Amp To Hp Calculator

Enter the horsepower, amps, and voltage into the calculator to determine the missing variable.

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Amp to HP Formulas

Converting amps to horsepower requires knowing the voltage of the circuit and the type of electrical system (DC, single-phase AC, or three-phase AC). The constant 746 in each formula represents the number of watts in one mechanical horsepower. For AC circuits, the power factor (PF) and motor efficiency also factor into the calculation.

DC Circuit Formula

HP = (V \times I) / 746

DC motors have no power factor component because voltage and current are always in phase. This formula gives the electrical input power in horsepower. To find mechanical output, multiply by motor efficiency.

Single-Phase AC Formula

HP = (V \times I \times PF \times \eta) / 746

Single-phase AC motors are common in residential and light commercial settings. Power factor accounts for the phase difference between voltage and current waveforms in AC circuits. Typical single-phase motor power factors range from 0.6 at light load to 0.95 at full load.

Three-Phase AC Formula

HP = (V_{L-L} \times I \times PF \times \eta \times 1.732) / 746

The factor 1.732 (the square root of 3) accounts for the power contribution of all three phases in a balanced three-phase system. V refers to line-to-line voltage. Three-phase motors above 5 HP are standard in industrial and commercial installations because they deliver smoother torque and higher efficiency than single-phase equivalents.

Variables

• HP = mechanical horsepower (1 HP = 746 watts)
• V = voltage in volts (line-to-line for three-phase)
• I = current in amps
• PF = power factor (0 to 1.0; typically 0.80 to 0.95 for motors)
• eta = efficiency as a decimal (typically 0.85 to 0.96 for modern motors)

Amps to HP at 240 V (Single-Phase)

Amps to HP at 460 V (Three-Phase)

NEC Full Load Amps by Motor Horsepower

The National Electrical Code (NEC) Tables 430.248 and 430.250 provide standard full load amp (FLA) values used for sizing conductors and overcurrent protection. These values represent typical operating currents, not nameplate ratings. Per NEC 430.6(A)(1), wire and branch circuit breakers must be sized using these table values rather than the motor nameplate FLA. The nameplate value is only used for sizing the overload relay.

Power Factor and Motor Efficiency

The simplified formula HP = (V x I) / 746 assumes a power factor of 1.0 and 100% efficiency, which never occurs in a real motor. Power factor represents how effectively a motor converts apparent power (volt-amps) into real working power (watts). Induction motors draw magnetizing current that does no useful work but still registers on an ammeter. At full load, a typical induction motor has a power factor between 0.82 and 0.92. At light loads (below 40% of rated capacity), power factor can drop to 0.50 or lower, meaning the motor draws significantly more amps than the simple formula would predict for its actual mechanical output.

Motor efficiency is the ratio of mechanical shaft power to electrical input power. Losses occur as heat in the stator windings (stator copper loss), rotor conductors (rotor copper loss), magnetic core (iron loss), and through friction and air resistance (mechanical loss). NEMA Premium Efficiency motors, required since 2016 for new 1 to 500 HP motors in the US, achieve efficiencies between 85.5% for a 1 HP motor and 96.2% for a 500 HP motor. Efficiency peaks near 75% of rated load. A motor running at 50% load operates at roughly 2 to 4 percentage points below its full-load efficiency, while a motor at 25% load can lose 10 or more percentage points.

Starting Current vs Running Current

Electric motors draw far more current during startup than during steady-state operation. This inrush current, also called locked rotor amps (LRA), typically runs 5 to 8 times the full load amps for standard induction motors. A 10 HP motor at 230V with a 28 amp FLA may draw 140 to 225 amps for the first few seconds of startup. This surge does not produce proportionally more horsepower because most of the energy goes into accelerating the rotor and overcoming static friction. When using the amp-to-HP formula, always use running (steady-state) current rather than starting current to get a meaningful horsepower value.

NEMA motor design letters classify starting current characteristics. Design B motors (the most common general-purpose type) have a locked rotor current of roughly 600% of FLA. Design C motors have the same LRA but higher starting torque for hard-to-start loads like conveyors and compressors. Variable frequency drives (VFDs) eliminate the inrush problem entirely by ramping voltage and frequency gradually, keeping starting current near or below the full-load rating.

Common Applications

Amp-to-HP conversions appear in several practical contexts. Electricians use them to verify that a motor's measured running amps align with its rated horsepower, which can reveal overloading, bearing failure, or voltage imbalance. HVAC technicians measure compressor amps to confirm capacity matches system specifications. Industrial maintenance teams compare measured amps against NEC table values when sizing replacement motors or evaluating whether existing wiring and breakers can handle a motor upgrade. In residential settings, the conversion is useful for determining whether a garage circuit (typically 120V, 15A or 20A) can support a specific power tool or shop motor, or whether 240V service is required.