How to select Circuit Breaker Sizes for SMDB?
Many engineers still oversize (or) undersize breakers leading to:
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Nuisance tripping
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Poor protection
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Unsafe systems
Here is a simple and practical method to selecting the right breaker for SMDB (Incomer & Outgoing):
Step 1: Calculate Maximum Demand
Apply demand/diversity factor and don’t depend on connected load.
Step 2: Convert Load to Current
Use 3-phase formula with proper PF (0.8–0.9).
Step 3: Size the Incomer Breaker
Always add ~25% margin
Use MCCB (adjustable type)
Step 4: Size Outgoing Feeders
Based on individual DB loads
Match breaker with cable capacity
Don’t forget these important checks:
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Cable vs breaker coordination
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Short circuit capacity (kA rating)
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Selectivity (MDB → SMDB → DB)
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Voltage drop (limit 3–5%)
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Future expansion (20–30%)