Analysis of ten misunderstandings in selecting AC power cable diameter

The design of the power supply and distribution system in the computer room has certain specifications. When users build a new power supply and distribution system in the computer room, they should select the appropriate AC line diameter through the design unit and strictly follow the design documents for construction. For the addition of general loads to the existing computer room, it is often designed and installed by the user.

Safe use of electricity is a basic requirement for power equipment installers and maintenance personnel. All installers and maintenance personnel need to understand the methods and principles of selecting AC cable diameters. In their daily work, maintenance personnel are not limited to discovering potential equipment failures, but should also pay attention to the risks of supporting equipment such as cables to achieve refined maintenance. In specific installation and maintenance work, many engineers have some misunderstandings about the selection of cable diameters, which need to be analyzed. Choosing the wrong cable diameter will increase construction or operation costs at best, and may bring huge safety hazards at worst.

The ten misunderstandings listed in this article are easy to occur in engineering and maintenance personnel. In fact, there are more influencing factors in the selection of wire diameter. When selecting the specific wire diameter, you should consult the electrician's manual or other relevant design specifications based on ambient temperature, allowable temperature rise, laying method, etc.

When the cable is thin, the cable has a large specific surface area, which is beneficial to heat dissipation; when the cable is thick, the cable has a small specific surface area, and heat is not easy to dissipate. When the same current passes through the wire per unit cross-sectional area, the thick cable has a higher temperature. If the cable temperature exceeds the allowable value, it will be dangerous. The following table shows the long-term continuous load current carrying capacity of plastic insulated copper core wires laid in the air (Chapter 14, page 99 of "Electrician's Manual", Shanghai Science and Technology Press, fourth edition, edited by Lv Ruliang et al., January 2002), the ambient temperature is 25℃, and the long-term allowable working temperature of the wire core is 70℃.

As can be seen from the table above, the current carrying capacity of thinner cables is much greater than 4A per square meter. As the cable diameter increases, the current carrying capacity per unit mm2 decreases significantly. Since the cable should not always operate at the highest temperature and there may be overcurrent or other factors, the current carrying capacity of the wire should be less than the current carrying capacity value in the table above when selecting.

From this point of view, it is more practical to understand the economic current density as 2 for thick cables and 4 for thin cables than to understand it as 2 is safer and 4 is more economical.

R=L/(σS)=100×2/(57×4)=0.88Ω

ΔU=IR=16×0.88=14.1V

The voltage drop of the connection circuit under the maximum working current shall not exceed the allowable value of the circuit (page 6 of "Electric Power Engineering Cable Design Specifications", GB50217-94). In this example, the voltage drop on the cable reaches 14.1/220=6.4%, which exceeds the requirement that the voltage drop on most equipment lines should not be greater than 5%. The load working voltage drops by 6.4%, and the corresponding working current increases by 1A. It is necessary to use a thicker cable (such as 6mm2) and recalculate the voltage drop until the voltage drop is less than 5%.

Myth 3: Only choose the wire diameter, not the wire type

When calculating the cable diameter, only the cross-sectional area of ​​the cable metal medium is determined. As long as the cross-sectional area is the same, regardless of the type of insulation layer and sheath, the cable itself has exactly the same properties (copper, communication room power cables generally do not use aluminum core wires). However, it is precisely because of the difference in insulation layer and sheath that the heat dissipation performance and allowable temperature rise are different. For example, the commonly used VV (polyvinyl chloride insulation) cable and JYV (cross-linked polyethylene insulation) cable, the former allows a temperature of 70°C, and the latter can reach 90°C. Therefore, the JYV cable allows a larger interception current. Under the same load current conditions, a smaller wire diameter can be selected. In addition, single-core and multi-core cables (referring to multi-core complete sets of cables with mutual insulation inside) have different heat dissipation conditions and different interception currents. For example, the cross-section of the copper core conductor is 50mm2. The current carrying capacity specifications of single-core and multi-core open-laid cables are shown in the following table when the ambient temperature is 25°C and the conductor temperature is 70°C (VV cable) and 90°C (JYV cable) respectively (data source: Beijing Cable Network).

