ElectroHub

Motor Full Load Current Calculator

The rating stamped on a motor is its shaft output. The supply has to deliver that plus the motor's own losses, at the motor's own power factor — so efficiency and power factor both divide, and leaving either out under-sizes every cable and breaker downstream.

Motor full-load current

The nameplate rating of a motor is its shaft output. The supply has to deliver that plus the motor's own losses, at the motor's power factor — so efficiency and power factor both divide. Leave either out and every cable and breaker downstream comes out too small.

Full-load current
Starting current (DOL)
Input power
Input apparent power
  1. 1.FormulaOutput kW = hp × 0.7355 (metric hp)Substitute= 5 × 0.7355Result= 3.677 kW
  2. 2.FormulaInput kW = output ÷ efficiencySubstitute= 3.677 ÷ 0.88Result= 4.179 kW
  3. 3.FormulaInput kVA = input kW ÷ power factorSubstitute= 4.179 ÷ 0.85Result= 4.916 kVA
  4. 4.FormulaI = kW × 1000 / (√3 × V × η × cos φ)Substitute= 3.677 × 1000 / (1.732 × 415 × 0.88 × 0.85)ResultI = 6.84 A
  • Direct-on-line starting draws roughly 6× full-load current — about 41 A here — for a few seconds. Protection has to ride through that without tripping, which is why motor circuits use a C or D curve rather than B.
Common trap: Efficiency and power factor are not interchangeable and neither is optional. A 7.5 kW motor at 88 % and 0.85 pf draws 14 A at 415 V — leave both out and you get 10.4 A, which is a whole cable size and a whole breaker size wrong.

The formula

I = kW × 1000 / (√3 × V × η × cos φ) [three phase]

kW
nameplate output power (hp × 0.7355 for metric horsepower)
V
line voltage
η
full-load efficiency, as a fraction
cos φ
full-load power factor
√3
present for three phase, absent for single phase

Worked example

A 7.5 kW three-phase motor on 415 V, 88 % efficient at 0.85 power factor.

  1. Input power = 7.5 / 0.88 = 8.52 kW
  2. Input apparent power = 8.52 / 0.85 = 10.03 kVA
  3. I = 7500 / (1.732 × 415 × 0.88 × 0.85)
  4. I = 13.95 A
  5. Direct-on-line starting draws roughly 6 × that — about 84 A for a few seconds

13.95 A full load, about 84 A while starting.

Where you'll use it

Sizing the cable, the breaker and the starter for a motor circuit, and checking a nameplate current that looks wrong. The starting figure is why motor circuits use a C or D curve breaker: a B curve would trip every time the motor started.

The laws behind it

Parts this applies to

Resistor (fixed)Potentiometer / Rheostat / TrimmerResistor Array / NetworkThermistor (NTC / PTC)Varistor / MOVSupercapacitorLED (light-emitting diode)MOSFET

Related calculators

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