Calculation method of load and load current of wire and cable

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1. Purpose and significance of load calculation

The statistical calculation of load is an important content in the design of low-voltage power supply and distribution system. The load calculation results play a decisive role in the reporting and installation of power supply capacity, the selection of power supply and distribution equipment and safe and economic operation. The purpose of load calculation is:

Calculate the load current and apparent power of the transformer in the substation as the basis for selecting the transformer capacity.

Calculate the load current flowing through the main electrical equipment (circuit breaker, disconnector, bus, fuse, etc.) as the basis for equipment selection.

Calculate the load current flowing through each line (power incoming line, high and low voltage distribution line, etc.) as the basis for selecting the section of line cable or conductor.

Calculate the peak load, which is used for setting calculation of protective appliances and checking the starting conditions of motors.

2. Load calculation method

At present, the demand coefficient method and binomial coefficient method are widely used in China to determine the load of electrical equipment. Among them, the demand coefficient method is the most simple method widely used in the world to determine and calculate the load;

The binomial coefficient method is more reasonable in determining the calculated load of branch trunk lines with small number of equipment and large difference in capacity of each equipment;

In building power distribution, load density method and unit index method are also commonly used to calculate load. The unit index method can be adopted in the scheme design stage; In the stage of preliminary design and construction drawing design, the demand coefficient method should be adopted.

3. Load calculation principle

During load calculation, the equipment power shall be calculated according to the following principles:

The rated power of electrical equipment with different working systems shall be converted into unified equipment power.

The equipment power of rectifier refers to the rated AC input power.

The equipment power of group electrical equipment shall not include standby equipment.

When the calculated active power of fire power is greater than that of fire, the general power and lighting load may be cut off at the same time. The total low-voltage equipment power and load shall be calculated according to the general power and lighting load not cut off plus the fire load. Otherwise, the fire load shall not be considered when calculating the total low-voltage load. When there is a load for both normal use in the fire load, this part of the load shall also be included in the general power and lighting load.

Single phase load shall be evenly distributed to three phases. When the total calculated capacity of single-phase load is less than 15% of the total calculated capacity of three-phase symmetrical load within the calculation range, it shall be calculated as three-phase symmetrical load; When it exceeds 15%, the single-phase load shall be converted to the equivalent three-phase load, and then added to the three-phase load.

4. Example: wire load

The load in the power system generally refers to the active power, while the load of the wire we talk about orally generally refers to the current it can withstand. At present, 1kW is equivalent to 2A current. So, how many watts is the 2.5 square wire load?

① 2.5 square line load calculation

Speaking of the standard load of wires and cables, it can be said that there are all kinds of things that netizens say on the Internet. Some people say that the load of 2.5 square wires is 2 kilowatts, while others say that the correct load of 2.5 square wires is 1.5 kilowatts. There are different opinions and parameters.

The load degree of the wire mainly depends on the current and the voltage level. Generally, the safety current carrying capacity of 1mm2 below 50A is 6a, the safety current carrying capacity of 1mm2 for 50 ~ 100A current is 5a, and the safety current carrying capacity of 1mm2 above 100A current is 2.5A; So many square copper cables can calculate the current. According to the three-phase AC formula: P = 1.732 * u * I * cos φ， Cos φ Is the power factor, select 0.8; This is the three-phase power calculation formula. Single phase: P = u * I * cos φ， Cos φ Is the power factor. The power factor is 1 for resistive load and 0.8 for inductive load

This bv wire is different from BVR multi strand flexible copper wire. Of course, it is different from the load of rubber sheathed cable YC. The load of rubber sheathed cable is better than that of BV / BVR / RVV and other plastic wires. In other words, the outer insulation is different, which will also affect the load of the wire. Generally, the load of 2.5 square meters of wire is 2 kW, while the full load of rubber sheathed cable can reach 3 kW without problem (it is estimated that no matter how large it is, it will not be safe)

② 1.5 square line load calculation

Several square meters are the nominal value of a wire specification specified in the national standard. The square of the wire is actually marked with the cross-sectional area of the wire, that is, the area of the circular cross-section of the wire, in square millimeters, which is generally divided into 0.5, 1, 1.5, 2.5, 4, 6, 10, 16, 25, etc.

Those below 10 (mm2) are generally called wires, and those above 10 (mm2) are called cables. The square number of wires is an oral term in the decoration and hydropower construction. It is often said that the square number of wires does not add units, that is, square millimeters.

Generally speaking, the empirical carrying capacity is that when the grid voltage is 220V, the empirical carrying capacity of each square wire is about one kilowatt.

Copper wire can carry 1-1.5kw per square meter, and aluminum wire can carry 0.6-1kw per square meter.

Therefore, an electric appliance with a power of 1 kW only needs one square meter of copper wire.

When it comes to current, the copper wire can carry 3a to 5A current per square meter during short-distance power transmission.

The heat dissipation is about 1.5 kW / mm2, which is not good.

5. Load current calculation

① Difference between calculated load current and rated current

Rated current refers to the current when the equipment works normally. Load current refers to the working current of the load of the equipment. For a power supply equipment, if the load current is greater than its rated current, it may cause overheating or even burning. For example, your wall socket is rated at 220V 10A. If you connect a 220V 15A electric kettle, the socket may be burned because its internal circuit can only withstand 10A current. For three-phase electrical equipment, the rated current usually refers to its line current, which is uniformly converted into line current when there are single-phase load and three-phase load at the same time. Three phase load current + single phase load current / 1.732

② Calculation of three-phase four wire unbalanced load current

In the three-phase four wire system, if it is an unbalanced three-phase load, its current and power of each phase can only be calculated by single phase, and its single-phase power calculation formula is p = u phase * I phase * cos Then the sum of the three-phase power is the total power. Similarly, on the contrary, according to this formula, if you know the single-phase power, you can also obtain the phase current. It should be pointed out that the unbalanced three-phase load should be star connected, that is, the zero line is necessary. In this case, the selection of switch can only refer to the phase with the largest current in the three phases, and the rated value shall be slightly greater than the maximum phase current value.

③ Calculation method of single-phase load current

If it is used in single-phase electric heating appliances, such as electric lamps and electric furnaces, it is to use power (W) ÷ voltage (220V) = current (a). For example, for 100W electric lamps, it is to use 100W ÷ 220V = 0.46a;

If it is used in single-phase motor, such as 1000W. Calculation method: 1000 ÷ (220) × zero point seven five × 0.75)=8.08A。

Cos φ: 0.75 for single-phase power factor;

η: The single-phase efficiency is taken as 0.75.