How To Calculate 3 Phase Motor Power
By The Calcumatix Team Reviewed by Calcumatix Editorial Review 3 min read
Quick Answer
To calculate the electrical input power of a 3-phase motor, you must convert its mechanical horsepower (HP) into kilowatts (kW) and account for the motor's efficiency. First, multiply the motor's HP by 0.746. Then, divide that result by the motor's efficiency rating (expressed as a decimal). For example, if you have a 50 HP motor with an efficiency rating of 92%, multiply 50 by 0.746 to get 37.3. Divide 37.3 by 0.92. The result is 40.54 kilowatts of required electrical input power.
Imagine you are upgrading an industrial ventilation system and need to install a new exhaust fan. The fan comes with a large 3-phase electric motor, and you need to verify exactly how much power it will draw from the electrical panel to ensure you do not trip the main breaker. The motor nameplate lists the mechanical horsepower, the voltage, the amperage, the power factor, and the efficiency rating. Converting that mechanical horsepower rating into true electrical power consumption requires combining several pieces of nameplate data. This scenario explains how to calculate the actual electrical input power required to run a 3-phase motor at full load. To skip the manual math, use the 3 phase power calculator to get the result instantly.
The Difference Between Input Power And Output Power
The horsepower (HP) rating printed on a motor’s nameplate represents its mechanical output power. It tells you exactly how much physical twisting force the motor can deliver to the fan belt or pump shaft. It does not tell you how much electricity the motor consumes from the wall.
Because no machine is perfectly efficient, the motor always consumes more electrical power than it produces in mechanical power. The energy lost to heat and friction is represented by the “Efficiency” percentage on the nameplate. The input power formula: Input Power (kW) = (HP × 0.746) / Efficiency.
To find the true electrical draw, you must convert the mechanical horsepower into kilowatts using the standard conversion factor (1 HP = 0.746 kW), and then divide by the efficiency decimal to account for the electrical losses. You do not use the power factor (PF) in this specific calculation, because the efficiency rating already accounts for the gap between mechanical output and true electrical input.
The Step-By-Step Motor Nameplate Calculation Procedure
Let’s walk through the exact steps required to determine the power consumption for your new industrial ventilation fan using the standard nameplate data. You just need to find the correct numbers and apply the conversion math.
Step by step:
- Locate the motor nameplate riveted to the side of the metal housing.
- Find the mechanical power rating in Horsepower (HP).
- Find the nominal Efficiency rating (often labeled “NEMA Nom. Eff.”).
- Convert the efficiency percentage into a decimal by dividing by 100.
- Multiply the HP by 0.746 to find the mechanical output in kilowatts.
- Divide that result by the efficiency decimal.
- The result is the total electrical Input Power in kilowatts (kW).
Worked example: Inputs: Your new fan motor nameplate states it is a 25 HP motor. It has a nominal efficiency rating of 91.5%. You need to know the electrical load.
Step 1 (locate plate): Data found. Step 2 (find HP): 25 HP. Step 3 (find efficiency): 91.5%.
Step 4 (convert to decimal): 91.5 / 100 = 0.915. Step 5 (convert HP to kW): 25 × 0.746 = 18.65 kW mechanical output. Step 6 (divide by efficiency): 18.65 / 0.915 = 20.38.
Result: 20.38 kW input power (rounded to two decimal places). Even though the motor only delivers 18.65 kW of physical work, your electrical panel must be capable of supplying 20.38 kW of real electrical power to keep it running. Explore the Calcumatix engineering calculators for related tools.
Sources and References
Frequently asked questions
Why didn't we use the power factor in the power calculation?
The power factor (PF) is required if you are calculating the "Apparent Power" (measured in kVA) or the full-load amperage (Amps). However, when you calculate the "Real Power" (measured in kW) based directly on the mechanical horsepower and efficiency, the math is already complete. The efficiency rating dictates the exact relationship between the physical output and the real electrical input.
How do I calculate the apparent power (kVA) of the motor?
To find the apparent power, take the real input power (in kW) that you calculated above, and divide it by the Power Factor (PF) listed on the nameplate. If your motor requires 20.38 kW of real power and has a power factor of 0.85, the apparent power is 23.97 kVA. You must size your transformers based on apparent power.
How do I calculate the full load amps (FLA)?
To calculate the amperage, you need the horsepower, efficiency, power factor, and line voltage. The formula is: Amps = (HP × 746) / (Voltage × 1.732 × Efficiency × Power Factor). Most electricians simply read the "FLA" rating directly off the nameplate rather than doing this complex math manually.
Does a 3-phase motor use less power than a single-phase motor?
If a single-phase motor and a 3-phase motor both have a 10 HP rating and identical efficiency, they consume the exact same amount of total electrical power. However, the 3-phase motor requires significantly thinner wires because it splits that total power evenly across three separate electrical conductors instead of cramming it all through one.
What happens if the motor is not running at full load?
The nameplate data is only accurate when the motor is running at exactly 100% of its rated mechanical capacity. If your 25 HP motor is only driving a 15 HP load, its power factor and efficiency both drop significantly. To find the exact power consumption of a partially loaded motor, you must use a digital power analyzer.