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How to choose and correctly calculate the cable cross-section

Choosing and correctly calculating the right cable cross-section for your electricity needs is an essential part of any electrical installation. This post provides an easy to follow guide on how to correctly select and calculate a suitable cable cross-section based on the power requirements of the electrical installation. It further examines the advantages and distinctive features of choosing the correct cable cross-section, such as lower operating and installation costs, improved safety, and fewer network disturbances.

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How to choose a cable for connecting household appliances on your own, ensuring the safety of the wiring, and at the same time not overpaying? What to be guided by when choosing and how to calculate the cable cross-section for a group of consumers? You can find out about this from this article..

How to choose and correctly calculate the cable cross-section

The cable cross-section is the cross-sectional area of โ€‹โ€‹the current-carrying conductor. In most cases, the cut of the cable core is round, and its cross-sectional area can be calculated using the formula for the area of โ€‹โ€‹a circle. But, given the variety of cable shapes, to describe its main physical characteristic, not a linear dimension is used, but the value of the cross-sectional area. This characteristic is standardized in all countries. In our country, it is regulated by the PUE โ€œElectrical Installation Rulesโ€.

Why is it necessary to select the cable cross-section

The correct selection of cable cross-section is, first of all, your safety. If the cable does not withstand the current load, it overheats, the insulation melts, and, as a result, a short circuit and a fire may occur.

How to choose and correctly calculate the cable cross-section

How to choose a cable of the desired cross-section, while avoiding cases when, when several devices are simultaneously turned on, the smell of melting insulation appears, and not to overpay extra money using wires with a large margin?

For the power supply of residential premises, two main types of cables are used: copper and aluminum. Copper is more expensive than aluminum. But in modern wiring, preference is given to it. Aluminum has a higher internal resistance and is a brittle metal that oxidizes quickly. Copper is a flexible material that is less prone to oxidation. Recently, aluminum cables have been used exclusively for the restoration of wiring in buildings of the Soviet era.

How to choose and correctly calculate the cable cross-section

For preliminary selection of the required cross-section of copper cable, it is assumed that a cable with a cross-section of 1 mm2 can pass an electric current through itself up to 10 A. However, further you will make sure that such a ratio is suitable exclusively for selecting a cross-section โ€œby eyeโ€, and is true for cross-sections of no more than 6 mm2 (using the proposed ratio, the current is up to 60 A). An electric cable of this cross section is quite enough to enter a phase into a standard three-room apartment.

Most electricians use cables of the following cross-sections to supply electricity to indoor consumers:

  • 0.5 mm2 โ€“ Spotlights;
  • 1.5 mm2 โ€“ basic lighting;
  • 2.5 mm2 โ€“ sockets.

However, this is acceptable for domestic consumption, provided that each electrical appliance is powered from its own outlet, without the use of twins, tees and extension cords..

It will be more correct to use special tables when selecting a cable, which allow you to select a cross-section based on the known power of the electrical appliance (kW), or according to the current load (A). The current load in this case is a more important characteristic, since the load in amperes is always indicated for one phase, while with a single-phase consumption (220 V), the load in kilowatts will be indicated for one phase, and with a three-phase one โ€“ for all three phases in total.

When choosing a cable cross-section, it is necessary to take into account the type of wiring: external or hidden. This is due to the fact that with hidden wiring, the heat transfer of the wire decreases, as a result of which a more intense heating of the cable occurs. Therefore, for hidden wiring, cables with a cross-sectional area of โ€‹โ€‹about 30% larger are used than with open.

How to choose and correctly calculate the cable cross-section

Table for selecting the cross-sectional area of โ€‹โ€‹a copper cable for open and hidden wiring:

Cross-sectional area Open wiring Concealed wiring
S I P I P
220 V 380 V 220 V 380 V
0.5 eleven 2.4 โ€“ โ€“ โ€“ โ€“
0.75 15 3.3 โ€“ โ€“ โ€“ โ€“
1 17 3.7 6.4 fourteen 3 5.3
1.5 23 five 8.7 15 3.3 5.7
2 26 5.7 9.8 nineteen 4.1 7.2
2.5 thirty 6.6 eleven 21 4.6 7.9
4 41 nine 15 27 5.9 ten
five 50 eleven nineteen 34 7.4 12
ten 80 17 thirty 50 eleven nineteen
sixteen one hundred 22 38 80 17 thirty
25 140 thirty 53 one hundred 22 38
35 170 37 64 135 29 51

Table for selecting the cross-sectional area of โ€‹โ€‹the conductor of an aluminum cable for open and hidden wiring:

Cross-sectional area Open wiring Concealed wiring
S I P I P
220 V 380 V 220 V 380 V
2 21 4.6 7.9 fourteen 3 5.3
2.5 24 5.2 9.1 sixteen 3.5 6
4 32 7 12 21 4.6 7.9
five 39 8.5 fourteen 26 5.7 9.8
ten 60 13 22 38 8.3 fourteen
sixteen 75 sixteen 28 55 12 20
25 105 23 39 65 fourteen 24
35 130 28 49 75 sixteen 28

S โ€“ cable cross-sectional area (mm2), I โ€“ current load (A), R โ€“ total power of electrical equipment (kW).

