Motor Load Calculator: Find Current & Power Use

This motor load calculator works out the electrical load of a three-phase AC induction motor from its rated power, voltage, efficiency, and power factor. You enter those four values, and it returns the full-load current in amps and the input power in kilowatts, which is the actual electrical demand the supply must handle.

Formula

The full-load current of a three-phase motor is calculated using the standard electrical power equation:

I = (P × 1000) / (√3 × V × η × PF)

Where:

• I = full-load line current in amperes (A)
• P = motor shaft power in kilowatts (kW) — the mechanical output rating on the nameplate
• V = line-to-line voltage in volts (V)
• η = efficiency as a decimal (e.g., 90% = 0.90)
• PF = power factor (dimensionless, typically 0.7–0.95)

The √3 factor appears because it is a three-phase system. The input power in kilowatts is simply the shaft power divided by efficiency: P_in = P / η.

Worked Example

Using the default values from the calculator — a 15 kW motor, 400 V supply, 90% efficiency, and 0.85 power factor:

1. Convert efficiency to decimal: 90 / 100 = 0.90
2. Calculate the denominator: √3 × 400 × 0.90 × 0.85 = 1.732 × 400 × 0.90 × 0.85 = 529.5
3. Convert kW to watts: 15 × 1000 = 15000 W
4. Divide: 15000 / 529.5 = 28.33 A

The calculator shows 28.33 A for full-load current and 16.67 kW for input power (15 / 0.90 = 16.67). This matches the worked steps.

When to Use This Calculation

This calculation is essential whenever you need to size conductors, circuit breakers, contactors, or overload relays for a motor circuit. It also helps estimate the actual electrical demand a motor places on a generator or transformer, and it is the starting point for energy cost estimates.

Typical situations include:

• Designing a new motor branch circuit
• Checking whether an existing cable or breaker can handle a replacement motor
• Estimating demand for a panel or sub-panel
• Comparing the efficiency of two candidate motors

Note that this gives the full-load current — the current drawn when the motor delivers its rated shaft power. At no load, a motor draws much less current (typically 30–50% of full-load current), and during starting it can draw 5–8 times full-load current for a few seconds.

How to Read the Result

The output current is the line current in each of the three phases. For a motor nameplate that says “FLA 28 A� (full-load amps), the calculated value should be close. If it is significantly higher, the motor may be overloaded, the voltage may be low, or the efficiency/power factor values may be inaccurate.

The input power tells you what the motor actually consumes from the supply. The difference between input power and shaft power is the heat loss in the motor. For the example, the motor consumes 16.67 kW to deliver 15 kW of mechanical power, so 1.67 kW is lost as heat.

For single-phase motors, the formula is simpler: I = (P × 1000) / (V × η × PF) — no √3 factor. This calculator is for three-phase only.

Common Mistakes and Misconceptions

The most frequent error is mixing up units. Motor power is often given in horsepower (hp) on older nameplates — you must convert to kilowatts first (1 hp = 0.746 kW). Voltage must be line-to-line, not line-to-neutral. Efficiency and power factor must be entered as percentages or decimals consistently — entering 90 instead of 0.90 will give a wildly wrong current.

Another common mistake is using the locked-rotor current or starting current in this formula. This calculator is for steady-state full-load operation only. Starting current is a separate value, usually listed on the nameplate as “LRA� (locked rotor amps) or “inrush.�

A frequent misconception is that a motor always draws its full-load current. In reality, the current varies with the mechanical load. A motor driving a light load draws less current, and the power factor also drops. The nameplate full-load rating is the maximum continuous current the motor is designed to draw safely.

Limitations of the Formula

This formula assumes a balanced three-phase supply and a motor operating at its rated voltage and frequency. Real-world conditions can differ:

• Voltage imbalance between phases increases current and heating
• Harmonics from variable frequency drives (VFDs) add distortion that raises RMS current
• Motor efficiency and power factor vary with load — nameplate values are at full load
• Altitude and ambient temperature affect motor cooling and permissible current

For precise sizing of protection devices, always consult the motor nameplate and the relevant electrical code (such as NEC or IEC) — the calculated current is a starting point, not a substitute for code tables that add service factors and starting-current considerations.

How to Choose Input Values

Get the motor power, voltage, efficiency, and power factor directly from the motor nameplate. If efficiency is not listed, a typical value for a modern three-phase motor is 85–95% depending on size and class (IE2, IE3, etc.). Power factor is usually between 0.80 and 0.92 for a loaded motor.

If you are sizing for a motor that will run at a different voltage than nameplate (e.g., 460 V vs 400 V), use the actual supply voltage. The current will be inversely proportional to voltage — a motor on a higher voltage draws less current for the same power.

For a motor driving a variable load, use the maximum expected shaft power, not the nameplate rating, if they differ. The nameplate rating is the safe maximum, but if the load never exceeds 10 kW on a 15 kW motor, use 10 kW for sizing conductors to avoid oversizing.

Frequently Asked Questions

What is the difference between motor load and motor current?
Motor load refers to the mechanical power the motor delivers (shaft power), usually in kW or hp. Motor current is the electrical current drawn from the supply, in amps. They are related through voltage, efficiency, and power factor — this calculator converts between them.

Can I use this calculator for single-phase motors?
No, this calculator is for three-phase motors only. For single-phase, use the formula I = (P × 1000) / (V × η × PF) without the √3 factor. The result will be higher for the same power because single-phase supplies deliver less power per amp.

Why is the calculated current different from the motor nameplate FLA?
Nameplate FLA is measured under specific test conditions and may include a service factor. Your calculated value uses the efficiency and power factor you entered, which may differ from the actual operating values. Also, if the motor is not fully loaded, actual current will be lower.

What is a good power factor for a motor?
A power factor of 0.85–0.92 is typical for a fully loaded three-phase induction motor. At light load, the power factor drops significantly (sometimes below 0.5). Low power factor means more current for the same useful power, which is why power factor correction capacitors are often installed.

How do I convert horsepower to kilowatts?
Multiply horsepower by 0.746. For example, a 20 hp motor is 20 × 0.746 = 14.92 kW. This is the mechanical output power, which is what you enter into the calculator.

Does voltage drop in the cables affect the motor current?
Yes. If the voltage at the motor terminals is lower than rated, the motor draws more current to maintain the same power output, which increases heating. This is why voltage drop calculations are critical for long cable runs — the motor must see at least 90–95% of rated voltage at full load.

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in p | Motor power | kW | 15 | 0
in v | Line voltage | V | 400 | 0
in e | Efficiency | % | 90 | 0
in pf | Power factor | - | 0.85 | 0
out i | Full-load current | A | (p * 1000) / (sqrt(3) * v * (e / 100) * pf) | 2
out pin | Input power | kW | p / (e / 100) | 2