Circuit Breaker Sizing Calculator
AS/NZS 3000:2018
Inputs
Load current from cable sizing calculation
After derating (temperature, grouping, etc.)
▶Advanced options
Leave blank for default (6 kA domestic)
Results
Recommended Breaker Rating
32
A
B-curve
B-curve selected for resistive loads (trips 3–5× In). Suitable for general-purpose and lighting circuits with minimal inrush.
Ib ≤ In
32 A
AS/NZS 3000 Cl 2.5.3
In ≤ Iz
32 A
AS/NZS 3000 Cl 2.5.3
Breaking Capacity
6 kA
AS/NZS 3000 Cl 2.5.5
Inrush Tolerance
1
AS/NZS 3000 Cl 2.5.4
Circuit Breaker Selection — AS/NZS 3000:2018
Circuit breaker sizing follows the fundamental rule: Ib ≤ In ≤ Iz, where Ib is the design current, In is the breaker rating, and Iz is the cable current-carrying capacity.
The protective curve (B, C, or D) depends on the load type and inrush characteristics. B-curve suits resistive and lighting loads, C-curve handles motors with standard starting current, and D-curve accommodates heavy transformers and large motors.
Disclaimer: This calculator is a guide only. Always verify with the current standards and consult a qualified electrical engineer.