Enter project values, then calculate. Use the export buttons after results appear.
This tool estimates current from load, then applies planning and derating multipliers.
- Three-phase current: I = P(W) / ( √3 × V × PF × η )
- Single-phase current: I = P(W) / ( V × PF × η )
- kVA option: I = S(VA) / ( √3 × V ) for three-phase
- Planning: I_design = I_base × Demand / Diversity × Continuous × Margin
- Derating: I_required = I_design / (TempMult × InstallMult)
- Breaking capacity: choose next Icu ≥ 1.10 × Fault(kA)
- Enter connected load and select the correct unit.
- Set voltage, phase, power factor, and efficiency.
- Apply demand and diversity based on project intent.
- If the feeder is continuous, enable the continuous factor.
- Enter ambient temperature and installation condition for derating.
- Provide the fault current from a short-circuit study.
- Click calculate, then export the report as CSV or PDF.
These examples illustrate typical outputs for common feeder scenarios.
| Scenario | Input Summary | Typical Recommendation |
|---|---|---|
| Workshop Feeder | 75 kW, 415 V 3φ, PF 0.90, η 0.95, Demand 80%, Diversity 1.10, 45°C enclosed, Fault 35 kA | Often 400 A frame with 50 kA class |
| HVAC Main | 250 kW, 400 V 3φ, PF 0.85, η 0.92, Demand 90%, Diversity 1.05, 40°C ventilated, Fault 50 kA | Often 800 A frame with 65–85 kA class |
| Small Retail DB | 120 kVA, 415 V 3φ, Demand 70%, Diversity 1.20, 35°C enclosed, Fault 25 kA | Often 400–630 A frame with 36–50 kA class |
Reminder: Always verify with manufacturer catalogs and coordination studies.
An Air Circuit Breaker (ACB) is typically used as an incomer or main breaker where higher currents, better coordination, and adjustable protection are required. A practical selection starts with the expected operating current, then verifies that the breaker frame rating and breaking capacity safely cover real site conditions.
First, convert the connected load into current. For three-phase systems, current is derived from power, voltage, power factor, and efficiency. If your load is given in kVA, the calculation uses apparent power and voltage. After that, apply demand and diversity to reflect how loads actually run together. Finally, add continuous-duty and growth allowances so the breaker does not operate near its limits during normal operation.
Real installations also need derating. Higher ambient temperatures, tight enclosures, and congested trunking can reduce effective current-carrying capability. This calculator applies a practical derating multiplier, then selects the next standard ACB frame rating that is above the derated requirement. The goal is a selection that remains stable across seasons and typical panel thermal conditions.
Breaking capacity is equally critical. The breaker short-circuit rating (Icu) must exceed the prospective fault current at the breaker location, ideally with a selection buffer. Use a short-circuit study value from your consultant or design software, and select the next higher Icu class available from your manufacturer range. Confirm selectivity and coordination using time-current curves against downstream devices.
In professional design, selection is not only about amperes. Check the breaker’s service short-circuit rating (often listed as Ics) and any short-time withstand capability (commonly expressed as a short-time current rating and time, such as 1 second). These characteristics influence whether the ACB can coordinate with downstream breakers without nuisance tripping and whether it can withstand high fault energy during selective delays. If your scheme uses zone-selective interlocking or time grading, ensure the chosen frame supports the required trip unit and communication accessories.
Also confirm project deliverables: number of poles, neutral protection requirements, metering needs, shunt trip, undervoltage release, auxiliary contacts, and draw-out or fixed mounting. Many tenders specify form of separation and maintainability expectations, where draw-out ACBs provide faster isolation and inspection. Finally, verify that cable sizes, busbar ratings, and panel temperature rise are compatible with the selected frame rating and the intended Ir setting.
Example (same as the default form values):
Input: 75 kW, 415 V, three-phase, PF 0.90, efficiency 0.95, demand 80%, diversity 1.10, continuous factor 1.25, safety margin 15%, ambient 45°C, tight enclosure, fault level 35 kA. Result: base current ≈ 122.04 A, design current ≈ 127.58 A, required after derating ≈ 149.22 A, recommended 400 A frame with 50 kA Icu class.
1) What is the difference between frame rating and Ir setting?
The frame rating (In) is the maximum rating of the breaker body. Ir is the adjustable long-time pickup. Ir should
match your design current while staying within the breaker’s adjustable range.
2) Should I always add a safety margin?
A margin is recommended for growth and measurement uncertainty. Keep it realistic; oversizing can reduce
protection sensitivity. Balance margin with coordination requirements and downstream device ratings.
3) Why does ambient temperature affect ACB selection?
Higher temperature reduces heat dissipation, increasing internal conductor temperature. Manufacturers apply
derating to maintain safe operation. Use site worst-case ambient, especially in non-air-conditioned electrical rooms.
4) Which fault current value should I enter?
Use the prospective short-circuit current at the ACB terminals from a short-circuit study. Do not use transformer
nameplate values unless the network impedance and cable lengths are already included.
5) Is the suggested trip setting final?
No. It is a starting point for coordination. Always validate with manufacturer time-current curves, upstream and
downstream devices, and cable thermal limits before freezing settings.
6) Can I use kVA instead of kW?
Yes. kVA reflects apparent power and does not require power factor. If you use kW, enter realistic power factor
and efficiency for a more representative running current.
7) Why is the recommended frame sometimes much higher than calculated current?
ACBs are sold in standard frame sizes. When the calculated requirement is below the smallest standard, the next
available frame is selected. Use Ir adjustment and coordination to maintain protection performance.