As can be seen from the table above, the current carrying capacity of multi-core cables is smaller than that of single-core cables, and the current carrying capacity of VV cables is smaller than that of YJV cables. These factors need to be taken into account when designing cables. When multiple single-core cables are bundled and laid in parallel, the current carrying capacity should also be calculated based on the single-core cable multiplied by a derating correction factor less than 1. The following table shows the current carrying correction factors when multiple cables are laid in parallel in "Factory Power Supply", and the cables are 100mm apart.

Myth 4: Give priority to cables with large long-term safe current carrying capacity

Generally speaking, from the perspective of cable insulation performance, environmental protection performance and weather resistance, YJV cable has a large current carrying capacity and performs better than VV cable in all aspects, and should be given priority in engineering design.

In fact, although YJY cable has the advantages of large current carrying capacity, small cable diameter, light weight, and easy installation, under the same cross-sectional area conditions, the reason why YJY cable has a larger current than VV cable is simply because it can withstand higher temperatures. With the same cross-sectional area, copper quality and conductivity, YJY cable can be thinner than VV cable in terms of wire diameter when transmitting the same current, but the line resistance increases, line loss and voltage drop also increase, which may not be cost-effective for long-term operation.

Myth 5: The larger the wire size in parallel, the larger the wire diameter will be.

Large computer rooms have large load capacity and need to provide large current. If a single wire is selected, a power supply cable with a thick wire diameter is undoubtedly required, which is not convenient for construction, and there may not even be a thick enough wire available. It is allowed to connect multiple wires in parallel. Since the current carrying capacity per square meter of small wires is greater than that of thick wires, parallel connection may be more economical.

Theoretically, the current between parallel wires is distributed according to the cross-sectional area. As long as the wires are made of the same material (such as copper wire), they can be directly connected in parallel. However, in actual projects, it is best to use the same wire diameter. If the wire diameters are very different, the current distribution may be biased due to the resistance of the terminal and the inductive reactance that is not proportional to the cross-sectional area of ​​the cable. One wire may distribute too much current and exceed the safe current carrying capacity. In addition, if inconsistent wire diameters are used, it is necessary to carefully check whether the current on the wire is less than the safe current carrying capacity. The unit current carrying capacity of thin wires can only be calculated based on thick wires.

Myth 6: Only based on load current, not considering short-circuit current

Selecting the wire diameter of the AC input cable based only on the load current actually poses a safety risk. For example, a building is powered by a transformer with a power S of 315KVA, and the transformer Z value is 5%. Now it is planned to add a 3P air conditioner (single-phase) in the distribution room. It is found that there is a circuit breaker CB3 with a rated capacity of 500A in the distribution cabinet that is idle and unused. It is planned to introduce one-phase AC power to the air conditioner through this circuit breaker, as shown in the figure below. The engineering staff selected the wire diameter according to the economic current density method, taking the economic current density as 4A/mm2, the air conditioner working current as 12A, and the cable cross-sectional area S as 4mm2, and installed a 16A circuit breaker on the air conditioner side as the air conditioner input switch. A

16A

315KVA/Z=5% Information from: Power Transmission and Distribution Equipment Network

CB1/500A Information from: Power Transmission and Distribution Equipment Network

CB2/500A Information source: http://tede.cn

CB3/500A

Other loads

3P air conditioning

R=L/(σS)=50×2/(57×4)=0.44Ω Information from: Power Transmission and Distribution Equipment Network

Assuming that the power supply capacity of the power grid is infinite, the transformer short-circuit current IST is: Information source: http://tede.cn

IST=S/(3U×Z)=315×1000/(220×3×5%)=9545A Information

The single-phase equivalent resistance RT of the transformer auxiliary winding is:

Assuming that the sum of the resistances of all conductors and connectors from the transformer output to CB3 is 0.05Ω, if a short circuit occurs at point A at the end of the cable, the short-circuit current IS is

IS=U/R=220/(0.023+0.05+0.44)=429A

Since the circuit breaker tripping current is 500A, the circuit breaker does not trip after the cable end is short-circuited, and the cable burns out or even catches fire.

