Rated current across the cable cross-section. How to choose cable cross-section

Difference between cable and wire

The question, by the way, is not simple. In particular, in accordance with SN, from the times of the USSR to the present day, work with cables is more expensive than with wires. However, there was no very clear classification in this regard, either in past times or today. Different sources provide a variety of perspectives. In practice, the characteristic “cable” or “wire” is assigned by GOST/TU for the production of a specific brand. In particular, the GDP brand cable from Odeskabel OJSC differs from the PVS brand wire only in the configuration of the sheath: the GDP cable is flat, and the PVS wire is round. And in no reference book about cables is the shape of the cable/wire sheath indicated as an insignificant factor. Therefore, you need to look at the certificate - it will certainly be stated there: this is a cable or wire.

Calculation of cable cross-section

There are reference plates indicating what cross-section of aluminum/copper conductor is needed for the assigned load. However, most electricians use a simple formula (consider a load of 8 kW): a copper cable cross-section of 1 mm2 can pass through 10A or 2.2 kW (power = 10A x 220V).

Therefore, the load in 8 kW in A will be equal to 36 A(load = 8 kW/220V), and for such a volume of current a cable with a cross-section of equals 4mm2.

This calculation is more or less suitable for cables with a cross-section of no more than 6 mm2. For large cross-sections, tables of “Permissible current loads” are needed.

With an equal load, the cross-section of the aluminum cable should be almost 30% larger than that of copper. The cable cross-section is the cross-sectional area of ​​the core that conducts current.

The cross-section of a round current-carrying cable core is obtained according to the formula for the area of ​​a circle S = π × r2, where the number π=3.14, and r is the radius.

When there are a couple of wires in the core, then the cross-section of the core will be equal to the sum of the cross-sections of all the wires. The radius of the wire is measured with a caliper, and for very thin wires with a micrometer. How much cross-sectional margin is required? The reserve will undoubtedly not be superfluous. However, you need to know the limit.

For example, the limit of ordinary household socket-switches is 16A (3.2 kW = 16A x 220V) and connecting the socket using a 4 mm2 cable, with a throughput of 8 kW, is an inappropriate expenditure of finances.

And also, a cable with a cross-section of 4 mm2 will not fit into every socket.

Rational cross-sections in household electrical networks for copper: 1.5-2.5 mm2 for sockets and 0.75-1.5 mm2 for lighting.

Which cable to choose: copper or aluminum?

Many “experts” will say with absolute certainty - copper. Why? For the consumer, copper, compared to aluminum, is advantageous in that sooner or later copper does not deteriorate so quickly, and this is very significant when replacing lamps, etc. Whether it is necessary to pay three times more for this is a decision for the consumer.

You only need to connect copper and aluminum cables using a terminal block so that the aluminum does not come into contact with the copper.

Because due to some physical phenomena at the point of contact between aluminum and copper, the current resistance increases after some time. As a result, the connection point heats up extremely intensely, the cable breaks, a short circuit occurs, and in extreme cases, a fire. By the way, the connection of any heterogeneous materials with different resistance leads to a similar result.

As a result, there is no need to sharpen the wiring with the first wire you come across by twisting it.

Depending on the area of ​​use of the cable, the conductor is made from a variety of materials: first copper and aluminum, then nichrome, steel, etc. When you are not assured of the uniformity of the material of the cables being connected, use a terminal block.

Which cable is optimal: flexible or rigid?

A rigid cable is usually a single-core cable, and a flexible cable is usually a multi-core cable. The greater the number of wires in the core and how thinner each wire is, the more elastic the cable.

Based on flexibility, the cable is divided into 7 classes: monocore is class 1, and class 7 is the most flexible.

As the flexibility class of the cable increases, its price increases. A rigid cable is used for inserting into walls and laying in the ground, and a flexible cable is used for connecting maneuverable devices or electrical appliances. From an operational point of view, it doesn’t matter which cable to choose - rigid or flexible. From an installation point of view, every electrician has his own wishes. By the way: the ends of the flexible cable, which are embedded in sockets (switches), must certainly be soldered or crimped using special ends. For a rigid cable, a similar procedure is not necessary. To connect lighting equipment, it is better to purchase a flexible cable, because lighting devices are often replaced, and a rigid cable is more likely to break when connecting new electrical equipment.

How to independently determine the quality of the cable?

Many manufacturers do not always comply with cable manufacturing standards. Their main “trick” is to underestimate the cross-section of the conductor. And sometimes significantly. Of course, it is difficult to examine the cross section at the point of purchase. In the store you can measure any wire with a caliper and a micrometer.

You can also come across cables with a reduced sheath thickness or with a sheath made of low-quality material, and this reduces the service life of the cable.

