A motor current calculator for full-load amps
This motor current calculator turns a motor rating in kW or horsepower into the full-load current it draws, for single-phase and three-phase supplies. Enter the shaft power, line voltage, power factor and efficiency and it shows the electrical input power, the apparent power in kVA and the current in amps, with every step of the working.
It is the quick answer to questions such as "how many amps does a 15 kW motor draw at 415 V" or "what is the current of a 20 HP motor", and it also gives a table of the usual motor sizes at the same voltage, power factor and efficiency so you can compare ratings at a glance.
The values are computed from the power formula. They are a planning estimate: the motor nameplate, and published tables such as NEC Tables 430.248 and 430.250 or the IEC equivalents, take precedence when you choose cables, overload relays and protective devices.
How to use the Motor Full-Load Current Calculator
- Enter the motor power and pick kW, HP or metric PS.
- Choose three-phase or single-phase and type the line voltage, such as 415, 400, 460 or 230 V.
- Enter the power factor and efficiency from the nameplate (typical values are already filled in).
- Read the full-load current, the 125% figure often used as a starting point for cable and protection checks, and the direct-on-line start-up estimate.
- Copy the working, or scan the table for other motor sizes.
Motor Full-Load Current Calculator features
kW, HP and PS
Mechanical horsepower is converted at 745.7 W, metric horsepower at 735.5 W.
Single- and three-phase
The √3 factor is applied only for three-phase, using line-to-line voltage.
Working shown
Shaft power, input power, kVA and amps are listed step by step.
Rating table
Common motor sizes at your voltage, power factor and efficiency.
The motor current formula
The shaft power of a motor is less than the power it takes from the supply, because of losses. Input power is therefore Pin = Pout ÷ η. For a three-phase motor the line current is I = Pin × 1,000 ÷ (√3 × V × PF), and for a single-phase motor I = Pin × 1,000 ÷ (V × PF), with P in kW and V in volts.
Worked example: a 15 kW three-phase motor at 415 V, power factor 0.85 and efficiency 90%. Input power is 15 ÷ 0.9 = 16.67 kW; apparent power is 16.67 ÷ 0.85 = 19.61 kVA; current is 16,667 ÷ (1.732 × 415 × 0.85) = 27.3 A. A 20 HP motor is 14.91 kW at the shaft, because 1 HP is 745.7 W.
Using the result
- Cable and protection sizing normally starts from the full-load current multiplied by a factor set by your wiring rules (125% is a common starting point). Follow the code that applies to your installation.
- Starting current for a direct-on-line start is often quoted as 5 to 8 times full-load current; star-delta, soft starters and drives reduce it. The multiple here is your own input.
- Real full-load current can differ by several percent from this estimate because power factor and efficiency change with loading and between manufacturers.
Motor Full-Load Current Calculator FAQ
How do I calculate motor current from kW?
Divide the kW by the efficiency to get input kW, then I = kW × 1,000 ÷ (√3 × V × PF) for three-phase. A 15 kW, 415 V motor at PF 0.85 and 90% efficiency draws about 27.3 A.
How do I calculate motor current from HP?
Convert HP to kW (1 HP = 0.7457 kW) and use the same formula. A 20 HP motor is 14.9 kW at the shaft.
Which power factor and efficiency should I use?
Use the nameplate or datasheet values. If you have only a rough idea, 0.8 to 0.9 power factor and 85 to 95% efficiency are common for standard industrial motors, but they vary with size and load.
Is this the same as the NEC table value?
No. NEC tables list assumed full-load currents for sizing conductors and protection and are copyrighted, so they are not reproduced here. This page computes the current from the formula; where a code table applies, use the table.
Why is the current higher at a lower voltage?
For the same power the current rises as voltage falls: halving the voltage doubles the current.
Does this include the service factor?
No. The service factor lets a motor run above its rating for short periods; it is not part of the full-load current.