⚡ CircuitSecrets
MCB / MCCB / ACB Selection Calculator
Professional breaker sizing from load current or load power — with ambient derating, tripping curve, and breaking-capacity verification per IEC 60898/60947.
MCB ≤100A
MCCB 100–630A
ACB >630A
Ambient Derating
Curve B/C/D
Icu Check
Protection Sizing
Breaker Input
Input Mode
Load Current
Load Power
Load Type
Lighting
Curve C
Heater / Resistive
Curve B
Motor — DOL Start
Curve D
Motor — Star-Delta / VFD
Curve D
General Distribution
Curve C
Load Current
Circuit Conditions
Protection Result
Breaker Selection
Ib — A
▶
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MCB/MCCB
▶
Load
Recommended Breaker
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Amperes — select inputs and calculate
Nearby Standard Ratings
Standard Rating Ladder
| Rating (A) | Type | Typical Icu |
|---|
► Formulas & Selection Logic
Design Current
Continuous duty: Ib = I × 1.25
Non-continuous: Ib = I × 1.0
3-phase power: I = P / (√3 × V × PF × η)
1-phase power: I = P / (V × PF × η)
Ambient derate: In(req) = Ib ÷ derating factor
IEC 60364-433: Ib ≤ In ≤ Iz
Curves: B resistive · C lighting/distribution · D motor inrush
► Practical Notes & Compliance
This tool gives a first-pass selection only. Verify against manufacturer datasheets and the applicable local code (IEC 60947/60898, NEC, or BNBC).
Motors: the rating above sizes thermal/overload protection. Instantaneous (magnetic) or electronic trip settings must be separately checked against actual starting current.
Cable coordination: the downstream cable must independently satisfy Ib ≤ In ≤ Iz — a correctly sized breaker does not guarantee a correctly sized cable.
For final circuits feeding sockets or damp-area lighting, add RCD/RCBO protection alongside overcurrent protection.
⚠ Note: Ratings based on standard reference conditions (30°C ambient). Apply derating for grouping and installation method as needed. Breaking capacity shown without a fault-level entry is a typical class value only — always verify with a short-circuit study before installation.
