Abstract:The limiting temperature rise of busway trunking systems is directly related to the current-carrying capacity of conductors and potential safety hazards. Therefore, it is essential to mark the limiting temperature rise, as it stands as the most critical safety technical parameter of busway trunking systems.

■ Keyword: Limiting Temperature Rise

With the rapid development of China's economy and modernization drive, electricity load has been increasing significantly. In recent years, the replacement of cables with busway trunking systems has become a common practice in developed countries, and China has also formed a clear development trend in this direction. However, some designers, users, and quality supervisors have insufficient understanding of the limiting temperature rise – the most critical safety technical parameter of busway trunking systems – leading to potential safety hazards in projects and waste of investment. Below are discussions on several issues related to the limiting temperature rise of busway trunking systems.

In fire accidents in China, those caused by electrical issues account for over 60%. The electrical equipment responsible for such fires includes cables, wires, high-low voltage switchgear assemblies, transformers, busway trunking systems, and electrical components. Most of these fires are triggered by short circuits resulting from insulation material aging due to prolonged overheating. The standard technical term for heat generation detection in this context is "limiting temperature rise".

Therefore, to ensure the safe operation of power supply systems and achieve energy conservation and emission reduction, the limiting temperature rise of busway trunking systems is an indispensable technical parameter for evaluating such products, which deserves great attention from designers, supervisors, Party A construction units, and acceptance units.

■ Keyword: Limiting Temperature Rise

Low-voltage power transmission trunk lines include wires, cables, branch cables, busway trunking systems, bare busbars, and piercing cables. Due to differences in heat dissipation performance among these products, their current-carrying capacity per square millimeter varies: for the same product with the same conductor specification, the temperature rise differs when passing the same current; for conductors with the same cross-sectional area, temperature rise also varies due to different structural designs .

Naturally, higher temperature rise leads to increased resistance, greater voltage drop, and higher power loss. For example, a 35mm² wire has low temperature rise when passing 80A current and meets standards at 100A. However, if the current reaches 120A or 150A, the temperature rise will exceed the standard, causing rapid aging of insulation materials and eventually leading to short circuits. When a 35mm² wire carries 100A current, the current density is 2.85A per mm²; in contrast, a 6mm² wire carrying 38A current has a current density of 6.3A per mm². If the 6mm² wire operates at the same current density of 2.85A per mm² (carrying 18A), its voltage drop and power loss will be much lower than those of the 35mm² wire – this is precisely because the conductor's temperature rise decreases, leading to reduced power loss .

The same principle applies to busway trunking systems. Thus, it is incorrect to calculate the conductivity of busway conductors based on current density (current-carrying capacity per mm²). Instead, the current-carrying capacity is closely related to factors such as structural design, heat dissipation, skin effect, impedance, and inductive reactance . Therefore, China's national standard GB 7251.2-2006 (equivalent to the international electrotechnical standard IEC 60439-2:2000) specifies that the current-carrying capacity of busway trunking systems shall be determined by the rated current under the limiting temperature rise .

■ Requirements for Temperature Rise in Busway Standards

The international electrotechnical standard IEC 60439-2:2000 and China's national standard GB 7251.2-2006 (both titled Low-voltage switchgear and controlgear assemblies - Part 2: Particular requirements for busbar trunking systems (busways)) have consistent provisions: the allowable temperature rise of busway trunking systems is determined based on the heat resistance class of insulation materials . For example, if the insulation material of a busway trunking system is Class F (with heat resistance ≥ 155℃), its allowable temperature rise is 115K under permissible ambient conditions (155℃ minus the ambient temperature of 40℃) .

Therefore, the current-carrying capacity of a busway trunking system can only be determined after a full-load test, and limiting temperature rise is indeed its most critical technical parameter. According to the test standards for China's mandatory China Compulsory Certification (CCC), a limiting temperature rise of ≤ 70K for busway trunking systems is considered safe and reasonable.

