Calculator Input
Formula Used
Three phase current from kW:
I = P × 1000 ÷ √3 × V × PF × Efficiency
Three phase current from kVA:
I = kVA × 1000 ÷ √3 × V
Motor power conversion:
kW = HP × 0.746
Adjusted breaker current:
Required A = Base A × Continuous Factor × Demand Factor × Derating Factor × Future Margin
Standard selection:
The calculator selects the next standard breaker rating above the adjusted required current.
Example Data Table
| Load Type | Voltage | Power | Power Factor | Base Current | Suggested Breaker |
|---|---|---|---|---|---|
| Small Pump Motor | 400 V | 7.5 kW | 0.85 | 13.4 A | 20 A |
| Workshop Load | 415 V | 25 kW | 0.80 | 43.5 A | 70 A |
| Industrial Panel | 480 V | 75 kW | 0.90 | 100.3 A | 150 A |
| HVAC Feeder | 400 V | 50 kVA | N/A | 72.2 A | 100 A |
How to Use This Calculator
Select the input method first. Use kW for real power loads. Use kVA when apparent power is known. Use amps when measured current is already available. Use HP for three phase motors.
Enter the line-to-line voltage. Then enter the power factor and efficiency where needed. Add a continuous load factor when the equipment runs for long periods. Many designers use 125% for continuous loads.
Use demand factor when not all connected loads operate together. Use ambient derating when temperature, enclosure grouping, or installation conditions reduce breaker performance. Add future margin when later expansion is expected.
Press the calculate button. The result appears above the form. Review the base current, adjusted required current, selected breaker size, and safety margin. Use CSV or PDF export for project notes.
Three Phase Circuit Breaker Sizing Guide
Why breaker sizing matters
A three phase circuit breaker protects conductors, equipment, and distribution panels from excessive current. It must be large enough to carry the normal load. It must also be small enough to interrupt unsafe current before damage spreads. A poor selection can cause nuisance trips, heat, cable stress, or unsafe operation. This calculator helps estimate a practical breaker rating from common electrical inputs.
Understanding load current
Three phase current depends on voltage, load power, power factor, and efficiency. A balanced load shares current across all three phases. Real installations may not be perfectly balanced. Motors, compressors, welders, and HVAC equipment can also draw higher current during starting. For this reason, the calculated value should be reviewed with equipment data and local code requirements.
Continuous load allowance
A continuous load runs for a long period without stopping. Designers often apply an increased sizing factor for these loads. The calculator includes a continuous factor field. A value of 125 percent is common for many planning cases. This helps prevent overheating and nuisance tripping during extended operation.
Derating and margin
Ambient temperature, panel grouping, enclosure type, and conductor conditions can affect safe operation. Derating adjusts the required current upward when conditions are harder. Future margin adds spare capacity for small expansions. These two settings make the estimate more flexible for real projects.
Standard breaker selection
Breakers are sold in standard ampere ratings. The calculator rounds the adjusted current to the next listed size. This gives a clear recommendation for planning. Final selection should also check cable ampacity, voltage rating, interrupting capacity, trip curve, coordination, and manufacturer instructions.
Practical design note
This tool is for estimation and comparison. It is not a replacement for a qualified electrical design. Always compare the result with local electrical codes, equipment nameplates, conductor tables, and protection coordination studies. Large motors and critical feeders may need special protection settings.
FAQs
1. What is a three phase circuit breaker?
It is a protective device used on three phase systems. It disconnects all phases when current exceeds the safe limit or a fault occurs.
2. Why is power factor important?
Power factor affects current. A lower power factor needs more current for the same real power, so the breaker may need a higher rating.
3. Should I always use 125% for continuous loads?
It is commonly used for continuous load planning. However, final sizing should follow local code, equipment instructions, and conductor ampacity rules.
4. Can this calculator size motor breakers?
Yes, it can estimate motor breaker size from horsepower. Motor starting current and trip curve selection still need separate engineering review.
5. What does ambient derating mean?
Ambient derating adjusts sizing for heat or installation conditions. Higher temperatures can reduce safe carrying capacity and require a larger breaker.
6. Is the recommended breaker always final?
No. The result is an estimate. Final selection must check cable size, fault level, trip curve, voltage rating, and local electrical standards.
7. What is demand factor?
Demand factor accounts for loads that do not run together at full rating. It can reduce the calculated current for realistic operation.
8. Why does the tool round up?
Breakers come in standard sizes. Rounding up selects the next available rating that can carry the adjusted calculated load current.