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Circuit Breaker Sizing Calculator

Work out circuit breaker sizing instantly with clear inputs, formula shown and shareable results.

About Circuit Breaker Sizing Calculator

The breaker rating must sit between the design current and the derated cable ampacity. The curve then decides the instantaneous trip threshold: type B trips at 3-5 times rating, type C at 5-10 and type D at 10-20, so high-inrush loads need C or D to avoid nuisance tripping.

Formula

Breaker selection

Ib <= In <= Iz; magnetic trip = In x curve multiple; inrush must stay below the trip threshold

Worked example

With the default values below, Circuit Breaker Sizing Calculator returns Breaker rating of 32. Change any input above and every figure here is recalculated in your browser.

Inputs used

Design current
26 A
Breaker curveone of Type B (3-5x), Type C (5-10x), Type D (10-20x)
Type C (5-10x)
Cable ampacity after derating
32 A
Inrush multiple
6 x

Results

Breaker rating
32 A
Magnetic trip current
320 A
Expected inrush current
156.00 A
Nuisance trip assessment
Inrush is below the magnetic trip threshold

Step by step

  1. 1Values used
  2. 2Breaker selection
  3. 3Breaker rating= 32 A
  4. 4Magnetic trip current= 320 A
  5. 5Expected inrush current= 156.00 A
  6. 6Nuisance trip assessment= Inrush is below the magnetic trip threshold

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

What is Ib <= In <= Iz?

The fundamental coordination rule: design current no greater than device rating, device rating no greater than cable capacity.

When should I use type D?

For transformers, large motors and switch-mode power supply banks whose inrush would trip a type C. Note it also lengthens fault clearance time, which affects earth fault protection.

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