Breaker Size Calculator: engineer's guide
A breaker must protect the cable, not just carry the load. Sizing rules are set by the intersection of load, cable ampacity, and short-circuit withstand.
Open the Breaker Size calculator →
What this calculator does
The calculator returns a candidate breaker rating that satisfies Ib ≤ In ≤ Iz, applies the 125% continuous-load multiplier where required, and warns if the selected breaker exceeds the corrected cable ampacity.
Formulas used
- Continuous load rule:
In ≥ 1.25 × Icontinuous + Inon-continuous - Cable coordination:
Ib ≤ In ≤ Iz - Short-circuit energy limit:
I2t (breaker let-through) ≤ k2 S2 (cable withstand)
Variables
- Icontinuous = load current expected to persist ≥ 3 hours
- In = breaker frame or trip rating, amps
- k = cable-material constant (115 Cu/PVC, 143 Cu/XLPE, 76 Al/PVC, 94 Al/XLPE)
- S = conductor cross-section, mm²
- t = fault clearance time, seconds
Worked example
Scenario: Continuous lighting load 80 A on 25 mm² Cu XLPE (Iz after derate 95 A), fault clearance 0.1 s at 15 kA.
- Required In ≥ 1.25 × 80 = 100 A. Choose 100 A MCCB.
- Check coordination: 80 ≤ 100 ≤ 95 fails — the 100 A breaker exceeds the corrected cable ampacity.
- Either upsize the cable to 35 mm² (Iz ≈ 120 A) or use a 90 A breaker if load fluctuation allows.
- Short-circuit check: 15,000² × 0.1 = 22.5 MA²s; k²S² = 143² × 25² = 12.8 MA²s — cable withstand fails, so upsize the cable.
Common mistakes to avoid
- Missing the 125% continuous-load multiplier
- Selecting the smallest breaker that carries the load without checking cable coordination
- Ignoring the short-circuit I²t withstand of the cable
- Confusing frame rating with trip rating on adjustable MCCBs
Frequently asked questions
When is a load classified as continuous?
When it is expected to run at maximum current for three hours or more. Lighting, HVAC compressors, and elevator hold-loads typically qualify.
Do I use type B, C, or D characteristic?
Type B for resistive loads, Type C for typical inductive and mixed loads, Type D for motors and transformers with high inrush.
Does the breaker rating need to match a standard step?
Yes. Standard steps are 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200 A and up. Round down to the next available.
Standards and references
IEC 60947-2, BS 7671 Chapter 43, NEC Article 240
Related tools on this site
Open the calculator that pairs with this guide: Breaker Size Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.