■ Issues with Busway Trunking Systems Caused by High Temperature Rise

Busway trunking systems, like wires and cables, are used as trunk line equipment for power transmission. For example, a 35mm² wire can carry both a rated current of 80A and 125A; the key difference lies in that the temperature rises corresponding to the rated currents of 80A and 125A are completely different. The same applies to busway trunking systems: for the same busway, when the limiting temperature rises are 70K and 90K respectively, its current-carrying capacity differs by more than 15%.

In the busway market, the temperature rise values of busway trunking systems range from 55K, 70K, 90K to even higher. However, high temperature rise leads to several problems. It is recommended that users select busway trunking systems with a limiting temperature rise of preferably ≤ 70K or ≤ 55K.

3.1High temperature rise directly results in increased power loss.

3.2The higher the temperature rise, the faster the aging of insulation materials, which drastically shortens the service life of the busway trunking system.

3.3High temperature rise accelerates the aging of surrounding insulation equipment (such as wires and cables adjacent to or connected with the busway, or electrical insulating supports), and may even easily cause fire accidents.

3.4 High internal temperature rise of the busway trunking system leads to increased voltage drop.

3.5 High temperature rise also causes a decrease in the mechanical strength of the busway trunking system. When metal conductors are heated, their stress begins to relax, thereby reducing the mechanical strength.

3.6 It lowers the safety factor, and the high temperature of the busway enclosure may easily cause burns to people.

3.7 High temperature rise significantly affects the surrounding ambient temperature.

■ Causes of Temperature Rise

4.1 Low copper content and high resistivity of copper busbarsPeople often refer to the copper content and resistivity of copper plates, which are indeed related to the current-carrying capacity of busway trunking systems. Copper busbars with a copper content of 99.95% or ≥ 99.93% and a resistivity of ρ ≤ 0.01777 Ω·mm²/m are considered high-quality for busway applications. If the copper content is low, the resistivity will increase; in this case, the conductor size must be enlarged to ensure the required current-carrying capacity and meet the temperature rise standard. Otherwise, the temperature rise will be excessively high.

4.2 Poor heat dissipation of insulation materials and enclosure structureFor busway trunking systems with well-designed structures and insulation materials that enable good heat dissipation, their conductors can meet current-carrying requirements even after being derated according to design manuals or electrical handbooks. However, some products use resin-cast insulation or other insulation materials with poor heat dissipation; for air-insulated busways and compact busways with inadequate heat dissipation, a more significant derating is necessary.

Some products have extremely poor heat dissipation due to their structural design and insulation materials, yet their conductors are selected based on an ambient temperature of 30℃ (as specified in electrical handbooks)—this misleads users. It is understood that some of these products can only achieve 60% to 70% of their designed current-carrying capacity, posing severe safety hazards to China’s power supply system and causing substantial power losses. This issue warrants serious attention.

4.3 Concealment of overloadingIn some projects, as equipment is added, the load increases, or the originally designed busway fails to meet on-site demands. When procuring busways for such projects, some parties use capacity-changing sections to adjust capacity without implementing effective protective measures. Overloaded operation leads to high temperature rise; furthermore, after capacity adjustment, the head-end switch cannot provide adequate overload protection for the reduced current, resulting in safety risks.

4.4 Unstable connection joints and increased joint resistivityUnstable connections, poor contact at joints, and increased joint resistivity can all cause the temperature rise of busway trunking systems to rise.

4.5 Temperature rise is related to the skin effectInside a conductor, heat generated by resistance is difficult to dissipate, leading to a relatively high internal temperature. This causes valence electrons to move at a lower speed and the current path to flatten, resulting in relatively wider electron paths. Consequently, the surface resistance of the conductor is lower, and electrons move faster—this is one of the causes of the skin effect.

