Article — HP to Amps Calculator
HP to amps converter for motor sizing
To convert motor horsepower to amperage, divide the watts (HP × 746) by voltage, motor efficiency, and power factor. Single-phase: I = (HP × 746) / (V × eff × PF). Three-phase: divide additionally by √3. DC: skip the power factor. The result is full-load amps (FLA), and NEC Article 430.22 requires conductors sized at 125 percent of that.
The conversion is more than algebra; it ties together mechanical output, electrical input, and code-compliant wire selection. NEC Tables 430.247 through 430.250 publish standard FLA values that account for typical efficiency and power factor at each horsepower, so practical wiring rarely depends on a fresh calculation, but the formula explains where those numbers come from.
What HP to amps actually means
Horsepower is a unit of mechanical power, the shaft output of a motor. Amps is electrical current, what flows through the supply conductors. The two are linked by voltage (volts) and by how much of the electrical input the motor converts to mechanical work.
The 746 in every form of the equation comes from the original definition of mechanical horsepower by James Watt in 1782: 33,000 foot-pounds per minute. Converted to SI units, that is 745.7 W, rounded to 746 by NEC and most engineering tables.
The metric horsepower (PS in German, ch in French) is a slightly different unit at 735.5 W. European motor nameplates often list both. The difference is 1.4 percent, large enough to matter for precision work.
HP to amps formulas by phase
Three formulas cover the cases. The structure is the same in each: power output, converted to watts, divided by voltage and efficiency factors.
Single-phase: I = HP × 746 / (V × η × PF)Three-phase: I = HP × 746 / (V × √3 × η × PF)DC: I = HP × 746 / (V × η)Worked single-phase example. A 2 HP, 240 V, single-phase pump motor with efficiency 0.85 and PF 0.85: I = 2 × 746 / (240 × 0.85 × 0.85) = 1,492 / 173.4 = 8.6 A calculated. NEC Table 430.248 lists 12 A for a 2 HP, 230 V single-phase motor, which is conservative and accounts for variation across manufacturers.
Worked three-phase example. A 10 HP, 460 V, three-phase induction motor with eff = 0.92 and PF = 0.87: I = 10 × 746 / (460 × 1.732 × 0.92 × 0.87) = 7,460 / 638 = 11.7 A. NEC Table 430.250 lists 14 A; the spread accounts for motor design variation.
Motor efficiency and power factor
Efficiency is the fraction of electrical input that leaves as mechanical work. The rest is heat, friction, and magnetic losses. Power factor describes how much of the apparent power (volts times amps) ends up as real work versus circulating reactive power.
NEMA Premium and IE3 motors deliver the same mechanical output with substantially lower input current. The premium 5 HP example above draws about 8 percent less current than the standard motor at the same speed, which compounds across a year of continuous duty into real electricity savings.
HP to amps and NEC 430 wire sizing
NEC Article 430 governs motor branch circuits in the United States. The key rule is Article 430.22: conductors supplying a single continuous-duty motor must have an ampacity of at least 125 percent of the motor full-load current.
So a 10 A FLA motor requires wire rated for 12.5 A. A 28 A FLA motor (typical 5 HP single-phase at 240 V) requires 35 A wire, which is 8 AWG copper in a typical THHN run. Branch-circuit overcurrent protection per 430.52 sits at 250 percent for inverse-time breakers and 175 percent for non-time-delay fuses, both intentionally generous to ride through starting inrush.
Code-compliant wire sizing uses the FLA value in NEC Table 430.248 (single-phase) or 430.250 (three-phase), not the value you compute. NEC values incorporate worst-case efficiency and PF for the horsepower class. Computed values are useful for analysis, not for inspections.
Why three-phase reduces amps
Three-phase power delivers energy through three conductors carrying sinusoidal voltages offset by 120 degrees. At any instant, current can be returning on one phase while flowing out on another, so the total current any single conductor must carry is lower than the line-to-line equivalent.
The math: line current in a balanced three-phase system equals total power divided by V_line × √3 × PF, not by V_line × PF as in single-phase. The √3 (1.732) factor cuts current roughly 42 percent at the same voltage and power. For motors above about 3 HP, three-phase wins on copper cost and breaker size; under 3 HP, the simpler single-phase wiring usually wins.
Most U.S. industrial buildings use 208 V three-phase derived from a 120/208 V wye service, or 480 V three-phase for larger plants. Europe and most of the world use 400 V three-phase (230 V line-to-neutral). The HP-to-amps math is identical; only the voltage changes.
Starting current and breaker sizing
Motor starting current can reach 5 to 7 times the running FLA for 1 to 5 seconds. A 10 A FLA motor briefly draws 60 to 70 A on startup, which would trip a thermal breaker sized at running current.
NEC accommodates this with generous overcurrent rules. Inverse-time circuit breakers go up to 250 percent of FLA (430.52), instantaneous-trip breakers to 800 percent or 1100 percent in some cases. The branch-circuit overload protection (separate from short-circuit protection) sits at 115 to 125 percent of FLA, where the motor will actually trip if it sees a real overload.
For variable-frequency-drive (VFD) installations, starting current is controlled by the drive, not by the motor. Use the VFD nameplate input current for branch circuit sizing, plus 125 percent per NEC 430.122. The motor FLA still determines overload protection set inside the drive.
Common HP to amps mistakes
The formula is short; the inputs are where designs go wrong.
- Assuming 100 percent efficiency — a 1 HP motor does not draw exactly 1 HP × 746 / V watts. With eff = 0.85, it draws about 18 percent more.
- Ignoring power factor — an induction motor with PF = 0.8 draws 25 percent more current than the watts-only math suggests.
- Mixing mechanical and metric HP — 1 mech HP = 746 W, 1 metric HP (PS) = 735.5 W. Off by 1.4 percent.
- Using running current for breaker sizing — starting inrush is 5 to 7× FLA. Breakers must follow NEC 430.52, not running amps.
- Skipping NEC 125 percent on conductors — a motor that draws 10 A FLA legally requires 12.5 A wire. Inspectors will fail an undersized run even if it works.