Selection Calculation: A Five-Step Verification Process
The calculation for busway selection involves a five-step verification process: current calculation → ampacity selection → voltage drop verification → short-circuit verification → accessory configuration.
Industry experts note that the calculation follows a specific workflow: current calculation (determining current based on load capacity), ampacity selection (determining copper bar specifications based on current-carrying capacity), voltage drop verification (checking for drops over long distances), short-circuit verification (ensuring dynamic and thermal stability), and accessory configuration (start units, tap-off units, elbows, etc.).
Calculations are performed in accordance with GB/T 7251.8 (IEC 61439-8), with ampacity and voltage drop being the critical factors.
Five-Step Verification
- Current calculation: Calculate rated current (I=P/√3Ucosφ) based on load capacity (kW/kVA);
- Ampacity selection: Select busway specifications (copper bar dimensions/ampacity) based on current, ensuring a safety margin;
- Voltage drop verification: Verify voltage drop for long distances (>50m; limit ≤5%) and adjust cross-section if necessary;
- Short-circuit verification: Verify dynamic and thermal stability regarding short-circuit current (check busbars) to ensure safety;
- Accessory configuration: Configure start units, tap-off units, elbows, and plug-in units according to the circuit layout.
Key Calculation Points
First, calculate the current accurately.
Accurately determine the load current (applying demand factors) to provide a sound basis for selection.
Second, allow for a margin.
Include a margin in the ampacity rating (accounting for a 20%–30% load increase) to prevent overloading.
Third, verify the voltage drop.
Verify long-distance voltage drop (≤5%) to prevent low voltage at the terminal end.
Compliance is based on standards: calculations must align with GB/T 7251.8 (IEC 61439-8).
Pitfall Warnings
First: “Underestimating current.”
Underestimating load current leads to overheating from overloads;
ensure accurate calculations and include a safety margin.
Second: “Excessive voltage drop.”
Long-distance runs can result in excessive voltage drop (low terminal voltage);
verify and adjust accordingly. Third: “Missing accessories.”
Omitting tap-off units or elbows complicates installation;
ensure a complete set of accessories is specified.
Trends and Reminders
Busbar trunking systems are moving toward standardization, characterized by compact, high-current-capacity designs and integrated smart monitoring (e.g., online temperature monitoring).
For specifiers, the recommended approach is: “five-step verification, inclusion of safety margins, and adherence to standards.”
Accurate busbar calculations ensure a stable power supply.
