By Kevin Beck – Updated Mar 24, 2022

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In electrical engineering, power is the product of voltage and current:
P = V × I. When power is expressed in horsepower (HP), it’s simply a unit conversion, because 1 HP = 746 W. Understanding how to move between HP, watts, volts, and amps is essential for designing and troubleshooting both single‑phase and three‑phase systems.

Step 1: Convert Horsepower to Watts

Because amps and volts are SI units, we first translate HP to watts. For example, a 30 HP load is:

746 W × 30 HP = 22 380 W

Step 2: Identify the System’s Phase

Three‑phase systems introduce a factor of √3 (≈1.732). Engineers often use 1.728 for the correction factor. The power equation for a three‑phase circuit becomes:
P = 1.728 V I

Step 3: Apply Efficiency

Real‑world equipment rarely operates at 100% efficiency. If the system’s efficiency is 0.45 (45 %), the equation becomes:

P = 0.45 × 1.728 V I

Step 4: Solve for Voltage or Current

With the example numbers (P = 22 380 W, I = 800 A, efficiency = 0.45, three‑phase factor = 1.728), the voltage is:

V = 22 380 ÷ (1.728 × 0.45 × 800) ≈ 35.98 V

Rearranging the formula gives a handy rule of thumb for any HP value:

P (HP) = (Efficiency × Phase Factor × Voltage × Current) ÷ 746

Where Phase Factor is 1 for single‑phase or 1.728 for three‑phase. This equation lets engineers back‑calculate any missing variable with confidence.

Key Takeaways

• Always convert HP to watts before using SI units.
• Use the √3 factor for three‑phase power.
• Account for system efficiency to avoid over‑design.
• Apply the compact formula for quick conversions in design calculations.

By following these steps, you can reliably translate horsepower into the electrical parameters your projects demand.