When to Use This Calculator
Use this calculator for a quick motor running-current estimate when horsepower, voltage, efficiency, power factor, and phase are known. It is best for engineering checks and comparisons, not final motor branch-circuit sizing.
Enter Values
Formula Summary
Single-phase A = hp × 746 ÷ (V × efficiency × PF)
Three-phase A = hp × 746 ÷ (√3 × V × efficiency × PF)
This is an engineering estimate. Use the motor nameplate and applicable electrical code tables for equipment and conductor sizing.
Practical Examples
10 hp three-phase motor
10 hp, 460 V, three-phase, 90% efficiency, 0.85 PF
A = 10 × 746 ÷ (1.732 × 460 × 0.90 × 0.85)
A = 12.24 A
5 hp single-phase motor
5 hp, 230 V, single-phase, 88% efficiency, 0.82 PF
A = 5 × 746 ÷ (230 × 0.88 × 0.82)
A = 22.47 A
What This Estimate Represents
Horsepower is converted to mechanical watts, then divided by efficiency to estimate electrical real power. Voltage, power factor, and phase determine the corresponding running current.
Use the motor's rated voltage, full-load efficiency, and full-load power factor when those values are available.
Nameplate Current vs. Code-Table Current
The motor nameplate current can differ from both this formula estimate and electrical code table values. Different rules may use different currents for overload protection, branch-circuit conductors, short-circuit protection, and disconnect sizing.
Use the applicable code edition, manufacturer instructions, motor nameplate, and equipment listings for an installation. Starting current is much higher than calculated running current.
Common Mistakes
- Using mechanical output horsepower as if it were electrical input power without efficiency.
- Assuming running current covers starting current, locked-rotor current, or voltage dip.
- Using the formula result instead of required nameplate or code-table values for final installation sizing.
- Entering line-to-neutral voltage for a three-phase motor that is rated line-to-line.
Reference Notes
The calculation uses the standard horsepower-to-watts conversion and single-phase or balanced three-phase power relationships. For installations, compare against the motor nameplate, manufacturer instructions, equipment listings, and applicable electrical code rules such as NEC motor articles where adopted.