For inspection, it’s a good idea to have a piece of the “correct” cable with you as a standard. In stores you can come across a Chinese cable made of aluminum covered with copper (sold as copper with markings in Cyrillic).

It is easy to inspect such a cable: the cut of the current-carrying core on the cable gleams white - it is aluminum.

There are manufacturers who use low-quality copper or aluminum to reduce costs. Such cables have a much lower service life and conductivity than those required by GOST. It is possible to test the quality of the metal of a current-conducting core as follows:

• try bending and straightening the cable a couple of times. In factories, such a test is performed on a special bending mechanism under a certain bending radius. Of course, your number of bends will be less than those provided for in GOST. However, in any case, aluminum should withstand at least 7-8 bends, and copper - 30-40. After this, deformation of the insulation and wire breakage are possible. It is better to carry out the experiment at the end of the cable, so that you can simply cut it off later.
• a cable made of high quality copper/aluminum should bend and not spring;
• the copper/aluminum core on the stripped cable should have a bright (glare) color. When the vein is heterogeneous in color and there are hopeless spots, this indicates large impurities in the metal and its low quality.

Nevertheless, an amateur will not be able to determine the quality of the cable 100% on his own. In this case, there is only one recommendation - rely on the brand and purchase it in large, trusted stores.

What kind of insulation and sheath should the cable have?

It is best when the insulation and sheath of the cable are double insulated. A single-insulated cable has a service life of up to 15 years, while a double-insulated cable usually lasts 2 times longer. Typically, “insulation” and “sheath” are 2 different materials. The insulation is the layer of dielectric material that goes immediately behind the conductive core, and the sheath is all the layers on top of the insulation. The sheath is intended to protect the cable from various mechanical influences. The cable may have a couple of layers of sheath made of different types of material. Certain types of shell that may be useful:

1. Heat-resistant cables are designed for installation in rooms with high temperatures (saunas). Typically, the material used is fluoroplastic, with fiberglass on top. There are no special designations for such cables, i.e. if necessary, you need to seek help from reference books or catalogs, where the value of the “operating temperature” is indicated precisely;
2. non-flammable, marked “ng” - indicates the ability to self-extinguish when the flame disappears, but cannot withstand high temperatures
3. when the cable brand contains “FR” (fire resistant) and then E30, E90 or E120, then this cable can “function” in an open fire for 30, 90 or 120 minutes;
4. cables with a polyethylene sheath can be pulled both in the soil and in an open way (for example, along the walls of houses);
5. cables with insulation and sheath made of PVC (polyvinyl chloride) are used for pulling inside buildings (under plaster) or in cable ducts.

The most famous cable brands

1. wire PPV (copper), APPV (aluminum) in single insulation - for pulling inside walls;
2. cable PVS (copper), GDP (copper) in double insulation - for pulling inside buildings;
3. heat-resistant cables RKGM (copper) - up to 180°C, BPVL (tinned copper) - up to 250°C;
4. cable VVG (copper), AVVG (aluminum) - for stretching along the walls of houses and in the ground;
5. runway cable (copper) water-submersible - for pulling in water;
6. TPP cable (copper) telephone pair - for pulling in the ground;
7. TRP wire (copper) telephone distribution for subscriber communications (switching on the telephone)
8. twisted pair cable UTP, FTP - for organizing computer networks, turning on intercoms, etc.;
9. “Alarm” signal wire for connecting intercoms, security and fire alarms, etc.;
10. coaxial cable RG-6 for connecting TVs, antennas, CCTV cameras.

Internet cable

The concept of “Internet cable” generalizes many types of cable products. A variety of information cables are used to broadcast information. If you mean connecting to the Internet, then you need to check with the operator which cable needs to be laid across the walls. In this case, it is necessary to find out both the brand of the cable and the manufacturer in order to accurately determine compatible cable products.

For example, for the Internet they use a regular television cable TM Finmark, a twisted pair cable or an existing subscriber cable (the so-called “noodles”) to which the telephone is connected.

Optical cables can be laid on dedicated Internet lines.

Computer cable

The term is also general.

As a rule, a twisted pair cable is used to connect PCs with each other and with the server, but other information cables can also be used.

The technology of twisting two wires into a pair has been used in telephony since the last century. Due to the correctly calculated twisting pitch and the quality of the material, a maximum information transfer speed was achieved than that of a standard paired telephone cable. There are quite a few types of twisted pair cables, depending on the number of cores, the diameter of each core, installation locations, etc. Depending on the data transfer speed, twisted pair cables are divided into groups:

• 3rd category (standard telephone cable),
• 5th category (office networks),
• 6th category (new generation cable for changing category 5).

“Twisted pair”, which has gained the greatest popularity in our time, is a category 5 cable made of 8 twisted pairs of cores, the diameter of the core is at least 0.45mm and maximum 0.51mm.

