Calculator Inputs
Example Data Table
| Scenario | Phase | Voltage | Load | Demand | Continuous | Derating | Suggested |
|---|---|---|---|---|---|---|---|
| Small home service | Single | 230 V | 8.0 kW | 0.80 | 60% | 0.90 | 60 A |
| Workshop tools | Single | 230 V | 55 A (measured) | 0.90 | 30% | 1.00 | 80 A |
| Small commercial | Three | 400 V | 25 kVA | 0.75 | 50% | 0.90 | 50 A |
| Motor-heavy panel | Three | 480 V | Itemized list | 0.85 | 70% | 0.80 | Varies |
Formula Used
- Single-phase (kW): I = (kW × 1000) / (V × PF × Eff)
- Three-phase (kW): I = (kW × 1000) / (√3 × V × PF × Eff)
- Single-phase (kVA): I = (kVA × 1000) / V
- Three-phase (kVA): I = (kVA × 1000) / (√3 × V)
The calculator then applies these planning adjustments:
- Demand factor: I₁ = I × Demand
- Continuous share: I₂ = I₁ × ((1 − c) + 1.25c), where c = continuous% / 100
- Safety margin: I₃ = I₂ × (1 + Safety%/100)
- Derating: Required = I₃ / DeratingFactor
- Standard size: Select the next available standard breaker ≥ Required.
How to Use This Calculator
- Select phase and enter the supply voltage.
- Choose a load mode: total kW, total kVA, amps, or list.
- Enter power factor and efficiency if using kW or watts.
- Set demand factor to reflect realistic simultaneous usage.
- Set continuous load share for long-running loads.
- Add a safety margin and derating factor as needed.
- Click Calculate to see the recommended standard breaker.
- Use Download buttons to export a report.
FAQs
1) What is a main breaker?
A main breaker is the primary overcurrent device that disconnects and protects the service feeding a panel. It limits total current into the distribution system.
2) Why do continuous loads use 125%?
Many electrical standards treat loads running for extended periods as continuous. Using a 125% multiplier helps reduce overheating and nuisance trips under sustained demand.
3) Should I size the breaker to the exact calculated amps?
Usually you select the next standard rating above the required current. Exact matching is uncommon because breakers come in fixed sizes and adjustments add safety headroom.
4) What does the demand factor do?
Demand factor accounts for the fact that not all connected loads run at once. It reduces calculated current to a more realistic operating value for planning service size.
5) When should I use kVA instead of kW?
Use kVA when you only know apparent power from nameplates or utility data. Use kW when you have real power and want to include power factor and efficiency effects.
6) What is the derating factor?
Derating reflects reduced capacity due to temperature, bundling, enclosure limits, or installation conditions. A value below 1.00 increases the required breaker rating.
7) How does the motor allowance work here?
If enabled, the tool adds 25% of the largest motor current on top of summed currents. This is a simplified planning approach; motor circuit rules can be more specific.
8) Is this a replacement for code-compliant design?
No. It is a sizing aid for estimates and comparisons. Always verify conductor ampacity, protective device rules, and local requirements with a qualified professional.