Selecting busbar (or bus duct) size is a complex engineering issue involving safety, cost, and technical performance. It can’t be simply determined by looking up a table; it requires a systematic selection process.
The following are detailed steps and considerations for busbar sizing, which you can use to make your decision.
Core Principle: Size by Current, Verify Parameters by Application
The core of busbar sizing is current-carrying capacity, but the final size is determined by a variety of factors.
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Step 1: Determine Key Basic Parameters
Before you begin your selection, you need to clarify the following information:
- Rated Current (Ir): This is the maximum continuous operating current the busbar is required to carry. It should be determined based on the total calculated current of all downstream loads, with a certain margin. This is the most important parameter.
- System Voltage (U): This includes the voltage level (e.g., 400V, 690V) and type (AC/DC).
- Frequency (f): 50Hz or 60Hz. 4. Short-Circuit Current (Icw/Ipk): The expected short-circuit current at the busbar installation point. The busbar must be able to withstand the thermal effects and electrodynamic shocks generated by the short-circuit current without damage.
- Protection Rating (IP): Determined by the installation environment.
· Indoor, dry environment: IP30, IP40
· Indoor, humid, dusty environment: IP54
· Outdoor: IP65, IP66 - Installation Method:
· Horizontal / Vertical
· Wall-mounted / Ceiling-mounted / Underfloor
· Installation Clearance (affects heat dissipation) - Ambient Temperature (Ta): The maximum ambient temperature of the busbar duct.
Step 2: Core Selection Process
- Initial Selection Based on Rated Current and Ambient Temperature
This is the most basic step. Busbar manufacturers provide product catalogs with detailed “current-carrying capacity tables.”
· Consult the ampacity table: Use the manufacturer’s table to find the busbar model and size that corresponds to your required rated current (Ir) and ambient temperature (Ta).
· Understand temperature correction: Ampacity tables are typically based on a standard ambient temperature (such as 35°C or 40°C). If your actual ambient temperature (Ta) is higher, the busbar’s current-carrying capacity will decrease and must be multiplied by a correction factor (Kt).
· Corrected current = sample ampacity × Kt
· Requirement: Corrected current ≥ your rated current (Ir)
Example:
You need to carry 1000A at an ambient temperature of 45°C. A busbar model in the catalog has an ampacity of 1250A at 40°C, and the correction factor Kt at 45°C is 0.85.
Thus: Corrected current = 1250A × 0.85 = 1062.5A, which is greater than 1000A and meets the requirement.
- Verify Short-Circuit Withstand Capacity
During a short circuit, the huge current generates a significant amount of heat in a very short period of time. The busbar must be able to withstand this thermal stress.
· Check Parameters: A key parameter for busbars is the rated short-time withstand current (Icw), measured in kA/1s (or 3s). It indicates the effective value of the short-circuit current the busbar can withstand for one second.
· Verification Condition: The busbar’s Icw must be greater than or equal to the expected short-circuit current at the system installation point.
· If the short-circuit current is high, you may need to select a larger busbar (even if its current carrying capacity meets the requirements), or choose a busbar made of higher-strength material or a special design.
- Consider Voltage Drop
When transmitting power over long distances, voltage drop must be considered. Excessive voltage drop can cause the voltage at the end-of-line equipment to be too low, impacting operation.
Calculation formula: ΔU = K * I * L
ΔU: Voltage drop (V)
K: Voltage drop coefficient per unit busbar length (mV/A/m, available in the datasheet)
I: Operating current (A)
L: Busbar length (m)
Requirement: The calculated voltage drop should be within the allowable range (typically less than 5% of the system’s rated voltage).
If the voltage drop is excessive, a busbar with a larger cross-sectional area (i.e., larger dimensions) should be selected to reduce resistance.
- Dynamic stability (optional, but important for high-current systems)
This refers to the busbar’s ability to withstand the significant electrodynamic forces (mechanical stress) generated by short-circuit current without permanent deformation. This is typically guaranteed by the manufacturer during design, but for systems with particularly high expected short-circuit currents, the rated peak withstand current (Ipk) parameter requires special attention.
Step 3: Other Important Considerations
- Future Expansion Needs:
If there are plans to increase loads in the future, allow a 15%-25% margin when selecting busbars. Select busbars with a higher current rating to avoid future replacements.
- Busbar Type:
Air-insulated: Simple structure, low cost, suitable for low and medium currents.
Dense-pack: Compact structure, high current carrying capacity, good heat dissipation, and strong anti-dynamic stability, making it the mainstream choice today.
Fire-resistant/fireproof: Suitable for locations with strict fire protection requirements.
Select the appropriate type based on your application scenario (e.g., high-rise buildings, factory floors, data centers).
- Materials and Processing:
Conductor Materials: The mainstream materials are copper (excellent conductivity, high cost) and aluminum (lightweight, low cost, but requires a larger cross-sectional area for the same current carrying capacity).
Casing Materials: Typically steel or aluminum, with aluminum offering advantages in corrosion resistance and weight. · Insulation Material: Insulation performance and thermal class (e.g., Class B, Class F, Class H) directly affect current carrying capacity and lifespan.
- Brand, Standards, and Certifications:
· Select products that comply with national and international standards such as IEC 61439-6 and GB 7251.6.
· Choose reputable brands to ensure product quality and after-sales service.
Summary: Selection Process Checklist
You can follow the following checklist step by step:
- Determine rated current Ir = __ A
- Determine ambient temperature Ta = __ °C
- Consult the manufacturer’s catalog and, based on Ir and Ta, preliminarily select busbar specifications: __
- Verify short-circuit capacity: Is system short-circuit current _ kA ≤ busbar Icw _ kA?
- Verify voltage drop (if necessary): Calculate voltage drop ΔU = __ V. Is it within the allowable range?
- Confirm that protection level IP __ meets environmental requirements. 7. Confirm whether the installation method is compatible with the product.
- Consider future expansion: Do you need to reserve margin?
- Consider all costs and select the most cost-effective solution, provided all technical requirements are met.
Final Recommendation
Always consult with the busbar manufacturer’s sales or technical support engineers.
They have the most comprehensive product data and selection experience, and can provide the most accurate model recommendations and solution designs based on your specific project circumstances. Clearly presenting your requirements (all the parameters mentioned in step 1) is the most efficient and reliable way to select the right solution.
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