Table of loads for cable cross-section: selection, calculation

The correct choice of the cross section of the wiring affects the comfort and safety in the home. In case of overload, the conductor overheats and the insulation can melt, and cause fire or short circuit. But the cross section more than necessary to take profitable because it increases the price of cable.

Generally, it is calculated depending on the number of consumers, which first determine the total power used by the apartment, and then multiply the result by 0.75. In the EMP table applied loads on the cross section of the cable. It is possible to easily determine the diameter of strands, which depends on the material and passing current. As a rule, used copper conductors.

A Section of a cable core shall be calculated in the direction of increasing standard sizes. The most dangerous when it is low. Then the conductor constantly overheats, and the insulation of tubes. If you install the appropriate breaker, it will be frequent operation.

When inflating the section of the wire, it will cost more. Although a certain margin is necessary because in the future, as a rule, have to connect new equipment. It is advisable to apply a safety factor of 1.5.

Calculation of total power

Total consumption of the apartment the power goes to the main input, which is included in the switchboard, and after it is branched on the line:

• Lighting;
• Outlet groups;
• Separate powerful appliances.

Therefore, the large cross section of the power cable at the entrance. For return lines it is decreased depending on the load. First and foremost, is determined by the total power of all loads. It's easy, because engraved all appliances and passports to them, she indicated.

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All of the power add up. Similar calculations for each circuit. Experts suggest to multiply a reduction factor of 0.75. This is because all devices in the network are not included. Others offer select cross section larger in size. Due to this, provision is made for the subsequent commissioning of additional electric devices, which can be purchased in the future. It should be noted that this variant of calculation of the cable more reliable.

How to determine the wire size?

In all calculations, it appears the section of the cable. Diameter to define easier if you apply the formula:

• S = πD²/4;
• D = √(4×S/π).

Where π = 3,14.

The stranded wire you first need to count the number of wires (N). Then measure the diameter (D) of one of them, then define the cross-sectional area:

S = N×D²/1,27.

Stranded wire is used where flexibility is required. Cheaper solid conductors are used in a stationary installation.

How to choose a cable for power?

To find the wiring, use the table loads for the conductor:

• If the line is open type under voltage of 220 V, and the total power is 4 kW, is taken by the copper conductor cross-section 1.5 mm². This size is usually used for wiring lighting.
• With a capacity of 6 kW requires a larger cross-section conductor - 2.5 mm². The wire is used for sockets, which connect household appliances.
• Capacity of 10 kW requires the use of wiring in 6 mm². It is usually intended for the kitchen, where it is connected an electric stove. The supply line to such load produced by a separate line.

What are the cables better?

Electrical cable well known German brand NUM for office and residential premises. In Russia release of the cables, which are characteristics below, although they may have the same name. They can be distinguished by the stains of the compound in the space between tendons or lack of it.

The Wire is produced in solid and stranded. Each lived, as well as the whole twist outside of isolated PVC and the filler between them is made non-combustible:

• So, the NUM cable is used indoors as the insulation on the street is destroyed by sunlight.
• And as internal and external wiring harness is widely used cable brand VVG. It's cheap and reliable enough. For laying in the ground it is not recommended.
• Cable brand VVG is made flat and round. Between cores of the filler is not applied.
• Cable VVGng-P-LS do with the outer shell, does not support combustion. Cores are made of round cross-section up to 16 mm² and over – sector.
• Brand cables and PVA SHVVP are stranded and are mainly used for connecting household appliances. It is often used in domestic electrical wiring. On the street stranded conductors is not recommended due to corrosion. In addition, the isolation of the Flexural cracks at low temperature.
• On the street under the ground lay armoured and resistant to moisture cables Avbbshv and Vbsw. Armormade from two steel belts, which increases the reliability of the cable and makes it resistant to mechanical stress.

Load current

A More accurate result gives the calculation of the section of cable for power and current, where the geometric parameters associated with electrical.

For home wiring needs takes into account not only active but also reactive. Amperage is determined by the formula:

I = P/(U∙cosφ).

Reactive load creates fluorescent lamps and motors of appliances (refrigerator, vacuum cleaner, power tools, etc.).

Example of calculation of the cable cross-section current

Let's find out what if you need to determine the cross-section of copper cable to connect home appliances with a total capacity of 25 kW and three-phase machines to 10 kW. Such a connection is made with five-core cable is laid in the ground. The power house is made from three-phase network.

Taking into account the reactance, power in appliances and equipment is as follows:

• P_Gen. = 25/0,7 = 35,7 kW;
• P_Equipment. = of 10/0. 7 = 14.3 kW.

Defines the currents of the input:

• I_Gen. = 35,7×1000/220 = 162 A;
• I_Equi. = 14,3×1000/380 = 38 A.

If you distribute the single phase load evenly on the three phases, one will have to current:

I_F = 162/3 = 54 A.

At each phase, the current load will be:

I_F = 54 + 38 = 92 A.

All the equipment at the same time will not work. Taking into account the margin for each phase have current:

I_F = 92×0.75 mm×1,5 = 103,5 A.

At five-core cable takes into account only the phase cores. For cable laid in the ground, you can determine the current of 103.5 And cross-section 16 mm² (table of loads according to cable section).

Updated calculation of current saves money because it requires a lower section. Coarser calculation of cable power conductor cross-section is 25 mm², which will cost more.

The voltage Drop on the cable

Conductors have resistance, which must be considered. This is especially important for large cable length or when it is small section. The norms of PES for which the voltage drop on the cable must not exceed 5 %. The calculation is done as follows.

1. Is Defined conductor resistance: R = 2×(ρ×L)/S.
2. Is the voltage drop: U_Pad. = I×R. to the line in % it will be: U_% = (U_Pad./U_Lin.)×100.

In the formulas adopted notation:

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

The Factor of 2 indicates that the current flows through two cores.

Example of the calculation of cable voltage drop

For Example, it is necessary to calculate the voltage drop on the carrying cross-section of conductor 2.5 mm², with a length of 20 m. It is required to connect the welding transformer with a capacity of 7 kW.

• Resistance of the wire is: R = 2(0,0175 × 20)/2,5 = 0.28 Ohm.
• The current in the conductor: I = 7000/220 =31,8 And.
• The voltage Drop across carry: U_Pad. = 31,8 × 0,28 = 8.9 In.
• Percentage of voltage drop: U_% = (8,9/220) × 100 = 4,1 %.

Suitable for Carrying a welding machine according to the requirements of the rules of operation of electrical installations, because the percentage drop on it the voltage is in the normal range. However, its value in the supply lead remains large, which may adversely affect the welding process. Here it is necessary to check the lower allowable limit of the supply voltage for the welding machine.

Conclusion

To reliably protect the wiring from overheating during prolonged exceeding of the nominal current, the cable sizes are calculated according to permissible currents. The calculation is simplified if you use a table of the loads section of the cable. A more accurate result is obtained if the computation is determined by the maximum load current. And for stable and long-term operation circuit wiring install circuit breaker.