Inputs
Example Data Table
| Phase | Voltage (V) | Total Load (kW) | PF | Eff | Demand | Derating | Margin | Design Current (A) | Standard Rating (A) |
|---|---|---|---|---|---|---|---|---|---|
| Three-phase | 400 | 25.50 | 0.90 | 0.95 | 1.00 | 1.00 | 10% | ≈ 46.58 | 50 |
| Single-phase | 230 | 7.20 | 0.85 | 0.92 | 1.10 | 0.90 | 15% | ≈ 49.28 | 50 |
Formula Used
- Demand factor: Idemand = Irun × demand
- Derating: Iderated = Idemand / derating
- Continuous allowance: multiply by (1 + continuous%/100)
- Safety margin: multiply by (1 + margin%/100)
- Standard rating: choose the next higher common breaker rating
How to Use This Calculator
- Choose system phase and enter your line voltage.
- Enter loads in kW. Use general and motor fields as needed.
- Set PF and efficiency from datasheets or measurements.
- Apply factors like demand, derating, continuous allowance, and margin.
- Click calculate to see design current and breaker rating.
- Download CSV or PDF for records and approvals.
Load inventory and diversity
Breaker selection starts with a clean load list. Split lighting, receptacles, HVAC auxiliaries, and pumps into separate kW entries so you can review what is actually simultaneous. Many building panels operate below nameplate because not every circuit peaks together; a demand factor of 0.80–1.10 is commonly used for planning when measured data is not available. For construction temporary estimates keep a separate contingency line but do not double-count future circuits.
Power quality assumptions
Current rises when power factor or efficiency drops. A typical mixed commercial panel may see PF near 0.85–0.95, while modern variable-speed drives can change PF and harmonic content. Use equipment documentation or site measurements when possible, and treat unusually low PF as a signal to verify correction equipment and conductor sizing. Efficiency for small motors and fans may range from 0.88 to 0.96, and the same load will pull more amps when η is lower.
Continuous duty allowance
Loads expected to run for extended periods should be treated conservatively. Applying a continuous allowance of 25% helps reduce nuisance trips and limits thermal stress on terminals, busbars, and insulation. If your project defines a different rule, enter the percent that matches your specification and keep it consistent across submittals. For panelboards serving heating or process equipment, continuous operation is more likely than for office plug loads.
Derating and installation conditions
Real installations rarely match ideal catalog conditions. High ambient temperature, grouped cables, restrictive enclosures, or limited ventilation can reduce usable ampacity. A derating factor of 0.80–0.95 is often used to model harsh environments. When derating is applied, this calculator increases the required design current to maintain protection headroom.
Motor starting and trip curve choice
Motors can draw 5–7× running current at start, depending on the starting method and inertia. The calculator estimates inrush and suggests a trip curve category so the breaker is less likely to open during normal starts. Always confirm coordination with upstream protection, manufacturer guidance, and short-circuit studies for final approval.
FAQs
Should I size the breaker from kW or from measured amps?
Use measured running current when you have reliable site data. When planning, kW is acceptable if voltage, power factor, and efficiency are realistic. Recheck the result after equipment procurement and commissioning measurements.
What demand factor should I use for a mixed-use panel?
If you lack monitoring data, start near 1.00 for conservative design. For diversified loads with intermittent usage, many teams use 0.80–0.95, then validate with the owner’s load schedule and expected operating profile.
Why does derating increase the required breaker size?
Derating represents reduced heat dissipation or conductor capacity. By dividing by the derating factor, the calculator raises design current so the protective device is less likely to run hot or trip under sustained load.
How do I handle continuous loads correctly?
Enable the continuous allowance and enter the required percent. A common value is 25% for sustained operation. Apply the same rule consistently across similar panels, and confirm your local standard and project specifications.
Does the motor starting multiplier replace a coordination study?
No. It is a quick check to reduce nuisance trips during normal starts. Final selection should consider upstream devices, available fault current, cable protection, and manufacturer trip curves, typically confirmed in a coordination and short-circuit study.
Which standard breaker rating list does this tool use?
It uses a common progression of ratings (6A up to 3200A) and selects the next higher value above the design current. If your supplier uses different steps, update the list in the file to match your catalog.