Before connecting the load to the mains, it is important to make sure that the cores of the supply cable are sufficiently thick. In the case of a significant excess of permissible power, it is possible to destroy the insulation and even the core itself due to its overheating.
Calculation of the cable cross-section for power and current strength
Before calculating the cable cross-section for power, 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 700 W
- TV 160 W
- Water heater 1950 W
- Vacuum cleaner 600 W
- Iron 1750 W
- Total 10310 W = 10.3 kW
In sum, most modern apartments consume about 10 kW.Depending on the time of day, this parameter can be significantly reduced. However, when choosing the conductor cross-section, it is important to orient yourself toward a larger value.
It is necessary to know the following: the calculation of the cable cross-section for single-phase and three-phase networks is different. But in either case, the first three parameters should be taken into account:
- Current strength ( I),
- Voltage ( U),
- Power consumption ( P).
There are also several other variables, their meaning is different for each specific case.
Calculation of the wire cross-section for a single-phase network
The calculation of the power wire cross-section is carried out using the following formula:
Where,
- I - current strength;
- P - the power consumption of all electrical appliances in total;
- Ki is a concurrency factor, usually a standard value of 0.75 is accepted for calculations;
- U - phase voltage, it is 220( V), but can vary from 210 to 240( V);
- Cos( φ) - for household single-phase devices this value is unchanged and equal to 1.
If necessary, quickly calculate the current, you can omit the value of cos( φ) and even Ki. The obtained value will differ in the lower side( by 15%) if the following formula is used:
Having found the current according to the calculation formula, it is possible to proceed with the selection of the supply cable. More precisely, its cross-sectional area. There are special tables in which data are presented that allow you to compare the amount of current, power consumption and cross-section of the cable.
The data is very different for conductors made of different metals. To date, for residential wiring usually only a hard copper cable is used, aluminum is practically not used. Although in many old houses all the lines are laid with the use of aluminum.
The copper cable cross-section is selected according to the following parameters:
Calculating the conductor cross-section in an apartment - Table
It often happens that the result of calculations is a current between the two values shown in the table. In this case, use the nearest larger value. If, as a result of the calculations, the value of the current in the solid wire is 25( A), a cross section of 2.5 mm2 or more must be chosen.
Calculation of the cable cross-section for a three-phase network
To calculate the cross-section of a supply cable used in a three-phase network, the following formula should be used:
Where,
- I is the current that the cable cross-section will be selected for;
- U - phase voltage, 220( V);
- Cos φ - angle of phase shift;
- P - an indicator of the total power of all electrical appliances.
Cos φ in this formula is very important. Since it directly affects the current. For different equipment it is different, most often with this parameter can be found in the technical accompanying documentation, or it is indicated on the case.
The total power of consumers is very simple: all the capacities are added up, the value obtained is used for calculations.
A distinctive feature of the choice of cable cross-section for use in a three-phase network is that a thinner wire can withstand a high load. The required cross-section is selected according to the type table.
Selection of the cable cross-section for a three-phase network - Table
The calculation of the power wire cross-section in a three-phase network is performed using a value such as √3 .This value is necessary to simplify the appearance of the formula.
Thus, if necessary, you can replace the product of the root and phase voltage by the voltage linear. This value is equal to 380( V)( U linear = 380 V).
When selecting a cable cross-section, both for a three-phase network and for a single-phase network, the allowable continuous current must be taken into account. This parameter indicates the current strength( measured in amperes) that a conductor can withstand for an unlimited amount of time. It is determined by special tables, they are in the PUE.For aluminum and copper conductors, the data vary considerably.
Permissible current durations - Table
If the value indicated in the table is exceeded, the conductor starts to heat up. The heating temperature is inversely proportional to the current.
The temperature in a certain area may increase not only because of an incorrectly selected cross section, but also because of poor contact. For example, in the place of wire twisting. Quite often this happens as a result of direct contact of aluminum cables and copper ones. The surface of the metals is oxidized, covered with an oxide film, which significantly worsens the contact. At this point, the cable heats up.
