22 September, 2025 at 10:18

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A good thumb rule for circuit breaker selection (for LV distribution, up to 1 kV systems) is:

Breaker rating ≥ 125% of the full load current (FLC) of the circuit.
- For example, if your connected load current is 100 A, choose at least a 125 A breaker.
- This accounts for continuous operation and some margin.
Short-circuit capacity ≥ prospective fault

A good thumb rule for circuit breaker selection (for LV distribution, up to 1 kV systems) is:

Breaker rating ≥ 125% of the full load current (FLC) of the circuit.
- For example, if your connected load current is 100 A, choose at least a 125 A breaker.
- This accounts for continuous operation and some margin.
Short-circuit capacity ≥ prospective fault current at the point of installation.
- Check the available fault level (kA) at the panel and select a breaker with equal or higher breaking capacity.
Coordination with cable size:
- Breaker rating must be less than or equal to the cable’s current carrying capacity (ampacity) to ensure cable protection.
- Example: If cable ampacity is 150 A, breaker can be 125 A (not 200 A).
For Motors (DOL/Star-Delta):
- Breaker rating ≈ 1.25 × FLC
- Use motor protection circuit breakers (MPCB) or MCCBs with adjustable trip.
For Lighting/General Circuits:
- MCBs are commonly sized at 6A, 10A, 16A, 20A, 32A, etc., based on load current + 25% margin.

👉 Simple formula:

Breaker Size (A)≈1.25×Full Load Current (A)\text{Breaker Size (A)} \approx 1.25 \times \text{Full Load Current (A)}

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