples include power cords for household appliances and lighting fixtures.
So, all subsequent calculations will be carried out for the cross-section of the wire or cable core.
When assessing the conditions for the location of wires in the future, there may be options when you have to imagine the difference, for example, between three single-core wires stretched in one pipe, or one three-core cable.
On the left is the diameter of the conductor (core), measured in millimeters. On the right is the cross-sectional area of the conductor, measured in mm². All reference books usually use the cross-section parameter, since it is by this criterion that the classification of various brands of wires and cables is made.
But this is good if the brand of cable (wire) is known. If not, then the cross-section remains to be calculated based on the diameter, which can be measured with a caliper or micrometer.
The diameter of the core (wire) can be measured as usual. Sectional area - calculation only.
Everyone should probably remember the formula for the area of a circle. But nevertheless, we will present it just in case.
We invite you to familiarize yourself with: Cable power calculation formula
Sc = π × d² / 4 ≈ 3.14 × d² / 4 ≈ 0.785 × d²
The “approximately equal” sign is used only because the number π is rounded to hundredths, the well-known value π≈ 3.14. But in our case, such accuracy is more than enough!
Nothing complicated either. The vein is fluffed up so that it becomes possible to count the number of wires in the “pigtail”. And all that remains is to measure the diameter of one wire with a micrometer or caliper.
Sc = n × π × d² / 4 ≈ n × 3.14 × d² / 4 ≈ 0.785 × n × d²
where n is the number of wires in one core.
Calculation of the cross-section of copper wires and cables
Let's look at an example of calculating copper cables in a socket group installed in the kitchen.
We choose a wire with a cross-section of 2.5 mm2, but this requires 3-4 sockets. If there is only one socket, then the wire cross-section should be at least 2 times larger. There is enough wire in the room with a cross section of 1.5 mm2.
But it’s still better to make calculations and find out specific values.
Selection of cable cross-section by power
Another component that can affect the cross-section is temperature. The higher the temperature, the higher the electrical resistance of the conductor. Therefore, 18 C should be taken as the standard value.
Based on this temperature, tabular values have been compiled.
Since aluminum wires are used, for example, in street wiring, tables depending on the cross-section for current and power consumption have been compiled for aluminum cables as well.
In addition, if the wiring is intended for other purposes: connecting boilers in the bathhouse, heaters, stoves, then the wire cross-section is calculated individually.
How to choose conductor cross-sections
There are several more criteria that the cross-section of the wires used must meet:
- Length of cable. The longer the wire, the greater the current loss observed in it. This again occurs as a result of an increase in resistance, which increases as the length of the conductor increases. This is especially noticeable when using aluminum wiring. When using copper wires to organize electrical wiring in an apartment, the length, as a rule, is not taken into account - the standard margin of 20–30% (for hidden wiring) is more than enough to compensate for possible increases in resistance associated with the length of the wire.
- Type of wires used. There are 2 types of conductors used in household electricity supply - copper or aluminum based. Copper wires are of better quality and have less resistance, but aluminum wires are cheaper. In full compliance with the standards, aluminum wiring copes with its tasks no worse than copper, so you need to carefully weigh your choice before purchasing a wire.
- Electrical panel configuration. If all the wires supplying consumers are connected to one circuit breaker, then it will be the weak point in the system. A heavy load will lead to heating of the terminal blocks, and non-compliance with the rating will lead to its constant operation. It is recommended to divide the electrical wiring into several “beams” with the installation of a separate machine.
In order to determine the exact data for choosing the cross-section of electrical wiring cables, it is necessary to take into account any, even the most insignificant parameters, such as:
- Type and type of insulation of electrical wiring;
- Length of sections;
- Laying methods and options;
- Features of temperature conditions;
- Humidity level and percentage;
- The maximum possible value of superheat;
- The difference in the powers of all current receivers belonging to the same group. All these and many other indicators can significantly increase the efficiency and benefits of energy use on any scale. In addition, correct calculations will help avoid cases of overheating or rapid abrasion of the insulating layer.
In order to correctly determine the optimal cable cross-section for any human household needs, it is necessary in all general cases to use the standardized following rules:
- for all sockets that will be installed in the apartment, it is necessary to use wires with an appropriate cross-section of 3.5 mm²;
- for all spotlighting elements, it is necessary to use electrical wiring cables with a cross-section of 1.5 mm²;
- As for high-power devices, cables with a cross-section of 4-6 mm² should be used.