For example: Two copper busbar conductors (6×100mm and 10×60mm) have the same cross-sectional area of 600mm², but the former has a 19% higher current-carrying capacity than the latter—this is the effect of the skin effect. When carrying the same current, the former exhibits lower temperature rise, less power loss, and a smaller voltage drop than the latter. In other words, under the same temperature rise, the latter has a 19% lower current-carrying capacity than the former. This shows that it is completely incorrect to determine a conductor’s current-carrying capacity and power loss solely based on its cross-sectional area.

4.6 Misleading conductor calculationSome technicians calculate the conductor size for busway trunking systems (regardless of their structural design) based on tables in Electrical Handbooks (or Electrical Design Manuals) and infer the service life of busways from the current-carrying capacity per mm²—this approach is incorrect. The service life of copper or aluminum busway trunking systems depends primarily on their operating temperature: the higher the operating temperature, the faster the aging rate (this applies to copper/aluminum busbars and insulation materials alike. The creep strength, tensile strength, and oxidation rate of electrical copper and electrical aluminum are all closely related to temperature).

Due to differences in structural design, busways have varying heat dissipation capabilities, leading to different internal temperatures. When selecting conductors based on an ambient temperature of 35℃ (as per design manuals):

For compact busways with good heat dissipation, the current-carrying capacity must be derated by an additional 5% to 15% to meet the ≤ 70K temperature rise standard;

For compact busways with poor heat dissipation, the derating should be approximately 20%;

For air-insulated busways, the derating requirement is even more significant.

To summarize the above points: It is incorrect to determine the conductor size for busway trunking systems based solely on conductor cross-sectional area and current-carrying capacity per square millimeter—without considering product structure or calculating the skin effect.

■ Requirements for Conductor Temperature Rise by Relevant Entities

5.1 Design Institutes’ Design and Temperature Rise

Currently, most design institutes do not include an agreement on temperature rise in their designs—only specifying the rated current and whether the system is three-phase four-wire or three-phase five-wire. This constitutes a relatively generalized design. For example, a busway trunking system with a current rating of 1000A and a temperature rise of ≤ 55K, which uses Class F insulation materials, can be used in projects requiring a 115K temperature rise, and even affixed with a nameplate indicating a rated current of 1600A or above. Therefore, it is crucial to specify the busway’s temperature rise in the design. It is recommended that the limiting temperature rise of busway trunking systems (≤ 70K or ≤ 55K) be used as the basis for project quality. If all power equipment in China can be controlled to operate at a temperature rise of ≤ 55K (for 1000V systems), not only will power line losses be significantly reduced, but electrical fires will also decrease—achieving energy conservation and emission reduction while protecting people’s lives and property.

5.2 Temperature Rise Requirements by Project Supervisors, Quality Inspection Stations, and Power Acceptance Units

For most busway projects, the current-carrying capacity of busway trunking systems cannot be directly verified. According to national standard GB 7251.2 and international standard IEC 60439-2, the limiting temperature rise of busways is determined by the heat resistance class of insulation materials under permissible ambient conditions. However, since design institute drawings and Party A (the client) do not clearly specify the temperature rise value, it is impossible to confirm the busway’s current-carrying capacity. Furthermore, without a pre-agreed limiting temperature rise between the user and the supplier, there is no standard to determine how many Kelvin (K) of temperature rise qualifies a busway as “acceptable” for project use.

It is recommended that project supervisors, quality inspection stations, and power acceptance personnel:

Check the CCC certificate and verify whether the conductor specification and limiting temperature rise for various current ratings in the CCC test report are consistent with the actual product;

Use a conductor tester to measure the conductor’s electrical conductivity, and infer its copper content and resistivity;

Check the CCC certificate and technical parameters on the websites of certification centers or testing institutes to ensure consistency with the test report.

These steps will ensure the busway’s current-carrying capacity and low-temperature operation.

5.3 Verification of Busway Limiting Temperature Rise in National Mandatory CCC Certification

In China’s mandatory CCC certification, except for special products like fire-resistant busway trunking systems, all other busways are uniformly tested against a temperature rise standard of ≤ 70K. However, busways have a wide range of current specifications, and the testing and certification fees for each product can cost tens of thousands of yuan. To reduce the burden on enterprises, certification centers classify busways into units based on their short-circuit withstand strength, with each unit covering multiple rated current specifications.

