The Relationship Between Busbar Thickness and Inner Bend Radius

In the manufacturing of power distribution cabinets, high- and low-voltage switchgear, and busbar trunking systems, the bending and forming of copper and aluminum busbars constitutes a core process. Among all bending parameters, the relationship between the busbar thickness (T) and the inner bending radius (R) directly determines the quality of the busbar processing, its electrical conductivity, and the overall assembly precision of the cabinet. So, what exactly is the correlation between busbar thickness and the inner bending radius? Robin, a technical engineer at SUNSHINE—a renowned Chinese manufacturer of busbar processing equipment—addresses this very question.

During the bending process, the outer side of the busbar is subjected to tensile stress, while the inner side is subjected to compressive stress. Since the maximum degree of tension that the material can withstand is limited, the thicker the busbar, the greater the tensile stress generated on its outer side during bending; consequently, the required inner arc radius R must be larger.

To ensure that the busbar does not crack on the outer side or wrinkle on the inner side during bending, the inner arc radius must be greater than or equal to a critical value, R_min determined by the thickness:

R_min=K×T

T：Actual thickness of busbar material (mm)

K：Bending Coefficient (typically set to 1.5)

In actual industrial production,  R=1.5×T is widely recognized as the “golden ratio” that strikes a balance between preventing material damage and controlling springback.

Under special circumstances, the K-value primarily depends on the busbar material (copper or aluminum) and its temper (soft, semi-hard, or hard)：

• Soft-Temper Copper Busbar (M): K≈0.8~0.5  (Possesses excellent ductility; the inner bend radius can be less than the thickness.)

• Half-Hard Copper Busbar (Y2, Most Commonly Used):

  • When T ≤ 4 mm, K = 0.5 or 1.0

  • When 4 mm < T ≤ 8 mm, K = 1.0–1.25

  • When 8 mm < T ≤ 12 mm, K = 1.25–1.5

  • When T > 12 mm, K = 1.5–2.0
• Hard-Temper Copper/Aluminum Busbar (Y): K > 2.0 (Highly brittle; the inner bend radius must be increased.)

Outer Cracking: The immense tensile stress generated within the material thickness exceeds the tensile strength limit of the copper or aluminum, resulting in visible cracks or micro-cracks on the back of the busbar, which severely compromises the conductive cross-sectional area.

Inner-Side Wrinkling and Bulging: The material is subjected to extreme compression along an excessively tight inner radius, preventing it from flowing naturally. This results in the formation of wrinkles on the inner side, which compromises the flatness of subsequent overlaps with other copper busbars.

Resistivity Anomaly: Severe localized deformation leads to lattice distortion, causing an increase in local resistivity at the bend; during high-current operation, this is prone to triggering abnormal heating.

Relying solely on theoretical formulas is often sufficient only for standard operating conditions. However, during the actual processing of busbars—when contending with variations in material hardness across different batches, structural strain induced by punching, and high-intensity bending under heavy current loads—process parameters inevitably undergo changes. To address this scenario, SUNSHINE—China’s largest manufacturer of busbar processing equipment—conducted practical production experiments using its NC.40ZB-2000 CNC busbar bending machine, which features a full closed-loop servo control system and intelligent springback compensation. These experiments provided a detailed analysis of the dynamic relationship and physical behaviors exhibited by busbar thickness (T) and the inner bending radius (R) during actual operations:

Three brand-new T2 copper busbars from the same batch—each measuring 100 mm × 10 mm (with dimensions of 500 mm in length, 100 mm in width, and 10 mm in thickness)—were subjected to 90° flat bending while maintaining a constant busbar thickness of 10 mm. By interchanging the bending punches from SUNSHINE, bending experiments were conducted using three distinct inner arc radii: R = 5 mm (0.5T), R = 10 mm (1.0T), and R = 15 mm (1.5T).

 1. Extreme Destructive Testing: Busbar Bending with an Excessively Small Inner Arc Radius (R = 5 mm; i.e., 0.5T):

Process Operation: An R5 punch was installed on an NC.40ZB-2000 CNC busbar bending machine, and a single 90° bend was performed on a No. 1 copper busbar.

