Updated Electrical & Motor Sizing

Horsepower to Amps Calculator – AC & DC Motors

Convert horsepower to amps for AC single-phase, three-phase and DC motors. Choose US or IEC horsepower, set power factor and efficiency, apply NEC 125% sizing and even reverse amps back to horsepower.

AC 1-Phase & 3-Phase DC Motors US & IEC Horsepower NEC 125% Sizing

Interactive Horsepower to Amps Calculator

Use the tabs to convert horsepower or watts to amps for single-phase and three-phase AC motors as well as DC motors. Select US or IEC horsepower, choose typical power factor and efficiency values, apply NEC 125% sizing if needed and compare multiple motors side by side.

Use a value from about 0.7 to 1.0. Typical is 0.85–0.9.
For example, 0.9 for 90% efficient.

Single-phase AC current is estimated using: I = P / (V × PF) where P is the input electrical power in watts and includes motor efficiency. Use NEC sizing when selecting conductors or breakers according to code guidance.

Use line-to-line voltage (e.g., 400 V, 415 V, 480 V).

Three-phase AC current is estimated using: I = P / (√3 × V × PF) for input power P. This assumes a balanced three-phase system with line-to-line voltage.

For DC motors, current is estimated using: I = P / (V), where P is the input electrical power including efficiency: P = (mechanical power in watts) / η.

This mode estimates motor output horsepower from current, voltage, power factor (AC) and efficiency. It is useful when you only know the measured current and nameplate voltage.

Compare up to three motors side by side. Each scenario can be AC single-phase, AC three-phase or DC. The calculator uses the same horsepower standard (US or IEC) as selected at the top of the page.

Horsepower to Amps Calculator – US & IEC, AC & DC Motor Conversions

When you size motors, cables, contactors or breakers, you often need to convert between mechanical horsepower and electrical current. This Horsepower to Amps Calculator from MyTimeCalculator lets you move between HP, watts and amps for AC single-phase, three-phase and DC systems, with options for US and IEC horsepower definitions, power factor, efficiency and NEC 125% sizing.

All formulas are based on standard electrical engineering relationships between power, voltage, current, power factor and efficiency. You can work forward from horsepower to find expected full-load current or work backward from measured current to estimate installed horsepower.

How This Horsepower to Amps Calculator is Structured

The calculator is organized into five modes that reflect common sizing and troubleshooting tasks:

  • AC Single-Phase (HP → Amps): For single-phase circuits, typically small motors or residential applications.
  • AC Three-Phase (HP → Amps): For industrial three-phase motors using the √3 formula.
  • DC Motors (HP → Amps): For DC drives, traction motors and battery systems.
  • Amps → Horsepower: Reverse mode when you know current and voltage but not motor output.
  • Multi-Motor Comparison: Compare up to three motors at once with consistent assumptions.

A small global settings row lets you pick the horsepower standard (US or IEC) and set default power factor and efficiency presets. Each tab can override these defaults if needed.

US vs IEC Horsepower – Why It Matters

In US practice, one horsepower is normally taken as 746 watts, while the IEC “metric horsepower” is about 735.5 watts. The difference is small but noticeable in precise calculations. This calculator lets you choose:

  • US HP: 1 HP = 746 W
  • IEC HP: 1 HP ≈ 735.5 W

All forward and reverse calculations use the selected standard, so results align with the datasheets and standards you work with.

AC Single-Phase – Horsepower to Amps

For a single-phase motor, the real input power P (in watts) is related to current I, voltage V and power factor PF by:

  • P = V × I × PF

Mechanical output power is HP × W_per_HP (where W_per_HP is 746 W or 735.5 W). To get input power, divide by efficiency η. Combining these gives:

  • I = (HP × W_per_HP) / (V × PF × η)

The single-phase tab applies this formula, lets you choose HP or direct watts as the input, and then optionally multiplies the running current by 1.25 for NEC-style sizing.

AC Three-Phase – Horsepower to Amps

In a balanced three-phase system, real power is:

  • P = √3 × VL-L × I × PF

Again, using mechanical power = HP × W_per_HP and input power = mechanical / η, the full-load line current becomes:

  • I = (HP × W_per_HP) / (√3 × V × PF × η)

This is what the three-phase tab implements. It is especially useful for sizing three-phase feeders, MCCs and breakers based on motor HP.

DC Motors – Horsepower to Amps

For DC systems, the relationship is simpler because there is no power factor term:

  • P = V × I

With mechanical power and efficiency, you have:

  • Input power P = (HP × W_per_HP) / η
  • I = P / V

The DC tab applies these formulas directly and can also show an NEC-sized current for wiring and protection estimates.

Reverse Mode – Amps to Horsepower

When you only know current, voltage and approximate power factor and efficiency, it is often useful to estimate the underlying motor HP. The reverse tab works in the opposite direction:

  • For AC single-phase: P = V × I × PF
  • For AC three-phase: P = √3 × V × I × PF
  • For DC: P = V × I

It then multiplies by efficiency to obtain output mechanical power and divides by the chosen W_per_HP to report horsepower.

Power Factor and Efficiency Presets

Real motors rarely operate at PF = 1.0 and 100% efficiency. To make the calculator practical, you can use presets:

  • Power factor presets: 0.70, 0.80, 0.85, 0.90 for small, older or modern motors.
  • Efficiency presets: 80%, 85%, 90%, 93% to approximate standard, high and premium efficiency motors.

You can still overwrite these values manually for a specific motor’s nameplate efficiency or tested power factor.

NEC 125% Sizing Rule

Many electrical codes, including the NEC, require conductors and protective devices to be sized at 125% of continuous load or motor full-load current. The calculator’s NEC options simply multiply the calculated running current by 1.25 and display the result. Always verify final design against the latest code, local amendments and manufacturer recommendations.

Using the Multi-Motor Comparison Table

The comparison tab lets you enter three different motor scenarios — perhaps different voltages, phases or efficiencies — and quickly see how the required current changes. This is helpful when:

  • Comparing single-phase vs three-phase options.
  • Comparing standard vs premium efficiency motors.
  • Evaluating different supply voltages for the same horsepower.

Each row shows running amps and, if enabled, NEC-sized amps so you can quickly spot which option results in lower current and smaller conductor sizes.

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Horsepower & Current FAQs

Frequently Asked Questions About Horsepower and Amps

Clarify how power, voltage, power factor and efficiency interact in real motor circuits.

Nameplate full-load current values often include conservative assumptions and may reflect worst-case operating conditions. If your inputs for efficiency or power factor are optimistic, the calculated current can be lower. Always compare with the nameplate and follow manufacturer recommendations when sizing conductors and protection.

The basic relationships between power, voltage and current still apply, but VFDs introduce harmonic currents and additional considerations. This calculator is ideal for approximate sizing and understanding fundamentals, but detailed VFD design should follow drive manufacturer guidelines and harmonic studies where necessary.

Not always. If efficiency is unknown, you can start with typical values from motor efficiency tables or use standard presets and adjust if measured current is significantly different from expected. Older or poorly maintained motors often have lower efficiency than modern premium units.

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