TV cable

This is the common name for coaxial cable with a resistance of 75 ohms.

And " satellite cable" is a coaxial cable. Any 75 Ohm coaxial cable can be used to connect a satellite and any other antenna, and to connect to cable television. The only thing that matters is whether the cable is good or not.

Important characteristics of coaxial cable are signal attenuation and noise immunity.

All other characteristics of the cable are aimed at improving these 2 indicators themselves and are of secondary importance. In particular, our RK cable is made only from copper wire (sometimes even silver-plated), but the attenuation of the RK cable will be almost four times worse than that of any current RG cable made from inexpensive materials: steel and aluminum. This is achieved through special cable production technology.

Content:

Before connecting the load to the network, it is important to ensure that the supply cable cores are thick enough. If the permissible power is significantly exceeded, the insulation and even the core itself may be destroyed due to overheating.

Before calculating the cable cross-section by power, you should calculate the sum of the powers of the connected electrical appliances. In most modern apartments, the main consumers are:

• Refrigerator 300 W
• Washing machine 2650 W
• Computer 550 W
• Lighting 500 W
• Electric kettle 1150 W
• Microwave oven 700 W
• TV 160 W
• Water heater 1950 W
• Vacuum cleaner 600 W
• Iron 1750 W
• Total 10310 W = 10.3 kW

In total, most modern apartments consume approximately 10 kW. Depending on the time of day, this parameter can decrease significantly. However, when choosing a conductor cross-section, it is important to focus on a larger value.

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

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

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

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

Calculation of wire cross-section by power is carried out using the following formula:

I = (P × K u) / (U × cos(φ))

Where,

• I- current strength;
• P- power consumption of all electrical appliances in total;
• K and- simultaneity coefficient, usually the standard value of 0.75 is taken for calculations;
• U- phase voltage, it is 220 (V), but can range from 210 to 240 (V);
• Cos(φ)- for household single-phase appliances this value is unchanged and equals 1.

If you need to quickly calculate the current, you can omit the value of cos (φ) and even K and. The resulting value will differ downward (by 15%) if a formula of this type is applied:

I=P/U

Having found the current using the calculation formula, you can safely proceed to selecting the power cable. More precisely, its cross-sectional area. There are special tables that present data that allows you to compare the current value, power consumption and cable cross-section.

The data varies greatly for conductors made from different metals. Today, for residential electrical wiring, only hard copper cable, aluminum is practically not used. Although in many old houses all lines are laid using aluminum.

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

Calculation of wire cross-section in an apartment - Table

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

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

To calculate the cross-section of the power cable used in a three-phase network, you must use the following formula:

I = P / (√3 × U × cos(φ))

Where,

• I- current strength by which the cross-sectional area of ​​the cable will be selected;
• U- phase voltage, 220 (V);
• Cosφ- phase shift angle;
• P- an indicator of the total power of all electrical appliances.

Cosφ is very important in this formula. Since it directly affects the current strength. It is different for different equipment; most often this parameter can be found in the technical accompanying documentation, or it is indicated on the case.

The total power of consumers is found very simply: all powers are added up, the resulting value is used for calculations.

A distinctive feature of the choice of cable cross-sectional area for use in a three-phase network is that a thinner core can withstand a larger load. The required section is selected according to the standard table.

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

Calculation of the wire cross-section for power in a three-phase network is carried out using a value such as √3 . This value is necessary to simplify the appearance of the formula.

U linear = √3 × U phase

Thus, if necessary, you can replace the product of the root and phase voltage with linear voltage. This value is equal to 380 (V) (U linear = 380 V).

When choosing a cable cross-section, both for a three-phase network and for a single-phase network, it is necessary to take into account permissible continuous current . This parameter indicates the current strength (measured in amperes) that the conductor can withstand for an unlimited amount of time. It is determined using special tables, they are available in the PUE. For aluminum and copper conductors, the data differs significantly.

Permissible current duration - Table

When the value specified in the table is exceeded, the conductor begins to heat up. The heating temperature is inversely proportional to the current strength.

The temperature in a certain area can increase not only due to an incorrectly selected cross-section, but also due to poor contact.For example, in the place where wires are twisted. Quite often this happens as a result of direct contact between aluminum cables and copper cables. The surface of metals oxidizes and becomes covered with an oxide film, which significantly impairs contact. This is where the cable gets hot.

Current values ​​can be easily determined by knowing the rated power of consumers using the formula: I = P/220. Knowing the total current of all consumers and taking into account the ratio of the permissible current load for the wire (open wiring) per wire cross-section:

• for copper wire 10 amperes per square millimeter,
• for aluminum 8 amperes per square millimeter, you can determine whether the wire you have is suitable or whether you need to use another one.

