Breaker Rating Selection Calculator
Smallest standard breaker rating at or above the design current. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates breaker rating from design current. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Selection on current only. Breaking capacity, trip curve and discrimination are separate checks.
Cross-check: 85 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Selection on current only; breaking capacity, trip curve and discrimination are not checked.
What this calculator checks
- Smallest standard rating at or above the current × margin, and that it does not exceed the cable capacity you enter.
What remains unverified
- Breaking capacity against the prospective fault level.
- Trip curve, instantaneous pick-up and let-through energy.
- Selectivity with upstream and downstream devices.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 60364-4-43; IEC 60947-2, IEC 60364-4-43:2023 Ed 4.0; IEC 60947-2:2024 Ed 6.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Overcurrent protection and LV circuit-breaker product requirements are relevant. The calculator only rounds a user-margined current to a nominal rating; it does not check breaking capacity, trip curves, selectivity or local adoption. |
| Cross-check test cases | 85 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Design current | 400.94 A |
| Margin factor | 1 |
- Required = I × margin = 400.94 × 1 = 400.94 A
- Next standard rating ≥ 400.94 A is 500 A
Assumptions
- Standard ratings: 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 320, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3200, 4000 A
- Rating must also not exceed the cable current capacity
Selection on current only. Breaking capacity, trip curve and discrimination are separate checks.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Design current | A | greater than 0 A | |
| Margin factor | (number) | 1 or more, up to 2 | 1.0 selects on design current; 1.25 adds 25% for continuous loads |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Breaker rating | A | Shown to 0 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with design current
| Design current (A) | Breaker rating (A) |
|---|---|
| 40.1 | 50 |
| 100 | 100 |
| 200 | 200 |
| 401 | 500 |
| 802 | 1,000 |
| 1,600 | 1,600 |
| 4,010 | — |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60909-0 (short-circuit currents in three-phase AC systems)
- IEC 60947-2 (low-voltage circuit-breakers)
- IEC 60898-1 (circuit-breakers for household and similar use)
This is a first estimate. A protection study and the manufacturer's ratings and time-current curves decide the final selection. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter design current. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Standard ratings: 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 320, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3200, 4000 A. Rating must also not exceed the cable current capacity.
Which calculators should I use next?
Common next steps are Voltage Drop. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 85 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Circuit Breaker Formulas: Rating and Trip: Circuit breaker formulas: rating from design current and cable capacity, the 1.45 overload rule, trip ranges of B
- Cable Size Formulas: Current and Short-Circuit: Cable sizing formulas for current capacity with correction factors, conductor resistance and temperature
- Short-Circuit Formulas: Fault Level by MVA: Short-circuit formulas using the MVA method: transformer fault level, adding the source, three-phase fault current
- Overcurrent Relay Curve Formulas: Overcurrent relay formulas: the IEC operating time for the standard
- CT Burden Formulas: Lead Resistance and VA: Current transformer burden formulas: lead resistance, burden in VA for 5 A and 1 A secondaries
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