# 3-Phase Motor Amperage Calculator

Created by Kenneth Alambra
Last updated: Aug 01, 2022

If your motor's rating plate does not indicate the required amperage to run your motor, this 3-phase motor amperage calculator can help you with that. Powering a motor is not as simple as it sounds if you want to properly run it, especially if it has to provide a particular angular velocity and torque. To meet those desired outputs, we must follow the power, voltage, and amperage requirements of our motor. Keep on reading to learn:

• What a 3-phase motor is;
• How to calculate a 3-phase motor full-load current; and
• How to use this 3-phase motor amperage calculator.

## What is a 3-phase motor?

Motors are machines that convert electrical energy to rotational mechanical energy. There are lots of different types of motors, and one way to classify them is by the power source we use to power them. That could either be a single-phase or three-phase power source.

A single-phase motor runs on a single-phase alternating current and requires some applied movement on its rotor to start running. By utilizing a 3-phase power source, we can produce a magnetic field that results in a starting torque within the motor that starts the rotor's rotation once we power the motor. Also, what a 3-phase motor can do that a single-phase motor cannot is that it can produce high angular velocity and torque because of its 3-phase configuration. That is, given we provide its necessary power requirements.

In the next section of this text, let us discuss the relationship of the parameters we need to run a 3-phase motor.

## How to calculate a 3-phase motor current requirement

We can use Watt's Law to derive the 3-phase current formula, as shown below:

$\small I = \frac{P}{V\times \cos\phi\times \eta \times c}$

where:

• $I$ is the motor's full load amperage (in amps);
• $P$ is the motor power rating (in watts);
• $V$ is the motor voltage to run the motor (in volts);
• $\cos \phi$ is the motor's power factor (with a value from $0\ \text{to}\ 1$ for 3-phase motors, and $1$ for single-phase motors);
• $\eta$ is the motor efficiency (in percentage); and
• $c$ is a multiplier constant equal to $\sqrt 3$ for 3-phase line-to-line voltage type, $3$ for 3-phase line-to-neutral voltage type, and $1$ for single-phase load distribution.

## Sample 3-phase motor current calculation

Say we need to find the amperage of 3-phase motor with a 0.95 power factor, efficiency of 80% (or 0.8), and runs on a 220-volt 5-kilowatt power source. We have to substitute the variables on our 3-phase current formula with these values to find the amperage:

\small \begin{align*} I &= \frac{P}{V\times \cos\phi\times \eta \times c}\\\\ &= \frac{5\ \text{kW}}{220\ \text{V} \times\ 0.95 \times\ 0.8 \times \sqrt 3}\\\\ &= \frac{5000\ \text{W}}{289.599\ \text{V}}\\\\ &= 17.265\ \text{A} \end{align*}

That's how to calculate the full load current of a 3-phase motor.

🙋 If your motor power rating plate indicates a power rating in horsepower (hp), don't forget to multiply it first by 746 to convert it to watts before using it in our 3-phase current formula.

## How to use this 3-phase motor amperage calculator?

Using our 3-phase motor amperage calculator is easy and only requires a few simple steps listed below:

1. Select the load distribution of your power source. Since we're talking about 3-phase motors, we've already picked 3-phase as a default selection. But you can also use this tool as a single-phase motor calculator if you need to.
2. Choose the voltage type for your wiring. That could be line-to-line voltage or line-to-neutral voltage.
3. Enter your motor voltage requirement.
4. Input the power rating provided on your motor's specification.
5. Key in your motor's power factor (values range from 0 to 1) and its motor efficiency percentage rating.
6. Expect our tool to display your motor amperage requirement after entering the above parameters.
Kenneth Alambra
3-phase
Voltage type
Line to Line Voltage
Motor voltage (V)
V
Motor power rating (P)
kW
Power factor (cos Φ)
Motor efficiency (η)
%
Amperage (I)
A
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