From the above examples, it can be seen that when selecting cables, short-circuit current needs to be verified. When checking the power supply and distribution system, if thin wires are found to be connected to the back end of a large circuit breaker, it should be paid special attention. (Note: In addition to the short-circuit current, the ground fault current should also be calculated to verify whether the circuit breaker meets the requirements. Since this article only discusses the cable selection issue, it does not discuss how to select a circuit breaker.) Information from: www.tede.cn

Myth 7: Selecting lines based on load current without considering circuit breaker capacity

Depending on the nature of the load, the circuit breaker capacity is generally selected to be 1.15~1.5 times the load current. After the circuit breaker is selected, the overload trip current is determined (large circuit breakers often allow the trip current to be set). The generation of overcurrent is related to the power supply quality, load quality and operating status, and also to the leakage current. In the power supply system of the communication room, leakage protectors are usually not installed. If the sum of the leakage current and the load current does not exceed the rated current of the circuit breaker, the circuit breaker will not trip and the load will continue to operate.

In the case of a large leakage current, if the wire diameter is designed only according to the load current, the wire diameter may be too small, exceeding the safe current carrying capacity of the wire, causing the cable to overheat and pose a greater safety risk than the leakage current.

The correct approach is to select the capacity of the circuit breaker (including micro-breakers, fuses and other overcurrent protection devices are similar) according to the load current, then select the wire diameter according to the circuit breaker capacity, and then check whether the voltage drop meets the specification requirements.

Myth 8: Only considering construction costs, not total operating costs

When designing power distribution, the design unit will calculate the load current, line voltage drop, etc., and design according to the principle of minimum construction investment, with little consideration of operating costs. Taking 3P air conditioner as an example, if 4mm2 cable is selected, the power consumed by the cable is:

P = I2R = 122 × 0.44 = 63W

If a 6mm2 cable is used instead, the cable resistance value is:

R=L/(σS)=50×2/(57×6)=0.29Ω Information source: http://www.tede.cn

The power consumed in the cable is

According to Beijing cable prices, 2×6mm2 cables are 2.2 yuan/m more expensive than 2×4mm2 cables, and the price difference for 50 meters of cables is only 110 yuan. The initial investment of 6mm2 cables is greater than that of 4mm2 cables, but the investment can be recovered in less than one year, which is obviously more economical and saves more total operating costs. Whether it is more economical to use thicker cables needs to be calculated in the same way. If the investment can be recovered in three to five years, it is advisable to use thicker cables.

Myth 9: The selection of the neutral line does not take into account the third harmonic and unbalanced current

When the three-phase load is unbalanced, current will flow through the neutral line; when the three-phase is seriously unbalanced, the current of the neutral line is even greater than the phase current. Computers, energy-saving lamps and other electronic devices often generate third and third harmonics, and the harmonic current passes through the neutral line. For electronic devices with poor harmonic suppression, the third harmonic current may be greater than the phase current, and the neutral line current is very large. In addition, the third and above harmonics have a high frequency, and there is a skin effect when flowing through the wire, that is, the current mainly flows through the surface of the conductor, which is equivalent to reducing the cross-sectional area of ​​the wire, and the thermal effect is more obvious.

Conclusion

The selection of AC cables may seem simple, but in order to select safe and economical cables, many factors need to be considered comprehensively. Choosing a wire diameter that is too large may increase construction costs, while choosing a wire diameter that is too small may increase operating costs and may lead to serious safety risks.

At present, most power cables in the communication field are configured to be safe. As copper materials become increasingly expensive and cable costs account for an increasing proportion, it is necessary to choose economical cables. For systems in operation, it is advisable to cooperate with professional computer room evaluation agencies to implement computer room evaluation and necessary rectification to ensure power supply safety.