Currently, the unified regulation stipulates that for each unit, only the busway with the maximum current rating in the unit is tested; the other specifications are inferred by the enterprise and reviewed by the testing institute. Within the certified current range, the conductor specifications for the inferred currents must be determined based on the standard of “current-carrying capacity per mm²” of the test sample. If a conductor specification is smaller than that of the test sample, the enterprise must either have conducted a commissioned temperature rise test for it or have this conductor specification covered in another unit—otherwise, the certification will not be approved. This regulation is intended to control certification risks.

However, some testing institutes have approved test reports where enterprises arbitrarily filled in conductor specifications for the covered currents, without following the current-carrying capacity ratio of the test sample. For example, in a CCC type test report, a 2500A unit covers current ratings of 2000A, 1600A, and 1250A. The manufacturer tested a 2500A busway for this unit: the conductor specification of the 2500A sample was 6×250mm, with a limiting temperature rise of ≤ 70K. Based on this sample, the current-carrying capacity per mm² should be 2.0325A, so the recommended conductor specification for 2000A should be 6×165mm. However, the product description filled in by the manufacturer stated 6×125mm—obviously, the manufacturer could only achieve the 2000A current-carrying capacity by increasing the temperature rise.

This shows that certification centers struggle to control the conductor specifications for the covered currents in a unit. If there is a significant gap between the “current-carrying capacity per mm²” of the covered currents and that of the certified test sample, this issue requires serious consideration.

■ How to Ensure the Current-Carrying Capacity and Operational Safety of Busway Trunking Systems

China Quality Certification Center (CQC) and CCC testing institutions recommend strictly controlling the certification test process and publishing accurate, safe, and reliable data online—such as the limiting temperature rise, conductor specifications for different current ratings, and conductor materials of certified products. This facilitates user inquiries, safeguards people’s lives and property as well as users’ interests, and mobilizes the whole society to ensure project quality.

6.1 Design

When designing busway trunking systems, the limiting temperature rise must be marked on the drawings. Meanwhile, the technical requirements section of the drawings or the design plans should specify that one busway protection device (or temperature controller) is installed for each current level to monitor the operating temperature. It is recommended to install the device at the first joint of each current level.

Note: The protection device has two signal output points: one for overtemperature alarm and the other for current cutoff at limiting temperature. This ensures control over the internal temperature rise of the busway during operation.

6.2 Verification by Party A and Supervisors

Party A (the client) and supervisors can check the conductor specifications and temperature rise values in the CCC test report, or log on to the official website of China Quality Certification Center (CQC) to query other relevant technical parameters. Partial technical parameters are publicly available on CCC certificates and the website, such as IP protection rating, short-circuit withstand current (ICW = ____ kA), and rated current specifications. It is essential to ensure that the purchased busway trunking system is consistent with the certified parameters.

6.3 Limiting Temperature Rise Test

6.3.1 General Principle

The best way to ensure that the purchased busway trunking system can fully meet the requirements of safe, low-temperature, and low-loss operation under full-load conditions is to conduct a limiting temperature rise test.

6.3.2 Key Points of the Test

The detection positions for limiting temperature rise are also critical. The temperature rise of the busway’s incoming line section, conductors, socket interface conductors, joints, and enclosure must be tested. After operating under full-load current, the temperature rise is calculated by subtracting the ambient temperature from the stabilized maximum temperature, with the unit expressed in Kelvin (K).

When calculating the conductor specifications for other current ratings based on the “current-carrying capacity per mm²” of the qualified test sample, it is best to extrapolate from the maximum current to various smaller current specifications. If the current density (current per square millimeter) is feasible for the large-current busway, the conductor of the small-current busway with the same product structure and copper busbar thickness will definitely meet the requirements.