Calculation methods for the load and load current of wires and cables

1. Purpose and significance of load calculation

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

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

Calculate the load current flowing through each major electrical equipment (circuit breakers, disconnectors, busbars, fuses, etc.) as the basis for selecting the equipment.

Calculate the load current flowing through each line (power supply incoming lines, high and low voltage distribution lines, etc.) as the basis for selecting the cross-sectional area of the line cables or conductors.

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

2. Load calculation method

At present, the demand coefficient method and the binomial coefficient method are widely used in China to determine the load of electrical equipment. Among them, the demand coefficient method is the most commonly adopted method for determining and calculating the load internationally and is the simplest.

The binomial coefficient method is more reasonable when determining the load calculation of branch trunk lines with a small number of devices and significant differences in the capacity of each device.

In building power distribution, the load density method and the unit index method are also commonly used to statistically calculate the load. The unit index method can be adopted in the scheme design stage. In the preliminary design and construction drawing design stages, it is advisable to adopt the requirement coefficient method.

3. Load calculation principles

When conducting load calculation, the equipment power should be calculated in accordance with the following principles:

The rated power of electrical equipment with different operating systems should be converted to a unified equipment power.

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

The power of a group of electrical equipment should not include backup equipment.

When the calculated active power for fire protection is greater than that of the general power and lighting loads that may be simultaneously cut off during a fire, the total low-voltage equipment power and calculated load should be calculated by adding the fire protection load to the general power and lighting loads that have not been cut off. Otherwise, when calculating the total low-voltage load, the fire-fighting load should not be taken into account. When there are loads in the fire protection load that are used concurrently with normal times, such loads should also be included in the general power and lighting loads.

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

4. Give an example of wire load

The load in the power system generally refers to the active power, while the load of the wires we often talk about verbally usually refers to the current value it can withstand. Currently, 1KW is equivalent to a current of 2A. So, what is the load capacity of a 2.5-square-meter wire?

Load calculation for a 2.5-square-meter line

When it comes to the standard load of wires and cables, there are all kinds of opinions from netizens on the Internet. Some people say that the load capacity of a 2.5-square wire is 2 kilowatts, while others say that the correct one is that the load capacity of a 2.5-square wire is 1.5 kilowatts. There are various opinions and the parameters are even more uneven.

The load capacity of a wire mainly depends on the current and also on the voltage level. Under normal circumstances, the safe current-carrying capacity per square millimeter for currents below 50A is 6A, for currents between 50 and 100A, it is 5A, and for currents above 100A, the safe current-carrying capacity per square millimeter is generally 2.5A. Then a copper cable of such a large square can be used to calculate the current. According to the formula for three-phase alternating current :P=1.732*U*I*cosφ, where cosφ is the power factor, select 0.8. This is the formula for calculating three-phase power. For single-phase :P=U*I*cosφ, where cosφ is the power factor. The power factor is 1 for resistive loads and 0.8 for inductive loads.

This BV wire is different from BVR multi-strand flexible copper wire. Of course, its load capacity is also different from that of rubber-sheathed cable YC. The load capacity of rubber-sheathed cable is better than that of plastic wires like BV/BVR/RVV. That is to say, different outer insulation layers will also affect the load capacity of the wire. Generally, the load capacity of a 2.5-square wire is 2 kilowatts. However, for rubber-sheathed cables, a full load capacity of 3 kilowatts is no problem (it is estimated that if it is larger, it will not be safe).

② Load calculation for 1.5-square-meter lines

A few square meters is a nominal value for wire specifications stipulated by national standards. The square of a wire actually indicates the cross-sectional area of the wire, that is, the area of the circular cross-section of the wire, with the unit being square millimeters. Generally, it is divided into :0.5, 1, 1.5, 2.5, 4, 6, 10, 16, 25, etc.

Wires with a diameter of less than 10 square millimeters are generally called wires, while those with a diameter of more than 10 square millimeters are called cables. The square number of wires is an oral term in the construction of water and electricity during decoration. The commonly mentioned several square meters of wires are without a unit, that is, square millimeters.

Generally speaking, the empirical current-carrying capacity is approximately one kilowatt per square meter of wire when the grid voltage is 220V.

Copper wire can carry 1 to 1.5 kilowatts of electricity per square meter, while aluminum wire can carry 0.6 to 1 kilowatt of electricity per square meter.

Therefore, for an appliance with a power of 1 kilowatt, only one square meter of copper wire is sufficient.

Specifically, in terms of current, when short-distance power transmission is carried, a copper wire can generally carry a current of 3A to 5A per square meter.

If the heat dissipation conditions are good, take 5A per square millimeter; if not, take 3A per square millimeter. The load capacity of a 1.5-square-meter wire is approximately 1.5 kilowatts.

5. Calculation of load current

Calculate the difference between the load current and the rated current

The rated current refers to the current at which the equipment is operating normally. Load current refers to the working current of the load of the equipment. For a power supply device, if the load current exceeds its rated current, it may cause it to overheat or even burn out. For instance, if your wall socket is rated at 220V 10A and you connect a 220V 15A electric kettle, it might burn out the socket because the internal circuit can only withstand a current of 10A. For three-phase electrical equipment, the rated current usually referred to is its line current. When there are both single-phase and three-phase loads simultaneously, they are uniformly converted into line current. Three-phase load current + single-phase load current /1.732

② Calculation of unbalanced load current in three-phase four-wire systems

In a three-phase four-wire system, if the three-phase load is unbalanced, the current and power of each phase can only be calculated for a single phase. The formula for calculating the single-phase power is P=U phase *I phase *cos. Then the sum of the three-phase powers is the total power. Conversely, based on this formula, if the single-phase power is known, the phase current can also be calculated. It should be noted that unbalanced three-phase loads should be connected in a star configuration, that is, the neutral wire is essential. In this case, the selection of the switch can only refer to the phase with the largest current among the three phases, and the rated value should be slightly greater than the current value of the largest phase.

③ Calculation method for single-phase load current

If it is used in single-phase electric heating appliances, such as electric lamps and electric stoves, the formula is power (W)÷ voltage (220V)= current (A). For example, for a 100W electric lamp, it is 100 watts ÷220V=0.46A.

If it is used in single-phase motors, such as 1000W. Calculation method :1000÷(220×0.75×0.75)=8.08A.

Cosφ: The power factor for single-phase is 0.75.

η: For single-phase efficiency, take 0.75.