Motor current equation
This calculator estimates three-phase motor full-load current from output power, voltage, efficiency, and power factor.
where:
- Pout
- Motor output power[kW]
- hp
- Motor horsepower
- IFLC
- Estimated motor full-load current[A]
- V
- Line-to-line voltage[V]
- PF
- Motor power factor
- η
- Motor efficiency as a decimal
- IOL
- Overload relay current reference[A]
- Fol
- Selected overload reference factor
Nameplate full-load current should override calculated current for final overload relay, contactor, and motor protection settings.
How to calculate motor full-load current
The calculator estimates three-phase motor current from shaft output power, line voltage, power factor, and efficiency. Horsepower is first converted to kilowatts, then electrical input current is calculated using the balanced three-phase relationship.
- Enter motor output in kW or horsepower.
- Enter line-to-line voltage.
- Enter expected full-load power factor and efficiency.
- Calculate estimated full-load current.
- Use the motor nameplate and starter coordination data for final protection settings.
Motor current calculation example
A 7.5 kW three-phase motor at 400 V, 0.86 power factor, and 90% efficiency has an estimated full-load current of 14.0 A. A 115% overload reference corresponds to about 16.1 A, but the final relay setting must follow the motor nameplate and applicable protection rules.
IFL = 7,500 / (√3 × 400 × 0.86 × 0.90) = 14.0 A
Ioverload,ref = 14.0 × 1.15 = 16.1 A
| Input or result | Value |
|---|---|
| Motor output | 7.5 kW |
| Line voltage | 400 V |
| Power factor / efficiency | 0.86 / 90% |
| Estimated full-load current | 14.0 A |
| 115% overload reference | 16.1 A |
Motor kW vs. horsepower
Motor kW and horsepower normally describe mechanical output, not electrical input. The calculator uses 1 hp = 0.746 kW before dividing by efficiency and power factor to estimate supply current.
Motor current vs. starting current
Full-load current is not the same as starting current. Direct-on-line, star-delta, soft-starter, and variable-frequency-drive starting methods produce different current and torque behavior. Breaker, contactor, overload relay, and cable selection must account for the actual starting method and duty.
| Selection item | Main information required |
|---|---|
| Contactor | Utilization category, operational current, voltage, and duty |
| Overload relay | Motor nameplate current, service factor, class, and start time |
| Breaker or fuse | Starting current, short-circuit coordination, and breaking capacity |
| Cable | Nameplate current, installation derating, starting voltage drop, and fault withstand |
Use nameplate data for final motor protection
Manufacturer nameplate current and starter coordination tables take priority over a formula estimate. Confirm phase, frequency, duty, efficiency class, ambient conditions, starting method, and required Type 1 or Type 2 coordination where applicable.
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Assumptions
- Three-phase motor
- Nameplate data is not available
- Overload setting is a starting reference only
Important Warnings
- Motor protection settings must follow the motor nameplate, starter type, duty, and applicable standard.
- Starting current, utilization category, and coordination are not selected here.