What is the calculation method of load and load current of wire and cable?

1. Purpose and significance of load calculation: The statistical calculation of load in the design of low-voltage power supply and distribution system is an important content, and the calculation results of wire and cable load play a decisive role in the power supply capacity installation, selection of power supply and distribution equipment and safe and economic operation. The purpose of load calculation is to 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 breakers, isolating switches, busbars, fuses, etc.) as the basis for selecting equipment.

Calculate the load current flowing through each line (power supply line, high and low voltage distribution line, etc.) as the basis for selecting the line cable or wire cross-section.

Calculate the peak load, used to protect the electrical setting calculation and check the starting condition of the motor.

2. Load calculation method

At present, demand coefficient method and binomial coefficient method are widely used to determine the load of electric equipment in our country.

The binomial coefficient method is more reasonable in determining the load calculation of the branch trunk with fewer devices and large capacity difference of each device.

In the building distribution, the load density method and the unit index method are also commonly used to calculate the load. The unit index method can be used in the design stage. In the preliminary design and construction drawing design stage, it is advisable to use the demand coefficient method.

3, load calculation principle

When calculating the load, the power of the equipment should be calculated according to the following principles:

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

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

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

When the calculated active power of fire power is greater than the fire, the general power and lighting load may be removed at the same time to calculate the active power, and the total low-voltage equipment power and load should be calculated according to the unremoved general power and lighting load plus the fire load. Otherwise, when calculating the total low-voltage load, the fire protection load should not be considered. When there is a concurrent load in the fire load, the load should also be included in the general power and lighting load.

The single-phase load should be evenly distributed to the three phases. When the total calculated capacity of the single-phase load is less than 15% of the total calculated capacity of the three-phase symmetrical load in the calculation range, the calculation shall be based on the three-phase symmetrical load. When more than 15%, the single-phase load should be converted to the equivalent three-phase load, and then added to the three-phase load.

4, for example, wire load

The load in the power system generally refers to the active power, and the load of the wire we talk about verbally generally refers to the current value it can withstand, at present 1KW is equivalent to 2A current. So, how many watts is the 2.5 square wire load?

①2.5 square line load calculation

Speaking of the standard load of wire and cable, it can be said that the Internet is a variety of netizens. Some people say that 2.5 square wire load is 2 kilowatts, some people say that the correct is 2.5 square wire load 1.5 kilowatts. The arguments differ, and the parameters are even more mixed.

The load degree of the wire is mainly dependent on the current, but also depends on the voltage level. Under normal circumstances, the safe carrying capacity of 1 square millimeter below 50A is 6A, the safe carrying capacity of 1 square millimeter of 50~100A current is 5A, and the safe carrying capacity of 1 square millimeter above 100A current is generally 2.5A; So a square of copper cable can calculate the current. Wire and cable according to the three-phase AC formula :P=1.732*U*I*cosφ, cosφ is the power factor, choose 0.8; This is the formula for calculating the three-phase power. Single-phase :P=U*I*cosφ, cosφ is the power factor power factor for resistive load is 1, inductive load is 0.8.

This BV wire and BVR multi-strand soft copper wire is different, of course, and rubber sheath cable YC load is two kinds, rubber sheath cable load is better than BV/BVR/RVV plastic wire. That is to say, the outer insulation is not the same, which will also affect the load of the wire. The general 2.5 square wire load is 2 kilowatts, and the rubber cable, the full load can reach 3 kilowatts is no problem (it is estimated that it is not safe)

②1.5 square line load calculation

A few square is a nominal value of a line specification stipulated by the national standard, the square of the wire is actually the cross-sectional area of the wire, that is, the area of the circular cross-section of the wire, the unit is square millimeters, generally divided into :0.5, 1, 1.5, 2.5, 4, 6, 10, 16, 25 and so on.

The ones below 10(square mm) are generally called wires, and those above 10(square) are called cables. The square number of wires is a verbal term in the decoration of hydropower construction, and the square number of wires is often said to be without a unit, that is, square millimeters.

In general, the experience load is when the power grid voltage is 220V, the experience load per square wire is about one kilowatt.

Copper wire can carry 1-1.5 kilowatts per square, aluminum wire can carry 0.6-1 kilowatts per square.

Therefore, an appliance with a power of 1 kilowatt only needs to use one square of copper wire.

Specific to the current, the general copper wire can carry 3A to 5A current per square when the short distance is transmitted.

The heat dissipation condition is good to take 5A/ square millimeter, it is not good to take 3A/ square millimeter 1.5 square wire load is about 1.5 kilowatts.

5, load current calculation

① Calculate the difference between the load current and the rated current

Rated current refers to the current of the device when it is working normally. The load current refers to the working current of the load of the device. For a power supply equipment, if the load current is greater than its rated current, it may cause it to overheat or even burn. For example, your wall socket is rated 220V 10A, you connect a 220V 15A electric kettle, then it may burn the socket, because its internal line 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 is a 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, then its phase current and power are only single-phase calculation, and its single-phase power calculation formula is P=U phase *I phase *cos. Then the three phase power is added to the total power. In the same way, if the single-phase power can also be obtained according to this formula, 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 the switch of the wire and cable company can only refer to the phase of the large current in the three phases, and the rating should be slightly greater than the large phase current value.

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