How to use this calculator
- Choose the motor supply. Select three-phase AC, single-phase AC or DC so the correct current formula is used.
- Enter motor power and voltage. Use rated mechanical output power and line-to-line voltage for three-phase motors.
- Set efficiency and power factor. Use nameplate, datasheet or measured motor data where available.
- Compare design current. Review formula FLA, design current, service-factor current, selected-current utilization and margin.
How it works
Motor current starts with the rated mechanical output power. The real electrical
input power is higher because the motor is not perfectly efficient:
P_in = P_out / efficiency
For a balanced three-phase AC motor, current is then
I = P_in / (sqrt(3) x V_LL x PF)
Single-phase AC uses I = P_in / (V x PF), and DC uses
I = P_in / V. The calculator applies your design-current multiplier
and compares that result with the current limit you enter.
Use this page before downstream electrical checks: send the resulting current to the voltage drop calculator, compare conductor derating in the ampacity derating calculator, use the motor full-load current by HP page for formula-first searches, then check conduit fill and box fill for the installed wiring path. If the motor is fed by a transformer, use the transformer kVA calculator to check source apparent power.
Worked example
Verified against the live calculator
A 10 hp three-phase motor at 480 V, 90%
efficiency and 0.85 power factor has real input power of about
8.29 kW. Formula full-load current is
8,286 / (sqrt(3) x 480 x 0.85) = 11.7 A. With a
125% design multiplier, the comparison current is
14.7 A, leaving about 5.3 A margin on an entered
20 A reference limit.
Frequently asked questions
How do I calculate motor full-load amps from horsepower?
Convert horsepower to output watts, divide by motor efficiency, then divide by voltage and power factor. For three phase, divide by sqrt(3) times line-to-line voltage and power factor. In short: I = Pout / (sqrt(3) x V x PF x efficiency) for a three-phase motor.
Does this use NEC motor FLC tables?
No. This calculator estimates current from the motor power formula and your entered efficiency and power factor. NEC motor full-load-current tables, nameplate FLA, branch-circuit protection and overload settings are separate authority checks.
What voltage should I enter for a three-phase motor?
Enter the line-to-line voltage, such as 208 V, 230 V, 460 V, 480 V or 600 V. The three-phase formula already includes the sqrt(3) multiplier.
Why do efficiency and power factor matter?
Efficiency converts mechanical output power into real electrical input power. Power factor converts real input power into apparent power for AC current. Lower efficiency or lower power factor both increase current.
What is the design-current multiplier?
It is a user-entered multiplier applied to the formula FLA before comparing against the selected circuit or ampacity. The default 125% is a common screening value, not a complete motor-circuit rule.
Can I use this for single-phase or DC motors?
Yes. Single-phase mode uses I = Pout / (V x PF x efficiency). DC mode ignores power factor and uses I = Pout / (V x efficiency).
Method & assumptions
- Uses rated mechanical output power, not locked-rotor current or startup/inrush current.
- Three-phase mode assumes balanced line current and line-to-line voltage.
- Power factor is ignored for DC motors and required for AC motors.
- Formula FLA can differ from nameplate FLA and code-table full-load current.
- Does not size motor overloads, short-circuit protection, conductors, grounding, disconnects, starters, VFDs or control gear.