Capacity Selection: Load + Margin
The process for selecting power distribution cabinet capacity follows this formulaic approach: calculate load (using the demand factor method) → select transformer/incoming line capacity → select cabinet type (GGD/GCK/MNS) and busbar rated current → verify short-circuit capacity. Industry experts note that capacity selection is determined by the load: first, calculate the design load and current;
ensure the cabinet’s rated current (busbar ampacity) is greater than or equal to the calculated current, while allowing a 10%–20% margin; verify short-circuit capacity (cabinet withstand rating ≥ system short-circuit current);
and when in doubt, choose a larger capacity rather than a smaller one.
Capacity Selection Steps
- Step 1: Calculate load—tally equipment capacity, check demand factors, and calculate design load (Pjs) and design current (Ijs);
- Step 2: Select incoming line—choose transformer capacity (or incoming switch rating) based on design load, e.g., 400kVA/630A;
- Step 3: Select cabinet type—choose based on current and number of circuits (GGD economical type / GCK drawer type / MNS modular type);
- Step 4: Determine busbar rating—busbar rated current ≥ design current (e.g., 1000A/1600A/2000A), allowing for a margin;
- Step 5: Verify short-circuit rating—cabinet short-circuit withstand rating ≥ system prospective short-circuit current (e.g., 50kA) to ensure compatibility.
Key Selection Points
The primary criterion is the current rating.
Capacity selection should be based on calculated current (busbar rating ≥ calculated current) rather than the total installed capacity of the equipment.
Secondly, ensure sufficient margin.
Reserve a 10%–20% margin (to accommodate future expansion and surge loads) to prevent overheating under full load.
Thirdly, perform short-circuit verification.
Match short-circuit capacity (cabinet withstand rating) to prevent damage during faults.
Finally, select the appropriate cabinet type.
Choose the type based on the number of circuits and maintenance requirements (e.g., drawer-type units facilitate easier maintenance).
Pitfalls to Avoid
First, “insufficient capacity.”
Errors arising from selecting based on total installed capacity (without applying the demand factor/Kx) or inadequate margins can lead to overload and overheating.
Second, “short-circuit mismatch.”
If the cabinet’s short-circuit withstand rating is lower than the system’s fault current, it may suffer damage during an accident.
Third, “incorrect cabinet type selection.”
Using a fixed-type cabinet where a drawer-type is needed makes maintenance inconvenient.
Trends and Recommendations
Capacity selection is moving towards “digitalization,” utilizing load calculation software for selection and smart power distribution systems for real-time load monitoring and capacity optimization.
For users, the recommendation is to “select based on current, reserve sufficient margin, and verify short-circuit ratings.”
Proper capacity selection ensures safe operation.
