Circuit Breaker Sizing Calculator
Pick a standard circuit-breaker rating for a load using the wiring-rules coordination Ib ≤ In ≤ Iz. Enter the design current directly or let the tool work it out from the load power, apply a safety/growth factor, and get the next standard breaker up — with an optional check that it still protects your cable.
Enter Values
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Range 0–1
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the design (load) current Ib in amps, OR enter the load power in kW with the voltage (and power factor) to have it calculated.
- Leave the safety/growth factor at 1.25 for a continuous load, or set it to 1.0 if your figure is already the design current.
- Optionally enter the cable's current-carrying capacity Iz so the tool checks the breaker does not exceed it.
- Read the recommended standard breaker and the pass/fail cable-protection verdict.
How it works
Protection coordination requires Ib ≤ In ≤ Iz — the load current must not exceed the breaker rating, which must not exceed the cable's capacity. The tool multiplies the design current by the safety factor to get a required minimum rating, then chooses the next size up from the standard series 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250 A. If a cable capacity Iz is supplied, it confirms the chosen breaker satisfies In ≤ Iz.
Worked example
Worked example. A 20 A continuous load with a 1.25 factor needs at least 25 A of protection, so the recommended breaker is 25 A. With a cable rated Iz = 32 A, the check In ≤ Iz (25 ≤ 32) passes, so the 25 A breaker both carries the load and protects the cable.
Common mistakes
- Sizing the breaker to exactly the load current with no allowance — continuous loads normally use a 1.25 (125%) factor.
- Choosing a breaker larger than the cable can carry (In > Iz), which leaves the cable unprotected against overload.
- Sizing only for overload and ignoring the breaker's short-circuit rating (kA), the trip curve (B/C/D) and earth-fault-loop disconnection time.
Frequently asked questions
What does Ib ≤ In ≤ Iz mean?
It's the overload-protection rule: the design current Ib must be at or below the device nominal rating In, and In must be at or below the cable's continuous current-carrying capacity Iz. Meeting both means the device protects the cable without nuisance tripping.
Why the 1.25 factor?
A 125% allowance (factor 1.25) is a common design margin for continuous loads and provides headroom for growth. If your entered current is already the true design current with margin included, set the factor to 1.0.
Why did it jump to the next size up?
Breakers only come in standard ratings. The tool picks the smallest standard rating that is at least the required minimum, so a 22.5 A requirement rounds up to a 25 A breaker.
Is choosing the breaker enough to finish the design?
No. You still need to confirm the short-circuit (kA) rating exceeds the prospective fault current, that the trip curve suits the load, that Iz accounts for grouping/temperature derating, and that the earth-fault loop impedance gives a fast enough disconnection time.
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Cite this tool
Zerdly. (2026). Circuit Breaker Sizing Calculator. Retrieved from https://www.zerdly.com/tools/circuit-breaker-sizing-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.