HP to Amps Calculator
Enter your motor horsepower, supply voltage, efficiency, and power factor to find the full-load current in amps. Choose DC, single-phase AC, or three-phase AC to match your motor type. Results update as you type, and the steps panel shows exactly how the number is derived from the formula.
Formula
Worked example
A 5 HP three-phase induction motor runs on 460 V with 90% efficiency and a 0.85 power factor. Step 1: output watts = 5 x 746 = 3,730 W. Step 2: input watts = 3,730 / 0.90 = 4,144 W. Step 3: amps = 4,144 / (1.732 x 460 x 0.85) = 4,144 / 677 = 6.12 A. The NEC 125% sizing figure is 6.12 x 1.25 = 7.65 A.
What does HP to amps mean?
Horsepower (HP) measures the mechanical output of a motor, the useful shaft power it delivers. Amperes (amps) measure the electrical current drawn from the supply. These two quantities are linked by the supply voltage, the motor efficiency, and, for AC motors, the power factor. Knowing the full-load current is essential for selecting wire gauges, circuit breakers, motor starters, and disconnect switches. The conversion is not a fixed ratio because the same HP motor will draw different amps at 120 V than at 480 V, and a more efficient motor draws less current than a less efficient one of the same rating.
The formulas for each motor type
DC motors: I = (HP x 746) / (V x efficiency). Here 746 is the number of watts in one mechanical horsepower. Efficiency (as a decimal) accounts for the heat and friction losses inside the motor. Single-phase AC motors: I = (HP x 746) / (V x PF x efficiency). The power factor (PF, between 0 and 1) reflects the phase difference between voltage and current in an AC circuit. Real power equals apparent power only when PF equals 1.0, which almost never happens in practice. Three-phase AC motors: I = (HP x 746) / (sqrt(3) x V x PF x efficiency). The square root of 3, approximately 1.732, appears because three-phase power is distributed across three conductors with a 120-degree phase shift between them. Three-phase motors are more efficient and smoother than single-phase motors of the same HP, which is why industrial equipment almost always uses three-phase power.
NEC motor ampacity rules and wire sizing
The National Electrical Code (NEC) Article 430 governs motor branch circuits in the United States. A key rule is the 125% continuous-load factor: conductors feeding a motor must be rated at not less than 125% of the motor full-load current (FLC). So a motor drawing 10 A at full load needs conductors rated at 12.5 A minimum. Overcurrent protection (the branch-circuit breaker or fuse) is typically sized at 250% of FLC for inverse-time breakers, or 300% for fuses, to allow for startup inrush current without nuisance tripping. NEC Tables 430.247 through 430.250 list the FLC values for standard motors at common voltages; use those published values for code compliance even if your nameplate differs slightly, because they represent the maximum current for that HP/voltage combination.
Motor efficiency, power factor, and energy cost
Motor efficiency is the fraction of electrical input that becomes useful mechanical work. A 90% efficient motor wastes 10% as heat. NEMA Premium efficiency motors reach 91-96% for motors between 1 and 200 HP, meaning they draw measurably less current than standard-efficiency motors of the same rating, which reduces conductor and breaker sizes and cuts electricity bills. Power factor is an AC concept: a PF below 1.0 means the supply is delivering more volt-amps than the motor actually uses. Utilities often charge commercial and industrial customers a penalty for low power factor, because it forces them to oversize distribution equipment. Capacitor banks and variable-frequency drives can correct a lagging power factor toward 1.0, reducing apparent current and demand charges.
Typical full-load amps for three-phase induction motors at 460 V (per NEC Table 430.250)
| Motor HP | Full-load amps (460 V, 3-phase) |
|---|---|
| 0.5 | 1.1 |
| 0.75 | 1.6 |
| 1 | 2.1 |
| 1.5 | 3 |
| 2 | 3.4 |
| 3 | 4.8 |
| 5 | 7.6 |
| 7.5 | 11 |
| 10 | 14 |
| 15 | 21 |
| 20 | 27 |
| 25 | 34 |
| 30 | 40 |
| 40 | 52 |
| 50 | 65 |
| 60 | 77 |
| 75 | 96 |
| 100 | 124 |
| 125 | 156 |
| 150 | 180 |
| 200 | 240 |
Values for squirrel-cage and wound-rotor induction motors at 60 Hz, 460 V line-to-line, based on NEC Table 430.250. Actual nameplate amperage may differ.
Frequently asked questions
How many amps does a 1 HP motor draw?
It depends on the voltage, efficiency, and motor type. A 1 HP single-phase motor at 120 V with 85% efficiency and 0.85 power factor draws about 8.6 A. The same motor at 240 V draws about 4.3 A. A three-phase 1 HP motor at 460 V draws about 2.1 A (per NEC Table 430.250). Use this calculator to get the exact figure for your supply voltage and motor specs.
Why does the three-phase formula use 1.732?
1.732 is the square root of 3 (to three decimal places). In a balanced three-phase system, each phase is offset by 120 degrees. The total power is split across three conductors, and the relationship between line-to-line voltage and phase voltage introduces a factor of the square root of 3. This is why three-phase motors draw less line current than single-phase motors of equivalent power.
What is a typical motor efficiency?
Most modern NEMA Design B induction motors run between 85% and 95% efficient at full load. Smaller motors (under 1 HP) tend to be less efficient, sometimes 70-80%. NEMA Premium motors hit 91-96% for ratings from 1 to 200 HP. Your motor nameplate should list the efficiency rating; if not, 85-90% is a reasonable assumption for a standard commercial motor.
What is a typical power factor for an induction motor?
Induction motors at full load typically have a power factor of 0.80 to 0.90. At partial load, the power factor drops significantly, sometimes to 0.50 or below. This is why oversizing a motor is wasteful: a 10 HP motor running a 3 HP load will have a poor power factor, increasing current draw and potentially incurring utility penalty charges.
How do I size a breaker for a motor circuit?
Per NEC Article 430, the branch-circuit overcurrent device (breaker or fuse) is sized differently from the conductor. For inverse-time circuit breakers, the maximum size is 250% of the motor full-load current. For time-delay fuses, the maximum is 175%. This oversizing is intentional: it allows the motor to start without tripping the breaker due to inrush current (which can be 6-8 times the full-load current). The conductors still follow the 125% rule and must be protected by a properly rated device.
Can I use this to convert amps back to horsepower?
Yes. Rearrange the formulas: for DC motors, HP = (I x V x efficiency) / 746. For single-phase AC: HP = (I x V x PF x efficiency) / 746. For three-phase AC: HP = (I x 1.732 x V x PF x efficiency) / 746. Enter your known amps, voltage, efficiency, and power factor and work backward using these expressions.
What is the difference between FLA and FLC?
FLA (full-load amperes) is the actual current shown on the motor nameplate under rated conditions. FLC (full-load current) is the value from NEC Table 430.247 to 430.250 for that HP/voltage combination. The NEC requires you to use the table FLC values for sizing branch-circuit conductors and overcurrent protection, not the nameplate FLA, because the table represents the maximum possible current for that HP/voltage category. Use nameplate FLA only for selecting the overload relay setting inside the motor starter.