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Electric Motor Calculator
Estimate motor current, power, and efficiency relationships for electrician training and troubleshooting.
Professional workflow
Understand motor electrical relationships before checking nameplate and code data.
Single-phase: I = P / (V × PF × η) · Three-phase: I = P / (√3 × V × PF × η)
- Focuses on the motor values electricians commonly compare: horsepower/kW, voltage, phase, efficiency, and power factor.
- Helps explain why three-phase motors draw different current than single-phase loads.
- Designed as a planning and study aid before using motor nameplate data and code tables.
Motor Calculator
Calculate motor rated current and power requirements
Formula:
Three-phase: I = P / (√3 × V × η × PF)
Single-phase: I = P / (V × η × PF)
Use motor nameplate kW, voltage, efficiency, and power factor when available.
Motor starting current can be several times the running current, so protection sizing needs a separate check.
Choose single-phase or three-phase before comparing current with conductor and starter ratings.
Common powers:
💡 Typical motor efficiency 85-95%, power factor 0.7-0.9
Inputs to prepare
- Motor power or horsepower
- Supply voltage
- Single-phase or three-phase
- Efficiency
- Power factor
Worked example
Example: Estimate three-phase motor current
Given
- • Motor output = 5 kW
- • Voltage = 400 V
- • Power factor = 0.85
- • Efficiency = 90%
Steps
- 1. Use I = P / (√3 × V × PF × η).
- 2. Divide 5000 by √3 × 400 × 0.85 × 0.90.
Estimated current is about 9.4 A before checking nameplate FLA and code requirements.
Common use cases
- Motor current study problems
- Pre-checking feeder and starter discussions
- Understanding nameplate current vs. calculated estimates
How to read results
- Calculated current is an estimate; nameplate full-load current should control real work.
- Lower efficiency or lower power factor increases current demand.
- Starting current, overload protection, and conductor sizing require separate checks.
Code and safety notes
- Motor branch circuits have special rules for conductors, overloads, short-circuit protection, and disconnects.
- Use manufacturer nameplate data wherever available.
- Final equipment selection must follow local code and qualified professional review.
Frequently asked questions
What inputs are useful for motor calculations?
Voltage, horsepower or power, efficiency, phase, and power factor are commonly used.
Should I use nameplate data?
Yes. Nameplate and code tables should override estimates for real work.
Does three-phase differ from single-phase?
Yes. Three-phase power calculations include the √3 factor.
Is this useful for exam prep?
Yes. Motor current and power relationships are common electrician study topics.