Breaker Sizing Form
Use the inputs below to estimate a practical standard breaker size for feeders, motors, temporary boards, and construction power distribution.
Formula Used
Single phase current: I = P / (V × PF × Efficiency)
Three phase current: I = P / (√3 × V × PF × Efficiency)
kVA mode: I = kVA × 1000 / (Phase Factor × V)
HP mode: P(kW) = HP × 0.746
Demand adjusted current: Base Current × Demand Factor
Continuous current: Demand Current × 1.25 when continuous duty is selected
Derated current: Current after startup ÷ (Ambient Factor × Bundling Factor)
Final design current: Derated Current × (1 + Future Expansion) × (1 + Safety Margin), then round up to the next standard breaker size.
How to Use This Calculator
- Enter a load name so the result can be exported with a clear label.
- Choose whether your starting value is current, kW, kVA, or motor HP.
- Set voltage and phase to match the actual panel, feeder, or equipment supply.
- Enter power factor and efficiency when sizing from power instead of direct amperes.
- Apply demand, continuous duty, and startup inputs to reflect real operating conditions.
- Use derating values below 100% whenever ambient heat or cable grouping reduces capacity.
- Add future expansion and safety margin to reduce undersizing during later site changes.
- Submit the form to view the result section, chart, and download options above the form.
Example Data Table
| Scenario | Mode | Voltage | Phase | Load Input | Demand | Continuous | Recommended Breaker |
|---|---|---|---|---|---|---|---|
| Temporary Site Office Board | kW | 230 V | Single | 9.5 kW | 100% | Yes | 80 A |
| Tower Crane Auxiliary Feed | Current | 400 V | Three | 118 A | 100% | No | 150 A |
| Concrete Batch Motor Group | HP | 400 V | Three | 40 HP | 90% | Yes | 125 A |
These examples are illustrative planning cases. Final project selections should be checked against local code, breaker trip curves, conductor ampacity, and coordination studies.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates a practical standard breaker size from load data, operating duty, derating, expansion allowance, and safety margin. It is intended for planning and early construction review, not final stamped design.
2. When should I mark a load as continuous?
Use continuous when the load is expected to run for long periods near full demand. The calculator applies a 125% factor so the protective device is less likely to be undersized.
3. Why do derating values increase the selected breaker?
Derating below 100% means heat, grouping, or installation conditions reduce effective capacity. The calculator divides by these factors, which raises the current used for breaker selection.
4. Should motor loads use an inrush factor?
Yes, when startup current is meaningfully higher than running current. A higher inrush factor can move the recommendation to the next standard breaker size and better reflect motor starting behavior.
5. Can I size breakers from kVA instead of kW?
Yes. kVA mode ignores power factor because apparent power already includes it. This mode is useful when equipment schedules or nameplates list only kVA.
6. Does the recommended breaker also size the cable?
No. Breaker selection and conductor sizing must be checked together. The chosen protective device should never exceed the conductor ampacity after all code and derating rules are applied.
7. Why add future expansion and safety margin?
Construction projects often change before handover. These allowances give extra room for later connected loads, small revisions, and practical installation uncertainty without immediately redesigning the distribution path.
8. Is this enough for final approval?
No. Use it as a planning tool. Final approval should consider local regulations, protective device curves, fault levels, discrimination, equipment ratings, and a qualified engineer’s review.