Table of loads by cable section: selection, calculation

2024 Author: Howard Calhoun | [email protected]. Last modified: 2024-01-02 13:48

The comfort and safety in the house depends on the correct choice of the electrical wiring section. When overloaded, the conductor overheats and the insulation may melt, resulting in a fire or short circuit. But it is unprofitable to take a section larger than necessary, since the price of the cable increases.

In general, it is calculated depending on the number of consumers, for which the total power used by the apartment is first determined, and then the result is multiplied by 0.75. The PUE uses a table of loads for the cable section. From it, you can easily determine the diameter of the cores, which depends on the material and the passing current. As a rule, copper conductors are used.

The cross section of the cable core must exactly match the calculated one - in the direction of increasing the standard size range. It's most dangerous when it's low. Then the conductor constantly overheats, and the insulation quickly fails. And if you install the appropriate circuit breaker, then it will operate frequently.

If you overestimate the cross-section of the wire, it will cost more. Although a certain margin is necessary, since in the future, as a rule, you have to connect new equipment. It is advisable to apply a safety factor of the order of 1, 5.

Calculation of total power

The total power consumed by the apartment falls on the main input, which enters the switchboard, and after it branches into the line:

• lighting;
• socket groups;
• Individual powerful electrical appliances.

Therefore, the largest section of the power cable is at the input. On the outlet lines, it decreases, depending on the load. First of all, the total power of all loads is determined. This is not difficult, since it is indicated on the cases of all household appliances and in their passports.

All powers add up. Similarly, calculations are made for each contour. Experts suggest multiplying the amount by a reduction factor of 0.75. This is due to the fact that all devices are not connected to the network at the same time. Others suggest choosing a larger section. This creates a reserve for the subsequent commissioning of additional electrical appliances that may be purchased in the future. It should be noted that this cable calculation option is more reliable.

How to determine the wire size?

All calculations include the cable section. It is easier to determine it by diameter if you apply the formulas:

• S=π D²/4;
• D=√(4× S /π).

Where π=3, 14.

In a stranded wire, you first need to count the number of wires (N). Then the diameter (D) of one of them is measured, after which the cross-sectional area is determined:

S=N×D²/1, 27.

Stranded wires are used where flexibility is required. Cheaper solid conductors are used in permanent installations.

How to choose a cable by power?

In order to select the wiring, the load table for the cable section is used:

• If the open type line is energized at 220 V, and the total power is 4 kW, a copper conductor with a cross section of 1.5 mm² is taken. This size is usually used for lighting wiring.
• With a power of 6 kW, larger conductors are required - 2.5 mm². The wire is used for sockets to which household appliances are connected.
• 10 kW power requires 6 mm² wiring. Usually it is intended for the kitchen, where an electric stove is connected. Such a load is connected via a separate line.

Which cables are best?

Electricians are well aware of the cable of the German brand NUM for office and residential premises. In Russia, cable brands are produced that are lower in characteristics, although they may have the same name. They can be distinguished by the leakage of the compound in the space between the cores or by its absence.

The wire is produced in monolithic and stranded. Each vein andthe entire twist is PVC insulated from the outside, and the filler between them is made non-combustible:

• So, the NUM cable is used indoors, because the insulation on the street is destroyed by sunlight.
• And VVG brand cable is widely used as internal and external wiring. It is cheap and quite reliable. It is not recommended for laying in the ground.
• Wire brand VVG is made flat and round. No filler is used between the cores.
• The VVGng-P-LS cable is made with an outer sheath that does not support combustion. The cores are made round up to a section of 16 mm², and above - sectoral.
• PVS and ShVVP cable brands are made multi-wire and are used mainly for connecting household appliances. It is often used as home electrical wiring. It is not recommended to use stranded conductors on the street due to corrosion. In addition, the insulation cracks when bent at low temperatures.
• On the street, armored and moisture-resistant cables AVBShv and VBShv are laid underground. The armor is made of two steel tapes, which increases the reliability of the cable and makes it resistant to mechanical stress.

Determination of current load

A more accurate result is given by the calculation of the cable cross-section by power and current, where the geometric parameters are related to the electrical ones.

For home wiring, not only active load, but also reactive load should be taken into account. The current strength is determined by the formula:

I=P/(U∙cosφ).

Reactive load is created by fluorescent lamps and motors of electrical appliances (refrigerator, vacuum cleaner, power tools, etc.).

Example of current cable section calculation

Let's find out what to do if it is necessary to determine the cross-section of a copper cable for connecting household appliances with a total power of 25 kW and three-phase machines for 10 kW. Such a connection is made by a five-core cable laid in the ground. The house is powered by a three-phase network.

Taking into account the reactive component, the power of household appliances and equipment will be:

• P_{life.=25/0, 7=35.7 kW;}
• P_{rev.=10/0, 7=14.3 kW.}

The input currents are determined:

• I_{life.=35, 7×1000/220=162 A;}
• I_{rev.=14, 3×1000/380=38 A.}

If you distribute single-phase loads evenly over three phases, one will have a current:

I_{f=162/3=54 A.}

There will be a current load on each phase:

I_{f=54 + 38=92 A.}

All appliances will not work at the same time. Given the margin, each phase has a current:

I_{f=92×0.75×1.5=103.5 A.}

In a five-core cable, only phase cores are taken into account. For a cable laid in the ground, it is possible to determine for a current of 103.5 A the cross section of the cores is 16 mm²(load table for the cable cross section).

Refined calculation of the current strength saves money, since a smaller cross section is required. With a rougher calculation of the cable in terms of power,the cross section of the core will be 25 mm², which will cost more.

Cable voltage drop

Conductors have resistance that must be taken into account. This is especially important for long cable lengths or small cross-sections. PES standards have been established, according to which the voltage drop on the cable should not exceed 5%. The calculation is done as follows.

1. The resistance of the conductor is determined: R=2×(ρ×L)/S.
2. The voltage drop is found: U_drop.=I×R. In relation to the linear percentage, it will be:)×100.

Denotations are accepted in formulas:

• ρ – resistivity, Ohm×mm²/m;
• S – cross-sectional area, mm².

Coefficient 2 shows that the current flows through two wires.

Example of cable calculation by voltage drop

For example, it is necessary to calculate the voltage drop on a carrier with a conductor cross section of 2.5 mm², 20 m long. It is necessary to connect a welding transformer with a power of 7 kW.

• Wire resistance is: R=2(0.0175×20)/2.5=0.28 ohm.
• Current in the conductor: I=7000/220=31.8 A.
• Carry voltage drop: U_{drop.=31.8×0.28=8.9V.}
• Voltage drop percentage: U_{%=(8, 9/220)×100=4, 1%.}

The carrying is suitable for the welding machine according to the requirements of the rules for the operation of electrical installations, since the percentage of voltage drop on it is within the normal range. However, its value on the supply wireremains large, which can adversely affect the welding process. Here it is necessary to check the lower permissible limit of the supply voltage for the welding machine.

Conclusion

To reliably protect the wiring from overheating when the rated current is exceeded for a long time, the cable cross-sections are calculated according to the long-term permissible currents. The calculation is simplified if the load table for the cable section is used. A more accurate result is obtained if the calculation is based on the maximum current load. And for stable and long-term operation, an automatic switch is installed in the wiring circuit.