Calculator Inputs
Example Data Table
| Motor Power | Voltage (V) | Efficiency (%) | Power Factor | Load (%) | Running Current (A) | Starting Current (A) |
|---|---|---|---|---|---|---|
| 2.00 HP | 230 | 82 | 0.86 | 100 | 9.20 | 32.20 |
| 1.50 HP | 115 | 78 | 0.80 | 90 | 14.03 | 49.12 |
| 3.00 kW | 230 | 88 | 0.92 | 75 | 12.08 | 42.29 |
Formula Used
Rated output power: Output Power = HP × 746, or kW × 1000.
Loaded output power: Loaded Output = Rated Output × Load Factor.
Electrical input power: Input Power = Loaded Output ÷ Efficiency.
Running current: Current = Input Power ÷ (Voltage × Power Factor).
Apparent power: Apparent Power = Input Power ÷ Power Factor.
Starting current: Starting Current = Running Current × Starting Multiplier.
Service current: Service Current = Running Current × Service Factor.
Minimum ampacity estimate: Ampacity = Running Current × 1.25.
Breaker estimate: Breaker Current = Running Current × Breaker Multiplier, then rounded to the next common breaker size.
Energy use: Daily kWh = Input kW × Hours Per Day, and Monthly kWh = Daily kWh × Days Per Month.
How to Use This Calculator
Enter the motor power in horsepower or kilowatts. Choose the matching unit before continuing.
Input the supply voltage, motor efficiency, and power factor from the nameplate or design assumptions.
Set the load factor to reflect the expected operating load, not only the rated maximum.
Adjust service factor, startup multiplier, and breaker multiplier if your application needs different engineering assumptions.
Add operating hours, monthly working days, and energy rate when you want energy and cost estimates.
Press the calculate button. The result panel appears below the header and above the form.
Use the CSV and PDF buttons to export the calculated values for reports, quotes, or design notes.
Always verify final sizing with the actual motor nameplate, manufacturer guidance, and your governing electrical standard.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates running current, startup current, apparent power, service current, conductor ampacity, breaker sizing current, and optional monthly energy use.
2. Why are efficiency and power factor both required?
Efficiency converts mechanical output to electrical input. Power factor converts real input power into the apparent power that determines line current.
3. Should I enter rated load or actual load?
Use the expected operating load whenever possible. Actual load gives a more realistic current estimate than assuming the motor always runs at full nameplate output.
4. What is the starting current multiplier?
It estimates inrush current by multiplying the running current. Capacitor-start and other motor designs can draw several times normal operating current.
5. Is the breaker result a final protection recommendation?
No. It is a planning estimate only. Final protection selection must follow motor type, conductor rating, ambient conditions, and applicable electrical rules.
6. Can I use kilowatts instead of horsepower?
Yes. Select kilowatts in the unit field. The calculator automatically converts that value into watts before computing electrical demand and current.
7. Why does the current rise at lower voltage?
For the same power requirement, lower voltage requires higher current. That increases cable losses, voltage drop sensitivity, and protection sizing pressure.
8. Can this help estimate operating cost?
Yes. Enter daily hours, working days, and energy price. The calculator estimates monthly energy use and a simple monthly electricity cost.