When performing hidden power wiring (in a tube or in a wall), the given values ​​are reduced by multiplying by a correction factor of 0.8. It should be noted that open power wiring is usually carried out with a wire with a cross-section of at least 4 square meters. mm based on sufficient mechanical strength.

The above ratios are easy to remember and provide sufficient accuracy for using wires. If you need to know with greater accuracy the long-term permissible current load for copper wires and cables, then you can use the tables below.

The following table summarizes the data on power, current and cross-section of cable and conductor materials for calculations and selection of protective equipment, cable and conductor materials and electrical equipment.

Permissible long-term current for wires with copper conductors with rubber insulation in metal protective sheaths and cables with copper conductors with rubber insulation in lead, polyvinyl chloride, nayrite or rubber sheaths, armored and unarmoured.

* Currents refer to wires and cables with and without a neutral core.

Permissible continuous current for cables with aluminum conductors with rubber or plastic insulation in lead, polyvinyl chloride and rubber sheaths, armored and non-armored.

Note. Permissible continuous currents for four-core cables with plastic insulation for voltages up to 1 kV can be selected according to this table as for three-core cables, but with a coefficient of 0.92.

Summary table of wire cross-sections, current, power and load characteristics.

The table shows data based on the PUE for selecting cross-sections of cable and wire products, as well as rated and maximum possible currents of circuit breakers for single-phase household loads most often used in everyday life.

The smallest permissible cross-sections of cables and wires of electrical networks in residential buildings.

• Copper, U = 220 V, single phase, two-wire cable

• Copper, U = 380 V, three phase, three-core cable

* cross-sectional value can be adjusted depending on the specific cable laying conditions

The smallest cross-sections of current-carrying conductors of wires and cables in electrical wiring.

Cable power table 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 loss of its properties.

To calculate the conductor resistance, you can use the conductor resistance calculator.

For the transmission and distribution of electric current, the main means are cables; they ensure the normal operation of everything related to electric current, and how good this work will be depends on the right choice cable cross-section by power. A convenient table will help you make the necessary selection:

But in order to use the table, you need 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 laid.

Example of power calculation.

Let's say you are installing closed electrical wiring in a house using an explosive cable. You need to write down a list of equipment used on a piece of paper.

But how now find out power? You can find it on the equipment itself, where there is usually a label with the main characteristics recorded.

Power is measured in Watts (W, W) or Kilowatts (kW, KW). Now you need to write down the data and then add it up.

The resulting number is, for example, 20,000 W, which would be 20 kW. This figure shows how much energy all electrical receivers together consume. Next, you should consider how many devices will be used simultaneously over a long period of time. Let’s say it turns out to be 80%, in which case the simultaneity coefficient will be equal to 0.8. We calculate the cable cross-section based on power:

20 x 0.8 = 16 (kW)

To select a cross-section, you will need a cable power table:

If the three-phase circuit is 380 Volts, then the table will look like this:

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.

So, the known power of each electrical appliance in the house, the known number of lighting fixtures and lighting points allow us to calculate the total power consumed. This is not an exact sum, since most values ​​for the powers of various devices are averages. Therefore, you should immediately add 5% of its value to this figure.

Average power readings for common electrical appliances

And many believe that this is enough to select almost standard copper cable options:

• cross section 0.5 mm2 for wires for lighting spotlights;
• cross section 1.5 mm2 for lighting wires for chandeliers;
• cross-section 2.5 mm2 for all sockets.

At the level of household use of electricity, such a scheme looks quite acceptable. Until the refrigerator and electric kettle decided to turn on in the kitchen at the same time, while you were watching TV there. The same unpleasant surprise overtakes you when you plug in a coffee maker, washing machine and microwave into one outlet.

Thermal calculation using correction factors

For several lines in one cable channel, the tabulated values ​​of the maximum current should be multiplied by the appropriate coefficient:

• 0.68 — for the number of conductors from 2 to 5 pcs.
• 0.63 — for conductors from 7 to 9 pcs.
• 0.6 — for conductors from 10 to 12 pcs.

The coefficient refers specifically to the wires (cores), and not to the number of passing lines. When calculating the number of laid wires, the neutral working wire or grounding wire is not taken into account. According to PUE and GOST 16442-80, they do not affect the heating of wires during the passage of normal currents.

Summarizing the above, it turns out that in order to correctly and accurately select the wire cross-section, you need to know:

1. The sum of all maximum powers of electrical appliances.
2. Network characteristics: number of phases and voltage.
3. Characteristics of cable material.
4. Tabular data and coefficients.

At the same time, power is not the main indicator for an individual cable line or the entire internal power supply system. When selecting a cross-section, be sure to calculate the maximum load current, and then check it with the rated current of the home circuit breaker.