Horsepower to Volts and Amps Calculator

This horsepower voltage amps calculator estimates the current a motor draws from its horsepower rating and supply voltage. Enter horsepower, voltage, and whether the supply is single-phase or three-phase; it returns estimated amps, with efficiency and power factor included as assumptions.

Formula

For a single-phase or balanced three-phase motor, estimated line current is:

I = P_out / (V × η × PF × √N)

• I is estimated line current in amperes (A).
• P_out is mechanical output power in watts (W), calculated here as horsepower × 746.
• V is the supply voltage: volts (V); use line-to-line voltage for a three-phase supply.
• η is motor efficiency as a decimal, from 0 to 1.
• PF is power factor as a decimal, from 0 to 1.
• N is the phase factor: 1 for single-phase and 3 for three-phase.

Thus, single-phase current is P_out ÷ (V × η × PF), while balanced three-phase line current is P_out ÷ (√3 × V × η × PF). The calculator uses 746 watts per mechanical horsepower. This is an estimate based on the entered efficiency and power factor, not a motor nameplate reading.

Worked Example

Using the calculator defaults—5 hp, 230 V, single-phase, 0.85 efficiency, and 0.80 power factor:

1. Convert output horsepower to watts: 5 × 746 = 3,730 W.
2. For single-phase, N = 1 and √1 = 1.
3. Calculate the denominator: 230 × 0.85 × 0.80 × 1 = 156.4.
4. Divide output power by the denominator: 3,730 ÷ 156.4 = 23.849… A.

The estimated current, rounded to two decimal places, is 23.85 A. The second result is the mechanical output power: 3,730 W.

What the Calculator Can and Cannot Tell You

Horsepower describes power, voltage describes electrical potential difference, and amps describe current. They are related, but horsepower and voltage alone do not determine a motor’s current precisely. A motor loses some input power as heat and other losses, represented by efficiency; alternating-current motors also have a power factor that affects current. That is why the calculator asks for both values rather than treating horsepower divided by voltage as a complete answer.

The calculator’s phase selector applies the standard balanced-load relationship for single-phase and three-phase AC. The three-phase option uses √3 (about 1.732) in the denominator. It assumes the voltage you enter is the line-to-line voltage, commonly the voltage printed for a three-phase supply. Do not enter phase-to-neutral voltage in its place.

The result is useful for understanding a motor’s approximate running current or checking a calculation. It is not a substitute for the motor’s nameplate, manufacturer data, or a qualified electrical assessment. Actual current varies with the specific motor, operating load, supply conditions, temperature, and starting behavior.

How to Choose Inputs

• Horsepower: Enter the motor’s rated mechanical output in hp. Do not confuse hp with electrical input power or with a rating for an entire machine.
• Voltage: Enter the motor’s rated supply voltage in volts. If the supply and motor ratings differ, this estimate may not represent operation at the actual conditions.
• Phase: Choose single-phase or three-phase to match the motor supply. The formula assumes a balanced three-phase load.
• Efficiency: Use the motor’s stated efficiency as a decimal: for example, 85% is 0.85. If it is unknown, the default is only an example assumption.
• Power factor: Use the nameplate or manufacturer value when available. A value of 0.80 means 80%, not 0.80%.

Efficiency and power factor are not interchangeable. Both reduce the denominator in the current calculation, but they describe different characteristics of AC motor operation. Using guessed values can shift the estimate, so label the answer as approximate whenever either input is uncertain.

Units, Comparisons, and Common Mistakes

This calculator uses horsepower (hp), volts (V), watts (W), and amperes (A). For metric power ratings, 1 mechanical horsepower is approximately 0.746 kilowatts (kW), while 1 kW is 1,000 W. Convert kilowatts to watts before using the formula: for instance, 3 kW is 3,000 W. Do not treat horsepower and kilowatts as numerically equal.

A frequent shortcut is to divide 746 × hp by voltage. That omits efficiency, power factor, and—in a three-phase calculation—the √3 factor, so it can substantially misstate current. Another mistake is entering a percentage as though it were already a decimal: 85 must be converted to 0.85. Keep the voltage and power units consistent, and avoid rounding intermediate values; round only the displayed result.

Some motor references give full-load current tables rather than a calculation. Those values may be standardized estimates, while actual nameplate current depends on the motor design and rating. When the purpose is selecting or checking electrical equipment, use applicable nameplate information and have a qualified professional determine requirements. This calculator does not size conductors, breakers, fuses, or other protective equipment.

Frequently Asked Questions

How many amps does a 5 hp motor draw?
There is no single answer without voltage, phase, efficiency, and power factor. With this calculator’s defaults—230 V single-phase, 0.85 efficiency, and 0.80 power factor—the estimate is 23.85 A.

How do I calculate amps from horsepower and voltage?
Convert horsepower to watts using 746 W per hp, then divide by voltage multiplied by efficiency and power factor. For balanced three-phase, also multiply the denominator by √3.

Does three-phase use less current than single-phase?
At the same output power, voltage, efficiency, and power factor, the balanced three-phase formula includes √3 in its denominator and therefore gives a lower line-current estimate. Compare equivalent line voltages and motor conditions.

Can I calculate motor amps from horsepower and voltage alone?
You can make a rough estimate only by assuming efficiency and power factor. For a useful motor estimate, obtain those values from the nameplate or manufacturer information; otherwise the answer can differ from actual current.

Is this result suitable for choosing a breaker or wire size?
No. It estimates current from general motor relationships and does not determine safe equipment sizing. Have a qualified professional use the motor nameplate and applicable requirements for that decision.

Related

• AVI Converter for Mac: How to Convert AVI Files
• Google Tensor First Chipset for Pixel 6 & 6 Pro?
• Predicting Developer Fatigue with Oura Data & Random Forest

in hp | Motor power | hp | 5 | 0 | 100000
in v | Line voltage | V | 230 | 0.001
sel ph | Supply phase | Single-phase=1, Three-phase=3
in eff | Efficiency | - | 0.85 | 0.001 | 1
in pf | Power factor | - | 0.8 | 0.001 | 1
out amps | Estimated current | A | hp * 746 / (v * eff * pf * sqrt(ph)) | 2
out watts | Mechanical output power | W | hp * 746 | 0