How do I select the right circuit breaker rating?

The breaker rated current must satisfy: I_load ≤ I_breaker ≤ I_cable. Select the next standard rating above load current (6, 10, 16, 20, 25, 32, 40, 50, 63 A for MCBs). The breaker must also not exceed the cable's current carrying capacity.

What is the difference between trip curves B, C, and D?

Trip curves define the instantaneous magnetic trip threshold. B curve: 3-5× rated (resistive loads, lighting). C curve: 5-10× rated (general loads, small motors). D curve: 10-20× rated (motors, transformers with high inrush).

What is breaking capacity and why does it matter?

Breaking capacity (kA) is the maximum short-circuit current the breaker can safely interrupt. It must exceed the prospective fault current at the installation point. Typical values: 6 kA residential, 10-25 kA commercial, 36-50+ kA industrial.

What is the difference between MCB and MCCB?

MCB (Miniature Circuit Breaker): up to 125 A, fixed settings, for branch circuits. MCCB (Moulded Case Circuit Breaker): up to 2500 A, adjustable trip settings, higher breaking capacity, for feeders and main distribution.

How do I coordinate breakers for selective tripping?

Selectivity (discrimination) ensures only the breaker closest to the fault trips. Rules: upstream breaker should have higher rating, slower trip time, and the ratio of rated currents should typically be ≥ 1.6:1 for MCBs of the same curve type.

Free Engineering Tool

Circuit Breaker Selection Calculator

Determine MCB/MCCB rating, trip curve (B/C/D), and verify breaking capacity. Rule: I_load ≤ I_breaker ≤ I_cable.

IEC 60898IEC 60947B / C / D Curves

Results

Recommended Breaker Rating

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Trip Curve

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Min Breaking Capacity

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Breaker Type

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Coordination Check

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Selection Rule

Trip Curves (IEC 60898)

Curve	Magnetic Trip	Application
B	3–5 × I_n	Resistive loads, long cables
C	5–10 × I_n	General purpose, small motors
D	10–20 × I_n	Motors, transformers, high inrush

⚠️ Note: This tool gives guidance only. Final selection must be verified by a qualified electrical engineer considering all installation conditions.

Circuit Breaker Selection Calculator | MCB/MCCB Sizing | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Selection Rule

Trip Curves (IEC 60898)