Calculators
Calculate motor current
The nameplate states the shaft power, but sizing cable, contactor and protective device needs the current. Efficiency belongs in the calculation, because it is the difference between what the motor delivers and what it draws.
Result
- Rated current
- —A
- Apparent power
- —kVA
- Electrical active power
- —kW
- Starting current, direct on lineAssumed at 6 times rated current.
- —A
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A guide, not a design to standard. We give no warranty for the correctness of the results. The applicable standards and case-by-case verification govern.
The grey values are an example. Simply enter your own.
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Calculating is the easy part.
A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.
Method and questions
Motor current
Rated current of a three-phase motor from power, voltage and efficiency.
Discuss a projectWorked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- System
- Three-phase (3~)
- Shaft power
- 7.5 kW
- Nominal voltage
- 400 V
- Power factor cos φ
- 0.86
- Efficiency η
- 90 %
Result
- Rated current
- 14 A
- Apparent power
- 9.69 kVA
- Electrical active power
- 8.33 kW
- Starting current, direct on line
- 83.9 A
Formula and standards
Formula
- Three-phase I = (P / η) / (√3 · U · cos φ)
- Single-phase I = (P / η) / (U · cos φ)
- S = P / (η · cos φ)
Assumptions and standards
- P is the shaft power (mechanical output) from the nameplate.
- Starting current is assumed at 6 times rated current. A common guide value for direct-on-line starting. The actual figure is in the data sheet and lies between 4 and 8 times.
- Sizing the protective device also depends on the starting time.
Why is the current higher than the rule of thumb “2 × kW”?
The 400 V rule of thumb ignores efficiency and power factor. For a 7.5 kW motor with cos φ 0.86 and 90 % efficiency the rated current is around 14 A rather than 15 A. So the rule of thumb lands above or below depending on the motor.
Which cos φ should I use?
The nameplate value at rated load. At partial load it drops considerably. A motor running permanently at half load has a markedly worse power factor.
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