How do I work out amps when I have V and kw?

Question

415 voltage, 15 KiloWatts, 3 phase motor... How do I work out what current it will draw on load?

Answer 1

A kilowatt is 1000 watts after applying ohms law you would draw

approximately 26.145 amps.  per phase

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Answer 2

These previous answers are almost there.

Because you are asking about motors in kW's I have to assume that you are using the european standards for motor ratings.

This measures the output of the motor, NOT the input as the americans do with HP.

This means that when you are calculating the amps you MUST include the efficiency of the motor.

I use a conservative estimate of 0.76 pf and 0.89 eff

the formula should look like this

I= (Kw X 1000)/(E X pf X Eff)

where E= volts, pf=power factor, eff= efficiency

This yields a result of 30.78 Amps @415V

Its always better to assume the worst. (also if you are wiring this a long distance from your motor starter, you may want to include volt drop in the calculation)

Answer 3

The equation for 3 phase power is as follows:-

Power (Watts) = 1.7321 x Line Voltage (VL)  X Line Current (IL) X Power Factor

1.7321 is the Square root of 3 so P = Sqrt of 3 x VL x IL x PF

If power Factor is not available assume 0.8 as a starting point

Re-arrange formula Current = Power divided by (1.7321 x VL x IL x0.8)

IL= 15000/(1.7321 x 415 x 0.8) = 26 Amps per phase