If some doubts arise during the installation or calculation process, it is better not to act blindly. The ideal option would be to refer to the appropriate table of calculations and standards.
| Cross-section of conductors (mm) | Copper conductors of wires and cables | |||
| Voltage 220 V | Voltage 380 V | |||
| Current (A) | Power, kWt) | Current (A) | Power, kWt) | |
| 1,5 | 19 | 4,1 | 16 | 10,5 |
| 2,5 | 27 | 5,9 | 25 | 16,5 |
| 4 | 38 | 8,3 | 30 | 19,8 |
| 6 | 46 | 10,1 | 40 | 26,4 |
| 10 | 70 | 15,4 | 50 | 33 |
| 16 | 80 | 18,7 | 75 | 49,5 |
| 25 | 115 | 25,3 | 90 | 59,4 |
| 35 | 135 | 29,7 | 115 | 75,9 |
| 50 | 175 | 38,5 | 145 | 95,7 |
| 70 | 215 | 47,3 | 180 | 118,8 |
| 95 | 265 | 57,2 | 220 | 145,2 |
| 120 | 300 | 66 | 260 | 171,6 |
Aluminum cable cross-section table
| Cross-section of conductors (mm) | Aluminum conductors of wires and cables | |||
| Voltage 220 V | Voltage 380 V | |||
| Current (A) | Power, kWt) | Current (A) | Power, kWt) | |
| 2,5 | 22 | 4,4 | 19 | 12,5 |
| 4 | 28 | 6,1 | 23 | 15,1 |
| 6 | 36 | 7,9 | 30 | 19,8 |
| 10 | 50 | 11 | 39 | 25,7 |
| 16 | 60 | 13,2 | 55 | 36,3 |
| 25 | 85 | 18,7 | 70 | 46,2 |
| 35 | 100 | 22 | 85 | 56,1 |
| 50 | 135 | 29,7 | 110 | 72,6 |
| 70 | 165 | 36,3 | 140 | 92,4 |
| 95 | 200 | 44 | 170 | 112,2 |
| 120 | 230 | 50,6 | 200 | 132 |
The safety of the facility directly depends on the correctly selected cable cross-section - therefore, it is necessary to approach the selection procedure with all responsibility. It is also recommended to consult with specialists before purchasing wires - an experienced electrician will suggest the best option.
Savings when purchasing often backfire - often apartment or house owners purchase aluminum cable instead of copper, not taking into account the fact that its cross-section should be larger. As a result, the installed electrical wiring gets very hot, and within a fairly short period of time a complete replacement of the wires is required, which will take a serious toll on the homeowner’s wallet. In addition, this is also extremely dangerous - many people who like to save money ended up without a roof over their heads.
If you have doubts about your own abilities, it is recommended to contact a specialist - only in this case can you guarantee safety for residents and the duration of operation of the new electrical wiring.
Current loads in DC networks
For DC consumers, it is important that the voltage at the ends is not lower than 0.5 V.
There is a formula that determines how much the voltage at the ends will drop compared to the base voltage, depending on the length of the conductor, its specific resistance and current strength in c epi:
U = ((pl) / S) I where:
- U — direct current voltage, V;
- p — specific resistance of the wire, Ohm*mm2/m;
- l — wire length, m;
- S—cross-sectional area, mm2;
- I - current strength, A.
Knowing all the values, you can calculate the wiring cross-section. This rule also works for circuit breakers for networks.
The cable that transmits electric current is an important element of the entire system. Calculating its performance and power gives confidence in the security of the entire network and the devices connected to it.
What is needed for load calculations
The main indicator that helps to calculate the cross-section and brand of cable is the maximum permissible long-term load.
A simple arithmetic summation of the powers of all electrical appliances that will be connected to the network is necessary.
Having received the exact value of the current, you should refer to the tables that allow you to find a cable or wire of the required cross-section and material.
Calculation for premises
The total load in the room consists of power and lighting. Remember that the kitchen and bathroom are the most loaded lines, so you should be especially careful when choosing cables in these two locations.
And distribute the load across different socket groups, rather than using a pilot for 5-6 sockets.
Keep in mind that consumption and power should be calculated individually and using cable power tables for each room in the apartment.
Closed or open wiring
The load also depends on the type of wiring: closed or open.
For example, if the house is made of brick or concrete, the wiring should be covered with plaster, and if it is wooden, then the wiring should run along a special treated surface.
Thus, calculating the cross-section of wires is an important process. It will take little time, but you will be confident in the correctness and safety of the wires you have selected.
