3-Phase Motor Amperage Calculator

This three phase motor current calculation tool calculates the full load rated current from the motor nameplate rating output, supply voltage, power factor and efficiency. By entering the values on the name plate you can check the current values. If one value is missing it can be calculated back. For example if you enter a measured current value then it will calculate the motor's output, efficiency and power factor.

What is a 3-phase motor?

A 3-phase motor is an electric motor that runs on a three-phase alternating current power supply. It produces a rotating magnetic field by feeding current from three conductors whose phases are offset by one-third of a cycle (120 degrees) to three sets of windings, causing the rotor to turn.

Three-phase motors are used in nearly all industrial applications, such as pumps, compressors, fans and conveyors. For the same rated output, a three-phase network can provide more stable power supply and higher torque than a single phase network with thinner wires.

The nameplate of a motor will show the rated output, voltage, power factor and efficiency. If there is no nameplate or if the information on it is illegible, you can use this calculator to estimate the current and use that as a guide in selecting the appropriate cable size and protective devices.

How to calculate the current of an 3-phase motor?

The current in the conductor at full load of an induction motor is calculated by dividing the input power by the product of the factor root three, the voltage between the conductors, the power factor and the efficiency.

If the rated power is given in kilowatts it is multiplied by 1000 to convert it into watts and if it is given in horsepower then it is multiplied by 746.

The principle is similar for single-phase motors, but since there are only one conductor and not three, the square root of three factor does not appear in the formula.

In the formula above P is the rated power, V is the voltage, PF is the power factor (as a decimal) and η (eta) is also the efficiency (as a decimal). The calculation tools use base units so if you select kW or hp the conversion will be done automatically.

Example calculation:

Let's take a three-phase motor with a nominal power of 15 kW, voltage of 400 V, power factor of 0.86 and efficiency of 90 percent. First the nominal power is converted to 15000 watts, then the individual values are inserted into the formula.

Thus, the full load current of this motor is about 28 amperes. The following table summarizes the symbols used in the formulas and the values for this example.

Real power, apparent power and reactive power

The result area shows the load states divided into three components of the power triangle. The real power is given in kilowatts and corresponds to the actual useful power of the motor, including shaft power and losses. It is calculated by dividing the output power by the efficiency.

Apparent power is expressed in kilovolt-amperes (kVA) and represents the capacity that must actually be provided by a source of electrical energy, generator or transformer. It is calculated by dividing real power by the power factor. Reactive power is expressed in kVARs and is a component which does not perform actual work as it is used for excitation but does demand circuit capacity.

As the power supply capacity is determined by apparent power, current and kVA will increase even though shaft power stays the same when the power factor is low.

The difference between full-load current and starting current.

This tool is used to determine the full-load current, which is the current that flows when the motor is running stably at its rated load and speed. The starting current is different from this.

In 3-phase motors with direct coupling, an inrush current flows during the short acceleration phase when inertia is overcome, when the motor starts. This amounts to around five to seven times the rated current. The duration of this period is normally less than one second. By activating the estimation of the starting current, it can be shown and used as a guide for selecting power switches, fuses or soft starters in order to avoid unwanted triggering during startup.

Why motor current is important:

The full-load current is the basis for determining wire cross-sections in circuits, overcurrent relay settings and rating values of power switches and fuses. If cables are too thin they can overheat, and if the capacity of protective devices is too high it may be that they do not function properly with an inrush current.

If the current value is known then it can be used to check the suitability of the motor and power supply before connection or to see if a current in excess of the rating is being drawn due to wear or overloading. It can also give an indication as to how much load will be added by new equipment on the distribution board.

This calculator provides a rough estimate for planning purposes, based on the data from the nameplate. The actual power draw depends on load, voltage, temperature and condition of motor. Please consult local installation codes (e.g. NEC or IEC) and have your installation checked by a qualified electrician.