Motor kW to Resistance Calculator
Estimate the equivalent electrical resistance of a motor from its rated power (kW or W), supply voltage, phase type, and power factor using standard electrical engineering formulas.
Motor kW to Resistance Calculator
How to Use Motor kW to Resistance Calculator
Estimating the equivalent electrical resistance of a motor from its power rating helps engineers size protection devices, verify load characteristics, and cross-check motor ohms values. Follow these steps for accurate results:
- 1Enter motor power. Input the rated output power from the motor nameplate.
- 2Select power unit. Choose kW (kilowatts) or W (watts) from the dropdown.
- 3Enter voltage. Enter the supply voltage in volts (V) — use line-to-line voltage for three-phase systems.
- 4Choose phase type. Select Single Phase or Three Phase depending on your motor supply.
- 5Enter power factor. If Three Phase is selected, enter the motor power factor (0.5–1.0). Default is 0.85.
- 6Click Calculate. Press the Calculate button to run the resistance and current computation.
- 7Review equivalent resistance and current. Read off the equivalent resistance (Ω), full-load current (A), and input power (W) from the output cards.
How to Calculate Motor kW to Resistance
Motor equivalent resistance is derived from fundamental electrical power equations. Because resistance cannot be calculated from power alone, voltage and phase type are required. The motor current calculator uses related principles to determine full-load current in the same manner.
Single Phase Formula
For single-phase motors, the power equation is P = V² / R. Rearranging for resistance:
Where P is power in watts, V is supply voltage, and R is equivalent resistance in ohms (Ω).
Three Phase Formula
For three-phase motors, the active power relationship incorporates the power factor (PF) and the √3 factor. Starting from P = √3 × V × I × PF, the current is I = P / (√3 × V × PF), and the equivalent resistance is R = V / I, giving:
Where P is power in watts, V is line voltage, PF is power factor, and R is equivalent resistance in ohms (Ω). Use the motor efficiency calculator to account for input versus output power differences.
Example 1 — Single Phase
Given: Motor Power = 2.2 kW, Voltage = 230 V
- Convert: P = 2.2 × 1000 = 2200 W
- R = 230² / 2200 = 52900 / 2200 = 24.05 Ω
- Current: I = V / R = 230 / 24.05 = 9.56 A
The 2.2 kW motor draws approximately 9.56 A with an equivalent resistance of 24.05 Ω at 230 V single phase.
Example 2 — Three Phase
Given: Motor Power = 15 kW, Voltage = 400 V, PF = 0.85
- Convert: P = 15 × 1000 = 15000 W
- R = (1.732 × 400² × 0.85) / 15000 = (1.732 × 160000 × 0.85) / 15000 = 235552 / 15000 = 15.70 Ω
- Current: I = P / (√3 × V × PF) = 15000 / (1.732 × 400 × 0.85) = 15000 / 589.0 = 25.47 A
The 15 kW three-phase motor at 400 V and 0.85 PF has an equivalent resistance of approximately 15.70 Ω and a full-load line current of 25.47 A. Cross-check results using the motor voltage calculator or motor nameplate calculator.
Motor kW to Resistance Chart
This reference chart lists equivalent resistance and full-load current for standard motor power ratings at 230 V single phase, calculated using R = V² / P. Use the motor current calculator for three-phase current values.
| Motor Power (kW) | Power (W) | Equivalent Resistance (Ω) | Current (A) |
|---|---|---|---|
| 0.37 kW | 370 W | 143.0 Ω | 1.61 A |
| 0.75 kW | 750 W | 70.5 Ω | 3.26 A |
| 1.5 kW | 1500 W | 35.3 Ω | 6.52 A |
| 2.2 kW | 2200 W | 24.0 Ω | 9.56 A |
| 3.7 kW | 3700 W | 14.3 Ω | 16.08 A |
| 5.5 kW | 5500 W | 9.62 Ω | 23.91 A |
| 7.5 kW | 7500 W | 7.05 Ω | 32.62 A |
| 11 kW | 11000 W | 4.81 Ω | 47.82 A |
| 15 kW | 15000 W | 3.53 Ω | 65.16 A |
Note: Values calculated at 230 V single phase using R = V² / P. Actual motor resistance depends on winding configuration, temperature, and operating conditions. For three-phase values use the calculator above.
Unit Standardization: SI vs. Imperial Sizing in Motor kW to Resistance
When working with Motor kW to Resistance calculations, using consistent physical units is vital. Small translation errors between SI Metric units (like millimeters, kilowatts, and meters) and Imperial units (like AWG wire, horsepower, and feet) can lead to serious sizing errors:
| Dimension | SI Metric Unit | Imperial Unit | Conversion Conversion Factor |
|---|---|---|---|
| Power | Kilowatts (kW) | Horsepower (HP) | 1 kW ≈ 1.341 HP |
| Length | Meters (m) | Feet (ft) | 1 m ≈ 3.2808 ft |
| Flow Rate | Cubic meters/hr (m³/h) | Gallons/minute (GPM) | 1 m³/h ≈ 4.403 GPM |
Always perform unit checks before installing physical components for Motor kW to Resistance to ensure they match equipment specification sheets.
Motor kW to Resistance Calculator Frequently Asked Questions
Not directly. Resistance requires both power and voltage. Using electrical power relationships — R = V² / P for single phase and R = (√3 × V² × PF) / P for three phase — you can calculate an equivalent operating resistance. The result represents electrical behaviour at rated conditions, not the physical winding resistance of the motor.
Equivalent motor resistance is the calculated resistance that would draw the same power as the motor at a given operating voltage. It is derived from electrical power equations and represents the overall load impedance seen by the supply, not the DC winding resistance measured by an ohmmeter.
Resistance cannot be determined from power alone. Using Ohm's Law and power equations, R = V² / P, voltage is an essential variable. The same motor power at different supply voltages will yield different equivalent resistance values. Without voltage, the calculation is mathematically incomplete.
No. Winding resistance is the DC resistance of the copper conductors measured with a resistance meter. Equivalent resistance from this calculator is a computed AC operating value derived from power, voltage, and power factor. They are different quantities used for different engineering purposes.
In three-phase systems, power factor directly influences the equivalent resistance result. A higher power factor reduces the apparent current draw, resulting in a higher computed equivalent resistance. The formula R = (√3 × V² × PF) / P shows that resistance scales proportionally with power factor.
Yes. Select Three Phase from the phase type dropdown and enter the line voltage and power factor. The calculator applies the three-phase formula R = (√3 × V² × PF) / P to compute the equivalent resistance and full-load line current for three-phase induction motors.
Motor resistance is influenced by winding temperature, magnetic saturation, rotor slip, and mechanical loading. This calculator uses idealised power equations and cannot account for those real-world variations. Results should be treated as engineering estimates for design reference, not precision measurements.
A 2.2 kW single-phase motor at 230 V has an equivalent resistance of approximately 24.05 Ω, calculated using R = V² / P = 230² / 2200 = 52900 / 2200 ≈ 24.05 Ω. The corresponding full-load current is approximately 9.56 A. Actual winding resistance will differ.