Physical Phenomena: During the downward stroke of the bending machine, the reaction force was observed to be significantly elevated. At the bend location, severe “orange peel” texture—indicative of extreme material stretching—became visible to the naked eye; in localized areas, even minute transverse tear cracks appeared. Due to excessive compression at the inner arc, the material buckled upward, resulting in severe deformation of the contact surface. The actual thickness at the apex of the bend plummeted from 10 mm to 8.3 mm—a thinning rate of 17%. High-current conduction testing subsequently revealed a significant increase in electrical resistivity at this specific point, leading to an elevated temperature rise during operation.

2. Critical Safety Test: For a uniform-thickness inner bend radius of R = 10 mm (i.e., 1.0T):

Process Operation: The punch on the CNC busbar bending machine was replaced with an R10 punch, and Busbar No. 2 was subjected to a single 90° bend.

Physical Phenomena: Following the bend, no visible cracks were discernible to the naked eye on the outer surface of the busbar; however, tensile strain caused the surface to exhibit a distinct “orange peel” texture (a characteristic feature of grain deformation under tension), indicating that the material had reached the verge of its yield strength. The measured thickness at the apex of the bend was 9.1 mm—representing a thinning rate of approximately 9%—and due to stress concentration, the elastic springback angle increased upon the release of the bending force.(Related Article: The Impact of Bending Radius R and Springback Compensation on Busbar Bending Accuracy)

2. Golden Ratio Test (Inner Arc Radius R = 15 mm; i.e., at 1.5T):

Process Operation: Utilizing the intelligent multi-step programming function of the NC.40ZB-2000 machine, an R10 die was selected to perform “segmented, multi-point progressive bending.” The total 90° bending angle was divided into three distinct pressing positions, with only 30° of bending executed at each position. By leveraging the bending machine’s servo feeding system, the workpiece was offset laterally—relative to the centerline—by 8 mm in each direction (applying pressure sequentially at the three specific points: -8 mm, 0 mm, and +8 mm).

Physical Phenomena: The deformation on the outer side of the copper busbar was perfectly and uniformly distributed across three distinct, minute arc regions. The surface of the outer arc remained smooth and flat, completely free of cracks or “orange peel” texture. The inner arc was formed by the smooth, seamless blending of the three R10 arc segments. Measurements taken with a contour profiler revealed that the resulting composite inner arc geometry was functionally equivalent to a single large arc with a radius of R16.5 mm (approximately 1.65T). The material thickness at the apex of the bend zone measured 9.65 mm (representing a thinning rate of only 3.5%, a result far superior to industry standards). High-current temperature rise testing yielded entirely normal results.

Conclusion: This series of experiments conclusively validates the accuracy of R = 1.5T as the “golden ratio” for busbar bending. The results demonstrate that when the actual equivalent radius falls below 1.0T (as observed with Copper Bar No. 1), material damage is inevitable; conversely, when the equivalent radius is guided to approximately 1.5T—achieved through the multi-step process utilizing the NC.40ZB-2000 machine (as observed with Copper Bar No. 3)—the busbar attains optimal mechanical strength and electrical safety performance.

In this series of rigorous process validation experiments, SUNSHINE’s NC.40ZB-2000 CNC busbar bending machine demonstrated the formidable technical barriers that define high-end industrial CNC busbar machinery. As a renowned global brand in the electrical manufacturing sector, SUNSHINE maintains exceptionally high standards in the R&D and manufacturing of CNC busbar equipment—specifically regarding processing precision and stability, intelligent control and software capabilities, structural rigidity and versatility, and production efficiency. These standards enable us to fully meet our customers’ exacting requirements for busbar processing: high precision, zero burrs, and high efficiency.

SUNSHINE is not merely manufacturing a busbar machine; we are leveraging digital technologies to ensure that every copper busbar leaving our facility remains undamaged at the bend points—free from micro-cracks or excessive material thinning—thereby perfectly safeguarding the very lifeline of electrical distribution systems. We offer not only high-quality busbar processing equipment but also professional solutions designed to optimize your specific busbar processing workflows. If you have any inquiries regarding copper busbar processing techniques, equipment selection, or automation upgrades, we invite you to contact SUNSHINE’s team of technical experts; we will be pleased to provide you with a customized technical solution tailored to your busbar processing needs.