⚡ Breaker Size Calculator
🔧 Breaker Size Calculator (Worldwide Standards) — Determine the correct circuit breaker ampere rating for residential, commercial, and industrial applications.
Based on NEC (USA) continuous load rules (125% factor), power factor, ambient corrections, and adjustable safety margins.
Supports single/three-phase, global voltages (120V, 230V, 240V, 400V, 480V etc.) and international breaker sizes.
This advanced tool includes derating, design margin, and a live comparison graph. Always consult local electrical codes.
📐 Load & System Parameters
0.8–1 for motors, 1 for resistive loads
Extra % added to load current before sizing
NEC Table 310.16 ampacity correction (informative)
⚙️ USA (NEC) factor info: Continuous loads → 125% of load current. Ambient derating affects conductor sizing; our calculator uses it for wire recommendation. Design margin adds extra safety before finding nearest standard breaker.
🌍 Global Breaker Standards
Custom multiplier (e.g., 0.96 for multiple conductors)
Load Current (A)
0.00 A
Min. Required Breaker (after rules)
0.00 A
✅ Recommended Breaker Size
— A
📏 Recommended wire (copper, 75°C): —
📘 Calculation summary: Load current × continuous factor + margin → nearest standard breaker.
📊 Comparison: Load vs Protection Curve
*Standard breaker size vs Load Current and Minimum required rating (NEC).
📐 Detailed factor notes (USA / worldwide)
✅ Continuous Load (NEC 210.20): If load operates ≥3 hours, breaker rating = 125% of load current.
✅ Power Factor (PF): for inductive motors (PF 0.8-0.9) increases apparent current. PF=1 for resistive loads.
✅ Design Margin: additional % to account for future expansion or harsh conditions.
✅ Ambient temperature derating affects conductor ampacity, which influences final wire size; we provide copper wire recommendation based on final breaker ampacity (ampacity must ≥ breaker rating).
✅ Breaker standards: IEC uses 6-125A series, USA uses 15-200A series. Calculator automatically selects nearest standard size greater or equal to minimum required.
❓ Frequently Asked Questions
What is a breaker size calculator used for?
It determines the correct ampere rating of a circuit breaker to safely protect wires and equipment against overcurrent, based on load power, voltage, phase, and environmental conditions.
Why 125% for continuous loads (USA NEC)?
Continuous loads (≥3 hours) generate sustained heat. NEC requires breaker to be sized at 125% of continuous load current to avoid nuisance tripping and ensure safety margin.
Does this tool support three-phase & international voltages?
Yes. It supports single/three-phase from 120V up to 480V, and uses both American & IEC breaker tables. All countries can select appropriate voltage from dropdown.
How does ambient temperature affect breaker selection?
High ambient reduces cable current capacity. Our calculator uses derating factors to recommend proper wire gauge, while breaker sizing remains based on load + safety margins.