It is also necessary to make adjustments when selecting the cable cross-section, taking into account its length. To do this, choosing the cross-section of the cable from the table by current strength, we calculate its resistance, taking into account the length, using the formula:

R = p? L / S

Where:

  • R โ€“ wire resistance, Ohm;
  • p โ€“ material resistivity, Ohm? Mm2/ m (for copper โ€“ 0.0175, for aluminum โ€“ 0.0281);
  • L โ€“ cable length, m;
  • S โ€“ cable cross-sectional area, mm2.

Using this formula, you can get the resistance of one cable core. Since the current comes through one core and returns through the other, in order to obtain the value of the cable resistance, the resistance of its core must be multiplied by two:

Rtotal = 2? R

Next, you need to calculate the voltage loss:

dU = I? Rtotal

Where:

  • dU โ€“ voltage losses, W;
  • I โ€“ current strength, A;
  • Rtotal โ€“ calculated cable resistance, Ohm.

If the selection of the cable cross-section was carried out according to the total power of the equipment and the current strength is not known, it can be calculated by the formula:

I = P / U? cos ?โ€“ for single-phase 220 V network

I = P / 1.732? U? cos ? โ€“ for a three-phase 380 V network

Where:

  • R โ€“ total used power of electrical equipment (W);
  • U โ€“ voltage (V);
  • cos? = 1 (for living conditions) and cos? = 1.3 (for powerful electrical appliances).

Next, we calculate the losses as a percentage: (dU / U)? one hundred%.

How to choose and correctly calculate the cable cross-section

If the obtained value does not exceed 5%, then the cable cross-section, taking into account its length, is selected correctly. If it exceeds, it is necessary to select a cable of a larger cross-section (next in the row) from the table and calculate again.

These tables are applicable for cables in rubber and plastic insulation, the cable cross-section selected according to them will work effectively if it is produced in accordance with GOST.

Selection of cable for a group of consumers

To select the cable cross-section for a group of consumers (for example, an input cable to an apartment), you can use the formula to determine the permissible current load. Letโ€™s calculate the current load for a 220 V network, which is often used in household power supply:

I = P? K / U? cos ?

Where:

  • R โ€“ total used power of electrical equipment (W), U โ€“ voltage (V), TO โ€“ coefficient of accounting for simultaneous switching on of devices (taken equal to 0.75);
  • cos? = 1 (for living conditions) and cos? = 1.3 (for powerful electrical appliances).

Having calculated the permissible current load for a group of consumers, using the tables above, you can select a cable of the required cross section. If a long-term simultaneous switching on of all possible consumers (for example, electric heating) is assumed, the calculation of the permissible current load must be made without taking into account the coefficient K.

An example of the selection of a cable for a domestic boiler

Based on the foregoing, we will try to calculate and select a copper cable of the required cross-section for a single-phase electric boiler, with a 2.0 kW heating element, provided that the cable to it is laid in a box. The cable length will be 10 meters.

How to choose and correctly calculate the cable cross-section

It can be seen from the table that the value of 3.0 kW is close in power, which corresponds to a cable cross-section of 1 mm2. Letโ€™s calculate taking into account the cable length:

  • Letโ€™s calculate the current strength: I = 2000 W / 220 V? 1 = 9.09 A.
  • Letโ€™s calculate the cable core resistance: R = 0.0175 Ohm? Mm2/ m? 10 m / 1 mm2 = 0.175 Ohm.
  • Total cable resistance: Rtotal = 2? R = 0.35 ohm.
  • Calculating voltage losses: dU = 9.09 A? 0.35 ohm = 3.18 V.
  • We calculate losses as a percentage: (3.18V / 220V)? 100% = 1.45% (does not exceed 5%).

1 mm cable2 suitable for connecting the electric boiler specified in the example.

Manufacturers often indicate the required cable cross-sectional area for their equipment in the equipment instructions. If there is such an instruction, you must follow it..

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Comments: 2
  1. Delaney

    Choosing the right cable cross-section and calculating it correctly can be crucial for ensuring safety and efficient power transmission. Are there any recommended guidelines or formulas that one should follow when determining the appropriate cable cross-section for a specific electrical project? What factors should be taken into consideration during this process to ensure accurate calculations?

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  2. Wyatt Brooks

    Can someone please explain the process of selecting and accurately calculating the cross-section of a cable? Iโ€™m curious about the factors that should be considered and the potential implications of selecting the wrong cross-section. Additionally, are there any resources or tools available to help with this calculation? Any advice or insights would be greatly appreciated.

    Reply
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